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Air Insulated Substations

Air Insulated Substations

Various factors affect the reliability of a substation, one of which is the arrangement of the switching devices. Arrangement of the switching devices will impact maintenance, protection, initial substation development, and cost. There are six types of substation bus switching arrangements commonly used in air insulated substations:
1. Single bus
2. Double bus, double breaker
3. Main and transfer (inspection) bus
4. Double bus, single breaker
5. Ring bus
6. Breaker and a half

1. Single Bus Configuration

Single Bus Configuration

Single Bus Configuration

This arrangement involves one main bus with all circuits connected directly to the bus. The reliability of this type of an arrangement is very low. When properly protected by relaying, a single failure to the main bus or any circuit section between its circuit breaker and the main bus will cause an outage of the entire system. In addition, maintenance of devices on this system requires the de-energizing of the line connected to the device. Maintenance of the bus would require the outage of the total system, use of standby generation, or switching to adjacent station, if available. Since the single bus arrangement is low in reliability, it is not recommended for heavily loaded substations or substations having a high availability requirement. Reliability of this arrangement can be improved by the addition of a bus tiebreaker to minimize the effect of a main bus failure.

2. Double Bus, Double Breaker Configuration

Double bus, double breaker

Double bus, double breaker

This scheme provides a very high level of reliability by having two separate breakers available to each circuit. In addition, with two separate buses, failure of a single bus will not impact either line. Maintenance of a bus or a circuit breaker in this arrangement can be accomplished without interrupting either of the circuits. This arrangement allows various operating options as additional lines are added to the arrangement; loading on the system can be shifted by connecting lines to only one bus. A double bus, double breaker scheme is a high-cost arrangement, since each line has two breakers and requires a larger area for the substation to accommodate the additional equipment. This is especially true in a low profile configuration. The protection scheme is also more involved than a single bus scheme.

3. Main and Transfer Bus Configuration

Main and transfer bus configuration

Main and transfer bus configuration

This scheme is arranged with all circuits connected between a main (operating) bus and a transfer bus (also referred to as an inspection bus). Some arrangements include a bus tie breaker that is connected between both buses with no circuits connected to it. Since all circuits are connected to the single, main bus, reliability of this system is not very high. However, with the transfer bus available during maintenance, de-energizing of the circuit can be avoided. Some systems are operated with the transfer bus normally de-energized. When maintenance work is necessary, the transfer bus is energized by either closing the tie breaker, or when a tie breaker is not installed, closing the switches connected to the transfer bus. With these switches closed, the breaker to be maintained can be opened along with its isolation switches. Then the breaker is taken out of service. The circuit breaker remaining in service will now be connected to both circuits through the transfer bus. This way, both circuits remain energized during maintenance. Since each circuit may have a different circuit configuration, special relay settings may be used when operating in this abnormal arrangement.

When a bus tie breaker is present, the bus tie breaker is the breaker used to replace the breaker being maintained, and the other breaker is not connected to the transfer bus. A shortcoming of this scheme is that if the main bus is taken out of service, even though the circuits can remain energized through the transfer bus and its associated switches, there would be no relay protection for the circuits. Depending on the system arrangement, this concern can be minimized through the use of circuit protection devices (reclosure or fuses) on the lines outside the substation.
This arrangement is slightly more expensive than the single bus arrangement, but does provide more flexibility during maintenance. Protection of this scheme is similar to that of the single bus arrangement. The area required for a low profile substation with a main and transfer bus scheme is also greater than that of the single bus, due to the additional switches and bus.

4. Double Bus, Single Breaker Configuration

Double bus, single breaker configuration

Double bus, single breaker configuration

This scheme has two main buses connected to each line circuit breaker and a bus tie breaker. Utilizing the bus tie breaker in the closed position allows the transfer of line circuits from bus to bus by means of the switches. This arrangement allows the operation of the circuits from either bus. In this arrangement, a failure on one bus will not affect the other bus. However, a bus tie breaker failure will cause the outage of the entire system. Operating the bus tie breaker in the normally open position defeats the advantages of the two main buses. It arranges the system into two single bus systems, which as described previously, has very low reliability. Relay protection for this scheme can be complex, depending on the system requirements, flexibility, and needs. With two buses and a bus tie available, there is some ease in doing maintenance, but maintenance on line breakers and switches would still require outside the substation switching to avoid outages.

5. Ring Bus Configuration

Ring bus configuration

Ring bus configuration

In this scheme, as indicated by the name, all breakers are arranged in a ring with circuits tapped between breakers. For a failure on a circuit, the two adjacent breakers will trip without affecting the rest of the system. Similarly, a single bus failure will only affect the adjacent breakers and allow the rest of the system to remain energized. However, a breaker failure or breakers that fail to trip will require adjacent breakers to be tripped to isolate the fault. Maintenance on a circuit breaker in this scheme can be accomplished without interrupting any circuit, including the two circuits adjacent to the breaker being maintained. The breaker to be maintained is taken out of service by tripping the breaker, then opening its isolation switches. Since the other breakers adjacent to the breaker being maintained are in service, they will continue to supply the circuits. In order to gain the highest reliability with a ring bus scheme, load and source circuits should be alternated when connecting to the scheme. Arranging the scheme in this manner will minimize the potential for the loss of the supply to the ring bus due to a breaker failure. Relaying is more complex in this scheme than some previously identified. Since there is only one bus in this scheme, the area required to develop this scheme is less than some of the previously discussed schemes. However, expansion of a ring bus is limited, due to the practical arrangement of circuits.

6. Breaker-and-a-Half Configuration

Breaker and a half configuration

Breaker and a half configuration

The breaker-and-a-half scheme can be developed from a ring bus arrangement as the number of circuits increases. In this scheme, each circuit is between two circuit breakers, and there are two main buses. The failure of a circuit will trip the two adjacent breakers and not interrupt any other circuit. With the three breaker arrangement for each bay, a center breaker failure will cause the loss of the two adjacent circuits. However, a breaker failure of the breaker adjacent to the bus will only interrupt one circuit.

Maintenance of a breaker on this scheme can be performed without an outage to any circuit. Further- more, either bus can be taken out of service with no interruption to the service. This is one of the most reliable arrangements, and it can continue to be expanded as required. Relaying is more involved than some schemes previously discussed. This scheme will require more area and is costly due to the additional components.

Table of configurations
ConfigurationReliabilityCostAvailable area
.Single busLeast reliable — single failure can cause complete outageLeast cost — fewer componentsLeast area — fewer components
.Double busHighly reliable — duplicated components; single failure normally isolates single componentHigh cost — duplicated componentsGreater area — twice as many components
.Main bus and .transferLeast reliable — same as
Single bus, but flexibility in operating and maintenance with transfer bus
Moderate cost — fewer componentsLow area requirement —  fewer components
.Double bus, .single breakerModerately reliable — depends on arrangement of components and busModerate cost — more componentsModerate area — more components
.Ring busHigh reliability — single failure isolates single componentModerate cost — more componentsModerate area — increases with number of circuits
.Breaker and a.halfHighly reliable — single circuit failure isolates single circuit, bus failures do not affect circuitsModerate cost — breaker-and-a-half for each circuitGreater area — more components per circuit


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Power quality

Power quality

The term power quality seeks to quantify the condition of the electrical supply. It not only relates largely to voltage, but also deals with current and it is largely the corrupting effect of current disturbances upon voltage. Power quality can be quantified by a very broad range of parameters, some of which have been recognized and studied for as long as electrical power has been utilized. However, the advent of the term itself is more modern and it has created a useful vehicle for discussing and quantifying all factors that can describe supply quality. Power quality is yet another means of analysing and expressing electromagnetic compatibility (EMC), but in terms of the frequency spectrum, power quality charac- terizes mainly low-frequency phenomena. Perhaps because of this and because of the manner in which it affects electrical equipment, power quality has largely been dealt with by engineers with electrical power experience rather than those with an EMC expertise. In reality, resolving power problems can benefit from all available expertise, particularly since power quality disorders and higher frequency emissions can produce similar effects.

In 1989, the European Community defined the supply of electricity as a product, and it is therefore closely related to the provisions and protection of the EMC Directive (89/336/EEC), but in drawing a comparison between electricity and other manufactured product it is essential to recall a significant difference.

Electricity is probably unique in being a product which is manufactured, delivered and used at the same time. An electricity manufacturer cannot institute a batch testing process for example and pull substandard products out of the supply chain. By the time electricity is tested it will have been delivered and used by the customer whether it was of good quality or not.

Key parameters

The parameters that are commonly used to characterize supplies are listed in Table 1 together with the typical tolerance limits which define acceptable norms. Within Europe these power quality limits are defined by the EN 61000 series of standards in order to be compatible with the susceptibility limits set for equipment.

Table 1: Summary of power quality levels defined by EN 50160
.Power frequency (50Hz).Interconnected systems
.±1% (95% of week)
.+4% (absolute level)
.−6% (absolute level)
.Supply voltage variations on 230V nominal.±10% (95% of week based on
.10 min samples, rms)
.Rapid voltage changes.±5% Frequent
.±10% Infrequent
.Flicker.Pk=1.0 (95% of week)
.Supply voltage dips.Majority
.Few 10s
.Duration <1s
.Depth <60%
.Some locations
.Few 1000 per year of <15% depth
.Short interruptions.20–500 per year
.Duration 1s of 100% depth
.Long interruptions.10–50 per year
.Duration >180s of 100% depth
.Temporary power frequency overvoltage.<1.5kV
.Transient overvoltages.Majority
.Supply voltage unbalance.Majority
.<2%(95% of the week)
.>2%, <3%(95% of the week)
.Harmonic voltage distortion.THD <8%(95% of the week)
.Interharmonic voltage distortion.Under consideration
.Mains signalling.95 to 148.5kHz at up to 1.4Vrms (not in MV)

The more a supply deviates from these limits, the more likely it is that malfunction could be experienced in terminating equipment. However, individual items of equipment will have particular sensitivity to certain power quality parameters while having a wider tolerance to others. Table 2 provides examples of equipment and the power quality parameters to which they are particularly sensitive. Table 2 shows a preponderance of examples with a vulnerability to voltage dips. Of all the power quality parameters, this is probably the most troublesome to the manufacturing industry; and in the early 1970s, as the industry moved towards a reliance on electronic rather than electromagnetic controls, it was commonly observed how much more vulnerable the industrial processes were to supply disturbances.

Supply distortion (characterized by harmonics) is another power quality parameter that has received enormous attention, with many articles, textbooks and papers written on the subject. However, the modern practices that will be discussed later have reduced the degree to which this currently presents a problem. Other parameters tend to be much less problematic in reality, although that is not to say that perceptions sometimes suggest otherwise. Voltage surge and tran- sient overvoltage in particular are often blamed for a wide range of problems.

Table 2: Examples of sensitivity to particular power quality parameters
Equipment typeVulnerable power quality parameter Effect if exceededRang
.Induction motor .Voltage unbalance.Excessive rotor heating.<3%
.Power factor correction .capacitors.Spectral frequency .content.This is usually .defined .in terms of harmonic .distortion.Capacitor failure due to .excessive current flow or .voltage.Most sensitive if .resonance occurs.In resonant .conditions
.PLCs .Voltage dips.Disruption to the programmed .functionality.V tr
.Computing systems .Voltage dips.Disruption to the programmed .functionality.V
.Variable speed drives, .motor starters and .attracted .armature control .relays .Voltage dips.Disruption to the control system .causing shutdown. V
.Power transformers .Spectral frequency content of .load current.This is usually .defined in terms of harmonic .distortion.Increased losses leading to excessive temperature rise.At full load
.Devices employing .phase .control, such as .light .dimmers and .generator .automatic .voltage regulator .(AVRs) .Alteration in waveform zero .crossing due to waveform .distortion, causing multiple .crossing or phase asymmetry.Instability.Will depend .upon the r
.Motor driven .speed-.sensitive plant .Induction and synchronous .motor shaft speed are .proportional to supply .frequency. Some driven loads .are .sensitive to even small .speed variations.The motors themselves are .tolerant of small speed .variations.At high supply .frequencies (>10%) shaft .stresses may be excessive due .to high running speeds.Limits .depend on .the .sensitivity

However, very often this is a scapegoat when the actual cause cannot be identified. Even when correlation with switching voltage transients is correctly observed, the coupling introduced by poor wiring installations or bad earth bonding practices can be the real problem. Unlike the other power quality parameters, voltage transients have a high frequency content and will couple readily through stray capacitance and mutual inductance into neighbouring circuits. Coupling into closed conductor loops that interface with sensitive circuits such as screens and drain wires can easily lead to spurious events.


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Adjustment Of Protection Relays Parameters

Adjustment Of Protection Relays Parameters

The successful operation of an MV distribution system depends on the proper selection and setting of switchgear relays.

Protective relays are arguably the least understood component of medium voltage (MV) circuit protection. In fact, somebelieve that MV circuit breakers operate by themselves, without direct initiation by protective relays. Others think that the operation and coordination of protective relays is much too complicated to understand. Let’s get into the details and eliminate these misbeliefs.

Background information

The IEEE Standard Dictionary defines a circuit breaker as follows:

A device designed to open and close a circuit by nonautomatic means, and to open the circuit automatically on a predetermined overload of current without injury to itself when properly applied within its rating.

By this definition, MV breakers are not true circuit breakers, since they do not open automatically on overcurrent. They are electrically operated power-switching devices, not operating until directed by some external device to open or close. This is true whether the unit is an air, oil, vacuum, or [SF.sub.6] circuit breaker. Sensors and relays are used to detect the overcurrent or other abnormal or unacceptable condition and to signal the switching mechanism to operate. The MV circuit breakers are the brute-force switches while the sensors and relays are the brains that direct their functioning.

The sensors can be current transformers (CTs), potential transformers (PTs), temperature or pressure instruments, float switches, tachometers, or any device or combination of devices that will respond to the condition or event being monitored. In switchgear application, the most common sensors are CTs to measure current and PTs to measure voltage. The relays measure sensor output and cause the breaker to operate to protect the system when preset limits are exceeded, hence the name “protective relays.” The availability of a variety of sensors, relays, and circuit breakers permits the design of complete protection systems as simple or as complex as necessary, desirable, and economically feasible.

Electromechanical relays

Electromechanical relay

Electromechanical relay

For many years, protective relays have been electromechanical devices, built like fine watches, with great precision and often with jeweled bearings. They have earned a well-deserved reputation for accuracy, dependability, and reliability. There are two basic types of operating mechanisms: the electromagnetic-attraction relay and the electromagnetic-induction relay.

Magnetic attraction relays. Magnetic-attraction relays, have either a solenoid that pulls in a plunger, or one or more electromagnets that attract a hinged armature. When the magnetic force is sufficient to overcome the restraining spring, the movable element begins to travel, and continues until the contact(s) close or the magnetic force is removed. The pickup point is the current or voltage at which the plunger or armature begins to move and, in a switchgear relay, the pickup value can be set very precisely.

These relays are usually instantaneous in action, with no intentional time delay, closing as soon after pickup as the mechanical motion permits. Time delay can be added to this type of relay by means of a bellows, dashpot, or a clockwork escapement mechanism. However, timing accuracy is considerably less precise than that of induction-type relays, and these relays are seldom used with time delay in switchgear applications.

Attraction-type relays can operate with either AC or DC on the coils; therefore, relays using this principle are affected by the DC component of an asymmetrical fault and must be set to allow for this.

Induction relays. Induction relays, are available in many variations to provide accurate pickup and time-current responses for a wide range of simple or complex system conditions. Induction relays are basically induction motors. The moving element, or rotor, is usually a metal disk, although it sometimes may be a metal cylinder or cup. The stator is one or more electromagnets with current or potential coils that induce currents in the disk, causing it to rotate. The disk motion is restrained by a spring until the rotational forces are sufficient to turn the disk and bring its moving contact against the stationary contact, thus closing the circuit the relay is controlling. The greater the fault being sensed, the greater the current in the coils, and the faster the disk rotates.

A calibrated adjustment, called the time dial, sets the spacing between the moving and stationary contacts to vary the operating time of the relay from fast (contacts only slightly open) to slow (contacts nearly a full disk revolution apart). Reset action begins when the rotational force is removed, either by closing the relay contact that trips a breaker or by otherwise removing the malfunction that the relay is sensing. The restraining spring resets the disk to its original position. The time required to reset depends on the type of relay and the time-dial setting (contact spacing).

With multiple magnetic coils, several conditions of voltage and current can be sensed simultaneously. Their signals can be additive or subtractive in actuating the disk. For example, a current-differential relay has two current coils with opposing action. If the two currents are equal, regardless of magnitude, the disk does not move. If the difference between the two currents exceeds the pickup setting, the disk rotates slowly for a small difference and faster for a greater difference. The relay contacts close when the difference continues for the length of time determined by the relay characteristics and settings. Using multiple coils, directional relays can sense direction of current or power flow, as well as magnitude. Since the movement of the disk is created by induced magnetic fields from AC magnets, induction relays are almost completely unresponsive to the DC component of an asymmetrical fault.

Most switchgear-type relays are enclosed in a semiflush-mounting drawout case. Relays usually are installed on the door of the switchgear cubicle. Sensor and control wiring are brought to connections on the case. The relay is inserted into the case and connected by means of small switches or abridging plug, depending on the manufacturer. It can be disconnected and withdrawn from the case without disturbing the wiring. When the relay is disconnected, the CT connections in the case are automatically shorted to short circuit the CT secondary winding and protect the CT from overvoltages and damage.

Many relays are equipped with a connection for a test cable. This permits using a test set to check the relay calibration. The front cover of the relay is transparent, can be removed for access to the mechanism, and has provisions for wire and lead seals to prevent tampering by unauthorized personnel.

Solid-state relays

Solid state relay

Solid state relay

Recently, solid-state electronic relays have become more popular. These relays can perform all the functions that can be performed by electromechanical relays and, because of the versatility of electronic circuitry and microprocessors, can provide many functions not previously available. In general, solid-state relays are smaller and more compact than their mechanical equivalents. For example, a 3-phase solid-state overcurrent relay can be used in place of three single-phase mechanical overcurrent relays, yet is smaller than one of them.

The precision of electronic relays is greater than that of mechanical relays, allowing closer system coordination. In addition, because there is no mechanical motion and the electronic circuitry is very stable, they retain their calibration accuracy for a long time. Reset times can be extremely short if desired because there is no mechanical motion.

Electronic relays require less power to operate than their mechanical equivalents, producing a smaller load burden on the CTs and PTs that supply them. Because solid-state relays have a minimum of moving parts, they can be made very resistant to seismic forces and are therefore especially well suited for areas susceptible to earthquake activity.

In their early versions, some solid-state relays were sensitive to the severe electrical environment of industrial applications. They were prone to failure, especially from high transient voltages caused by lightning or utility and on-site switching. However, today’s relays have been designed to withstand these transients and other rugged application conditions, and this type of failure has essentially been eliminated. Solid-state relays have gained a strong and rapidly growing position in the marketplace as experience proves their accuracy, dependability, versatility, and reliability.

The information that follows applies to electromechanical and solid-state relays, although one functions mechanically and the other electronically. Significant differences will be pointed out.

Relay types

There are literally hundreds of different types of relays. The catalog of one manufacturer of electromechanical relays lists 264 relays for switchgear and system protection and control functions. For complex systems with many voltage levels and interconnections over great distances, such as utility transmission and distribution, relaying is an art to which some engineers devote their entire careers. For more simple industrial and commercial distribution, relay protection can be less elaborate, although proper selection and application are still very important.

The most commonly used relays and devices are listed HERE in the Table by their American National Standards Institute (ANSI) device-function number and description. These standard numbers are used in one-line and connection diagrams to designate the relays or other devices, saving space and text.

Where a relay combines two functions, the function numbers for both are shown. The most frequently used relay is the overcurrent relay, combining both instantaneous and inverse-time tripping functions. This is designated device 50/51. As another example, device 27/59 would be a combined undervoltage and overvoltage relay. The complete ANSI standard lists 99 device numbers, a few of which are reserved for future use.

Relays can be classified by their operating-time characteristics. Instantaneous relays are those with no intentional time delay. Some can operate in one-half cycle or less; others may take as long as six cycles. Relays that operate in three cycles or less are called high-speed relays.

Time-delay relays can be definite-time or inverse-time types. Definite-time relays have a preset time delay that is not dependent on the magnitude of the actuating signal (current, voltage, or whatever else is being sensed) once the pickup value is exceeded. The actual preset time delay is usually adjustable.

Inverse-time relays, such as overcurrent or differential relays, have operating times that do depend on the value of actuating signal. The time delay is long for small signals and becomes progressively shorter as the value of the signal increases. The operating time is inversely proportional to the magnitude of the event being monitored.

Overcurrent relays

Sepam protection relay

Sepam protection relay

In switchgear application, an overcurrent relay usually is used on each phase of each circuit breaker and often one additional overcurrent relay is used for ground-fault protection. Conventional practice is to use one instantaneous short-circuit element and one inverse-time overcurrent element (ANSI 50/51) for each phase.

In the standard electromechanical relay, both elements for one phase are combined in one relay case. The instantaneous element is a clapper or solenoid type and the inverse-time element is an induction-disk type.

In some solid-state relays, three instantaneous and three inverse-time elements can be combined in a single relay case smaller than that of one induction-disk relay.

Overcurrent relays respond only to current magnitude, not to direction of current flow or to voltage. Most relays are designed to operate from the output of a standard ratio-type CT, with 5A secondary current at rated primary current. A solid-state relay needs no additional power supply, obtaining the power for its electronic circuitry from the output of the CT supplying the relay.

On the instantaneous element, only the pickup point can be set, which is the value of current at which the instantaneous element will act, with no intentional time delay, to close the trip circuit of the circuit breaker. The actual time required will decrease slightly as the magnitude of the current increases, from about 0.02 sec maximum to about 0.006 sec minimum, as seen from the instantaneous curve. This time will vary with relays of different ratings or manufacturers and also will vary between electromechanical and solid-state relays.

Time delays can be selected over a wide range for almost any conceivable requirement. Time-delay selection starts with the choice of relay. There are three time classifications: standard, medium, and long time delay. Within each classification, there are three classes of inverse-time curve slopes: inverse (least steep), very inverse (steeper), and extremely inverse (steepest). The time classification and curve slopes are characteristic of the relay selected, although for some solid-state relays these may be adjustable to some degree. For each set of curves determined by the relay selection, the actual response is adjustable by means of the time dial.

On the inverse-time element, there are two settings. First the pickup point is set. This is the value of current at which the timing process begins as the disk begins to rotate on an electromechanical relay or the electronic circuit begins to time out on a solid-state relay.

Next the time-dial setting is selected. This adjusts the time-delay curve between minimum and maximum curves for the particular relay. A given relay will have only one set of curves, either inverse, very inverse, or extremely inverse, adjustable through the full time-dial range. Note that the current is given in multiples of pickup setting.

Each element, instantaneous or time delay, has a flag that indicates when that element has operated. This flag must be reset manually after relay operation.

Setting the pickup point

The standard overcurrent relay is designed to operate from a ratio-type CT with a standard 5A secondary output. The output of the standard CT is 5A at the rated nameplate primary current, and the output is proportional to the primary current over a wide range. For example, a 100/5 ratio CT would have a 5A output when the primary current (the current being sensed and measured) is 100A. This primary-to-secondary ratio of 20-to-1 is constant so that for a primary current of 10A, the secondary current would 0.5A; for 20A primary, 1.0A secondary; for 50A primary, 2.5A secondary; etc. For 1000A primary, the secondary current is 50A, and similarly for all values of current up to the maximum that the CT will handle before it saturates and becomes nonlinear.

The first step in setting the relay is selecting the CT so that the pickup can be set for the desired primary current value. The primary current rating should be such that a primary current of 110 to 125% of the expected maximum load will produce the rated 5A secondary current. The maximum available primary fault current should not produce more than 100A secondary current to avoid saturation and excess heating. It may not be possible to fulfill these requirements exactly, but they are useful guidelines. As a result, some compromise may be necessary.

On the 50/51 overcurrent relay, the time-overcurrent-element (device 51) setting is made by means of a plug or screw inserted into the proper hole in a receptacle with a number of holes marked in CT secondary amperes, by an adjustable calibrated lever or by some similar method. This selects one secondary current tap (the total number of taps depends on the relay) on the pickup coil. The primary current range of the settings is determined by the ratio of the CT selected.

For example, assume that the CT has a ratio of 50/5A. Typical taps will be 4, 5, 6, 7, 8, 10, 12, and 16A. The pickup settings would range from a primary current of 40A (the 4A tap) to 160A (the 16A tap). If a 60A pickup is desired, the 6A tap is selected. If a pickup of more than 160A or less than 40A is required, it would be necessary to select a CT with a different ratio or, in some cases, a different relay with higher or lower tap settings.

Various types of relays are available with pickup coils rated as low as 1.5A and as high as 40A. Common coil ranges are 0.5 to 2A, for low-current pickup such as ground-fault sensing; 1.5 to 6A medium range; or 4 to 16A, the range usually chosen for overcurrent protection. CTs are available having a wide range of primary ratings, with standard 5A secondaries or with other secondary ratings, tapped secondaries, or multiple secondaries.

A usable combination of CT ratio and pickup coil can be found for almost any desired primary pickup current and relay setting.

The instantaneous trip (device 50) setting is also adjustable. The setting is in pickup amperes, completely independent of the pickup setting of the inverse-time element or, on some solid-state relays, in multiples of the inverse-time pickup point. For example, one electromechanical relay is adjustable from 2 to 48A pickup; a solid-state relay is adjustable from 2 to 12 times the setting of the inverse-time pickup tap. On most electromechanical relays, the adjusting means is a tap plug similar to that for the inverse-time element. With the tap plug, it is possible to select a gross current range. An uncalibrated screw adjustment provides final pickup setting. This requires using a test set to inject calibration current into the coil if the setting is to be precise. On solid-state relays, the adjustment may be a calibrated switch that can be set with a screwdriver.

Setting the time dial

For any given tap or pickup setting, the relay has a whole family of time-current curves. The desired curve is selected by rotating a dial or moving a lever. The time dial or lever is calibrated in arbitrary numbers, between minimum and maximum values, as shown on curves published by the relay manufacturer. At a time-dial setting of zero, the relay contacts are closed. As the time dial setting is increased, the contact opening becomes greater, increasing relay operating time. Settings may be made between calibration points, if desired, and the applicable curve can be interpolated between the printed curves.

The pickup points and time-dial settings are selected so that the relay can perform its desired protective function. For an overcurrent relay, the goal is that when a fault occurs on the system, the relay nearest the fault should operate. The time settings on upstream relays should delay their operation until the proper overcurrent device has cleared the fault. A selectivity study, plotting the time-current characteristics of every device in that part of the system being examined, is required. With the wide selection of relays available and the flexibility of settings for each relay, selective coordination is possible for most systems.

Selecting and setting other than overcurrent relays are done in similar fashion. Details will vary, depending on the type of relay, its function in the system, and the relay manufacturer.

Relay operation

An electromechanical relay will pick up and start to close its contacts when the current reaches the pickup value. At the inverse-time pickup current, the operating forces are very low and timing accuracy is poor. The relay timing is accurate at about 1.5 times pickup or more, and this is where the time-current curves start. This fact must be considered when selecting and setting the relay.

When the relay contacts close, they can bounce, opening slightly and creating an arc that will burn and erode the contact surfaces. To prevent this, overcurrent relays have an integral auxiliary relay with a seal-in contact in parallel with the timing relay contacts that closes immediately when the relay contacts touch. This prevents arcing if the relay contacts bounce. This auxiliary relay also activates the mechanical flag that indicates that the relay has operated.

When the circuit breaker being controlled by the relay opens, the relay coil is deenergized by an auxiliary contact on the breaker. This protects the relay contacts, which are rated to make currents up to 30A but should not break the inductive current of the breaker tripping circuit, to prevent arcing wear. The disk is then returned to its initial position by the spring. The relay is reset. Reset time is the time required to return the contacts fully to their original position. Contacts part about 0.1 sec (six cycles) after the coil is deenergized. The total reset time varies with the relay type and the time-dial setting. For a maximum time-dial setting (contacts fully open), typical reset times might be 6 sec for an inverse-time relay and up to 60 sec for a very inverse or extremely inverse relay. At lower time-dial settings, contact opening distance is less, therefore reset time is lower.

A solid-state relay is not dependent on mechanical forces or moving contacts for its operation but performs its functions electronically. Therefore, the timing can be very accurate even for currents as low as the pickup value. There is no mechanical contact bounce or arcing, and reset times can be extremely short.

CT and PT selection
MV current transformer

MV current transformer

In selecting instrument transformers for relaying and metering, a number of factors must be considered; transformer ratio, burden, accuracy class, and ability to withstand available fault currents.

CT ratio. CT guidelines mentioned earlier are to have rated secondary output at 110 to 125% of expected load and no more than 100A secondary current at maximum primary fault current. Where more than one CT ratio may be required, CTs with tapped secondary windings or multi-winding secondaries are available.

CT burden. CT burden is the maximum secondary load permitted, expressed in voltamperes (VA) or ohms impedance, to ensure accuracy. ANSI standards list burdens of 2.5 to 45VA at 90% power factor (PF) for metering CTs, and 25 to 200VA at 50% PF for relaying CTs.

CT accuracy class. ANSI accuracy class standards are [+ or -] 0.3, 0.6, or 1.2%. Ratio errors occur because of [I.sup.2]R heating losses. Phase-angle errors occur because of magnetizing core losses.

CTs are marked with a dot or other polarity identification on primary and secondary windings so that at the instant current is entering the marked primary terminal it is leaving the marked secondary terminal. Polarity is not required for overcurrent sensing but is important for differential relaying and many other relaying functions.

PT ratio. PT ratio selection is relatively simple. The PT should have a ratio so that, at the rated primary voltage, the secondary output is 120V. At voltages more than 10% above the rated primary voltage, the PT will be subject to core saturation, producing voltage errors and excess heating.

PT burden. PTs are available for burdens from 12.5VA at 10% PF to as high as 400VA at 85% PF.

PT accuracy. Accuracy classes are ANSI standard [+ or -] 0.3, 0.6, or 1.2%. PT primary circuits, and where feasible PT secondary circuits as well, should be fused.

CTs and PTs should have adequate capacity for the burden to be served and sufficient accuracy for the functions they are to perform. However, more burden or accuracy than necessary will merely increase the cost of the metering transformers. Solid-state relays usually impose lower burdens than electromechanical relays.



Related articles

ANSI Standards For Medium Voltage protection

ANSI Functions For Protection Devices

In the design of electrical power systems, the ANSI Standard Device Numbers denote what features a protective device supports (such as a relay or circuit breaker). These types of devices protect electrical systems and components from damage when an unwanted event occurs, such as an electrical fault.

ANSI numbers are used to identify the functions of meduim voltage microprocessor devices.

ANSI facilitates the development of American National Standards (ANS) by accrediting the procedures of standards developing organizations (SDOs). These groups work cooperatively to develop voluntary national consensus standards. Accreditation by ANSI signifies that the procedures used by the standards body in connection with the development of American National Standards meet the Institute’s essential requirements for openness, balance, consensus and due process.

ANSI standards (protection) – index
Current protection functions
ANSI 50/51 – Phase overcurrentANSI 79 – Reclose the circuit breaker after tripping
ANSI 50N/51N or 50G/51G – Earth fault or sensitive earth faultDirectional current protection
ANSI 50BF – Breaker failureANSI 67 – Directional phase overcurrent
ANSI 46 -Negative sequence / unbalanceANSI 67N/67NC – Directional earth fault
ANSI 49RMS – Thermal overloadANSI 67N/67NC type 1
Directional power protection functionsANSI 67N/67NC type 2
ANSI 32P – Directional active overpowerANSI 67N/67NC type 3
ANSI 32Q/40 – Directional reactive overpowerMachine protection functions
Voltage protection functionsANSI 37 – Phase undercurrent
ANSI 27D – Positive sequence undervoltageANSI 48/51LR/14 – Locked rotor / excessive starting time
ANSI 27R – Remanent undervoltageANSI 66 – Starts per hour
ANSI 27 – Phase-to-phase undervoltageANSI 50V/51V – Voltage-restrained overcurrent
ANSI 59 – Phase-to-phase overvoltageANSI 26/63 – Thermostat, Buchholz, gas, pressure, temperature detection
ANSI 59N – Neutral voltage displacementANSI 38/49T – Temperature monitoring by RTD
ANSI 47 – Negative sequence voltageFrequency protection functions
ANSI 81H – Overfrequency
ANSI 81L – Underfrequency
ANSI 81R – Rate of change of frequency (ROCOF)

Current protection functions

ANSI 50/51 – Phase overcurrent

Three-phase protection against overloads and phase-to-phase short-circuits.
ANSI index ↑

ANSI 50N/51N or 50G/51G – Earth fault

Earth fault protection based on measured or calculated residual current values:

  • ANSI 50N/51N: residual current calculated or measured by 3 phase current sensors
  • ANSI 50G/51G: residual current measured directly by a specific sensor

ANSI index ↑

ANSI 50BF – Breaker failure

If a breaker fails to be triggered by a tripping order, as detected by the non-extinction of the fault current, this backup protection sends a tripping order to the upstream or adjacent breakers.
ANSI index ↑

ANSI 46 – Negative sequence / unbalance

Protection against phase unbalance, detected by the measurement of negative sequence current:

  • sensitive protection to detect 2-phase faults at the ends of long lines
  • protection of equipment against temperature build-up, caused by an unbalanced power supply, phase inversion or loss of phase, and against phase current unbalance

ANSI index ↑

ANSI 49RMS – Thermal overload

Protection against thermal damage caused by overloads on machines (transformers, motors or generators).
The thermal capacity used is calculated according to a mathematical model which takes into account:

  • current RMS values
  • ambient temperature
  • negative sequence current, a cause of motor rotor temperature rise

ANSI index ↑



Automation device used to limit down time after tripping due to transient or semipermanent faults on overhead lines. The recloser orders automatic reclosing of the breaking device after the time delay required to restore the insulation has elapsed. Recloser operation is easy to adapt for different operating modes by parameter setting.
ANSI index ↑

Directional current protection

ANSI 67N/67NC type 1
ANSI 67 – Directional phase overcurrent

Phase-to-phase short-circuit protection, with selective tripping according to fault current direction. It comprises a phase overcurrent function associated with direction detection, and picks up if the phase overcurrent function in the chosen direction (line or busbar) is activated for at least one of the 3 phases.
ANSI index ↑

ANSI 67N/67NC – Directional earth fault

Earth fault protection, with selective tripping according to fault current direction.
3 types of operation:

  • type 1: the protection function uses the projection of the I0 vector
  • type 2: the protection function uses the I0 vector magnitude with half-plane tripping zone
  • type 3: the protection function uses the I0 vector magnitude with angular sector tripping zone

ANSI index ↑

ANSI 67N/67NC type 1

Directional earth fault protection for impedant, isolated or compensated neutralsystems, based on the projection of measured residual current.
ANSI index ↑

ANSI 67N/67NC type 2

Directional overcurrent protection for impedance and solidly earthed systems, based on measured or calculated residual current. It comprises an earth fault function associated with direction detection, and picks up if the earth fault function in the chosen direction (line or busbar) is activated.
ANSI index ↑

ANSI 67N/67NC type 3

Directional overcurrent protection for distribution networks in which the neutral earthing system varies according to the operating mode, based on measured residual current. It comprises an earth fault function associated with direction detection (angular sector tripping zone defined by 2 adjustable angles), and picks up if the earth fault function in the chosen direction (line or busbar) is activated.
ANSI index ↑

Directional power protection functions

ANSI 32P – Directional active overpower

Two-way protection based on calculated active power, for the following applications:

  • active overpower protection to detect overloads and allow load shedding
  • reverse active power protection:
    • against generators running like motors when the generators consume active power
    • against motors running like generators when the motors supply active power

ANSI index ↑

ANSI 32Q/40 – Directional reactive overpower

Two-way protection based on calculated reactive power to detect field loss on synchronous machines:

  • reactive overpower protection for motors which consume more reactive power with field loss
  • reverse reactive overpower protection for generators which consume reactive power with field loss.

ANSI index ↑

Machine protection functions

ANSI 37 – Phase undercurrent

Protection of pumps against the consequences of a loss of priming by the detection of motor no-load operation.
It is sensitive to a minimum of current in phase 1, remains stable during breaker tripping and may be inhibited by a logic input.
ANSI index ↑

ANSI 48/51LR/14 – Locked rotor / excessive starting time

Protection of motors against overheating caused by:

  • excessive motor starting time due to overloads (e.g. conveyor) or insufficient supply voltage.
    The reacceleration of a motor that is not shut down, indicated by a logic input, may be considered as starting.
  • locked rotor due to motor load (e.g. crusher):
    • in normal operation, after a normal start
    • directly upon starting, before the detection of excessive starting time, with detection of locked rotor by a zero speed detector connected to a logic input, or by the underspeed function.

ANSI index ↑

ANSI 66 – Starts per hour

Protection against motor overheating caused by:

  • too frequent starts: motor energizing is inhibited when the maximum allowable number of starts is reached, after counting of:
    • starts per hour (or adjustable period)
    • consecutive motor hot or cold starts (reacceleration of a motor that is not shut down, indicated by a logic input, may be counted as a start)
  • starts too close together in time: motor re-energizing after a shutdown is only allowed after an adjustable waiting time.

ANSI index ↑

ANSI 50V/51V – Voltage-restrained overcurrent

Phase-to-phase short-circuit protection, for generators. The current tripping set point is voltage-adjusted in order to be sensitive to faults close to the generator which cause voltage drops and lowers the short-circuit current.
ANSI index ↑

ANSI 26/63 – Thermostat/Buchholz

Protection of transformers against temperature rise and internal faults via logic inputs linked to devices integrated in the transformer.
ANSI index ↑

ANSI 38/49T – Temperature monitoring

Protection that detects abnormal temperature build-up by measuring the temperature inside equipment fitted with sensors:

  • transformer: protection of primary and secondary windings
  • motor and generator: protection of stator windings and bearings.

ANSI index ↑

Voltage protection functions

ANSI 27D – Positive sequence undervoltage

Protection of motors against faulty operation due to insufficient or unbalanced network voltage, and detection of reverse rotation direction.
ANSI index ↑

ANSI 27R – Remanent undervoltage

Protection used to check that remanent voltage sustained by rotating machines has been cleared before allowing the busbar supplying the machines to be re-energized, to avoid electrical and mechanical transients.
ANSI index ↑

ANSI 27 – Undervoltage

Protection of motors against voltage sags or detection of abnormally low network voltage to trigger automatic load shedding or source transfer.
Works with phase-to-phase voltage.
ANSI index ↑

ANSI 59 – Overvoltage

Detection of abnormally high network voltage or checking for sufficient voltage to enable source transfer. Works with phase-to-phase or phase-to-neutral voltage, each voltage being monitored separately.
ANSI index ↑

ANSI 59N – Neutral voltage displacement

Detection of insulation faults by measuring residual voltage in isolated neutral systems.
ANSI index ↑

ANSI 47 – Negative sequence overvoltage

Protection against phase unbalance resulting from phase inversion, unbalanced supply or distant fault, detected by the measurement of negative sequence voltage.
ANSI index ↑

Frequency protection functions

ANSI 81H – Overfrequency

Detection of abnormally high frequency compared to the rated frequency, to monitor power supply quality.
ANSI index ↑

ANSI 81L – Underfrequency

Detection of abnormally low frequency compared to the rated frequency, to monitor power supply quality. The protection may be used for overall tripping or load shedding. Protection stability is ensured in the event of the loss of the main source and presence of remanent voltage by a restraint in the event of a continuous decrease of the frequency, which is activated by parameter setting.
ANSI index ↑

ANSI 81R – Rate of change of frequency

Protection function used for fast disconnection of a generator or load shedding control. Based on the calculation of the frequency variation, it is insensitive to transient voltage disturbances and therefore more stable than a phase-shift protection function.

In installations with autonomous production means connected to a utility, the “rate of change of frequency” protection function is used to detect loss of the main system in view of opening the incoming circuit breaker to:

  • protect the generators from a reconnection without checking synchronization
  • avoid supplying loads outside the installation.

Load shedding
The “rate of change of frequency” protection function is used for load shedding in combination with the underfrequency protection to:

  • either accelerate shedding in the event of a large overload
  • or inhibit shedding following a sudden drop in frequency due to a problem that should not be solved by shedding.

ANSI index ↑

Related book: Relay selection guide

Link: Register

Autor: Edvard Csanyi, CsanyiGroup


Related articles

With light, spaces can be defined and reinterpreted time and time again

With light, spaces can be defined and reinterpreted time and time again

Vertical illuminance is a component of lighting design that is vitally important to architecture. Its primary purpose is to make spatial proportions and spatial limits visible. The opposite is the conventional horizontal illuminance, which is frequently the result of a purely functional, utilitarian and quantitative approach to design. In this latter case, the spatial experience is often secondary to the immediate visual task. Vertical illuminance, however, can help complement the functional lighting design as well as become a starting point for architecturally orientated lighting concepts. Illuminated walls give the observer a bright and open spatial impression. The fascination of wallwashing arises not only from the perception of brightness but also from the clear spatial presentation, which organises the architecture thereby making the surroundings more comprehensible. From the point of view of perception psychology and aesthetics, wallwasher lighting is an important concept for constructing spaces with light. It is for this reason that it belongs to the essential repertoire of qualitative lighting design.

The functional advantages of wall illuminance are especially apparent when it comes to museum lighting. Appropriate illumination puts the artwork in the right light.

Lighting Options for Walls
The uniform light distribution from the ceiling to the floor

The uniform light distribution from the ceiling to the floor

Three different approaches to vertical illuminance give the lighting designer considerable artistic freedom for a differentiated approach to the lighting of walls. Of particular interest from the architectonic point of view is the practice of uniform wallwashing. An even light distribution from the ceiling to the floor emphasises the surface of the whole wall as a single unit. This approach achieves a bright spatial impression and brings the wall to the fore in its function as a delineating surface. A second approach is to use grazing light up against the wall, the brightness distribution decreases across the wall. This type of lighting in particular brings out the material nature and texture of the wall surfaces. Point-source luminaires generate brilliant lighting effects. Conversely, linear sources produce a soft and diffuse effect. The third method of illuminating walls is from point sources. The regular sequence of beam intersections or “scallops” forms a pattern and lends the wall surface a rhythm of brightness contrasts. Special lighting tools are available for each of the different wallwashing techniques. Uniform wallwashing places the highest demands on the lighting technology. Various designs of wallwasher are available for this area of architecture.

Through uniform wallwashing a clear spatial presentation occurs, which makes the architecture more legible and creates a bright spatial impression.

Internal areas

Wallwashing aids orientation in entrance areas: visitors understand the structure of a room faster and assimilate important information quicker. Particular details on walls such as inscriptions become even more noticeable.

Wallwashing aids orientation in entrance areas

Wallwashing aids orientation in entrance areas: visitors understand the structure of a room faster and assimilate important information quicker

It is not only the architectural design task that falls to vertical lighting in the indoor area but also that of a contribution towards perception. The classic visual tasks include recognising the environment and reading information on walls. The latter ranges from text information for orientation, paintings in galleries and museums through to merchandise in the world of shopping. Uniform wallwashing holds great artistic potential: the wall itself can only be emphasised with its spatial and material quality or used as a neutral background for wallmounted objects. The practice of reflecting light off walls produces a diffuse component of light in the room and this can be used as ambient lighting. In particular, the areas of exhibitions and retail, whose pictures or merchandise are changed frequently, require a flexible lighting concept for walls. Uniform wallwashing offers a technique for the illumination of objects which does not require the luminaires to be constantly re-aimed to cater for rotating exhibits. It can also provide excellent presentation lighting. In the retail area, uniform wallwashing ensures that displays above the shelves are adequately illuminated. Special wallwashers are suitable for largeformat, full-height, advertising information because they bring the advertisement’s entire surface to bear evenly and, by using high illuminance levels, are excellent for attracting attention.

Wall lighting in the daytime reduces the contrast of bright facades and room walls. At night, the even lighting of wall areas ensures a bright spatial impression is maintained.

The combination of wallwashers and spotlights

The combination of wallwashers and spotlights allows the general lighting to be supplemented by accent lighting

Luminaires installed directly against the wall do not achieve uniform brightness from top to bottom. The linear grazing light of fluorescent lamps produces a diffuse light on the wall and creates a clear definition at the edge of the room where the wall meets the ceiling. Regardless of whether it is large-format advertising surfaces or walls with artworks in museums that are to be illuminated – uniform, vertical illuminance is an effective concept to illuminate walls and objects.

Although the use of horizontal workplace lighting is widespread in the office and administrative area, wallwashing can make a valuable contribution to the user’s wellbeing in this area. Because the luminance influences the impression of brightness in the viewer’s field of vision, the lighting of walls takes on an important role especially in small offices. This not only concerns having sufficient illuminance for visual tasks in places such as shelves or cabinets, but also the lighting of walls and graphic art, which makes the workplace environment appear more attractive. In addition, wall lighting also has a positive effect on luminance contrasts in the room: a higher background luminance reduces the contrast of both computer screens and luminaires. People’s faces are also given an even and balanced modelling due to the higher component of diffuse light in the room. In conference rooms and in large classrooms and auditoriums, sufficient vertical illuminance is particularly important for vertical presentation surfaces, enabling information to be read at a distance.

External areas
Nocturnal facade lighting presents cities

Nocturnal facade lighting presents cities, local authority districts and private clients with a multitude of design possibilities for lighting

Nocturnal facade lighting presents cities, local authority districts and private clients with a multitude of design possibilities for lighting both individual buildings as well as groups of buildings in the context of public squares, courtyards or main roads. The spectrum ranges from simple lighting intended primarily to provide orientation and safety, through to presentational lighting solutions and scenic illumination for special occasions. For buildings that are visible from afar, such as skyscrapers or towers, vertical lighting has the important task of highlighting nocturnal landmarks. Where buildings around the edge of public squares are concerned, facade lighting helps to ensure that these buildings are recognisable after dark and promotes better spatial understanding. In the interest of a “Dark Sky”, the lighting designer can effectively avoid what is known as light pollution (i.e. light which is emitted directly into the night sky) by using highquality lighting technology and by arranging the luminaries appropriately.

Lighting the monuments and historical buildings

Lighting of monuments and historical buildings

Many monuments and historical buildings tell their story in bas-reliefs, ornamentation or sculptural decoration. It is only with light and shadow that the three-dimensional nature and texture of the surfaces can be appreciated. Architectural details such as the constituency of materials, joints or facade patterns are also amongst those important features which, when discernable, will characterise the appearance of a building or structure at night. The contrast on the surface can be influenced by the direction of the light and the type of luminaire. Moving a floodlight further away from the facade gives the surface a uniform but flat appearance because the formation of shadows is reduced. Conversely, luminaires positioned right next to the facade will produce extreme shadow, creating a dramatic impression. A mid-way position, as is the norm for wallwashers, gives a balanced appearance with an even light distribution on the surface, while still allowing the three-dimensional nature of any details to be easy to recognise. Correction filters alter the colour impression for certain ranges of colour only. Thus, for instance, the warmer tone of the Skintone filter can emphasise the colour of beige sandstone. When using coloured light, a nocturnal perception and atmosphere is created which is distinct and independent from the daytime appearance. Certain moods and contrasts can be created with coloured light, and these can be used for instance to delineate large facade surfaces or to distinguish different parts of a building from each other. The texture of basreliefs on facades and of other material features can be enhanced by the shadow effect of wallwashing. The wall lighting in the indoor area is continued outside: the grazing lighting on the wall and the illuminated wall of trees both add to the emotive ambience on the terrace.

Internal and external areas – seen holistically
The internal wallwashing is visible from outside

The internal wallwashing is visible from outside. Combined with transparent facades, the flowing transition from outside to inside gives great spatial depth

Vertical illuminance internally not only alters the atmosphere within a building but can also characterise the view of the building from the outside at night. When the indoor lighting is switched on with the onset of dusk, the reflection of the surroundings on transparent facades disappears. The facade gains spatial depth: the supporting structure appears as a contour and the people, furnishings and materials inside become more apparent. It is important for entrance areas and foyers to have a striking night-time appearance which is effective even when seen from a distance. This not only improves orientation but the wall lighting also guides the visitors to the building and conveys a prestigious and open spatial impression. An illuminance level higher than the surroundings or light of a different colour can emphasise such entrance areas when viewed from inside the building and make them stand out when viewing the facade. Shop window lighting in particular utilises the advantages of vertical illuminance in several ways, including that of using a bright background to attract the attention of the consumers within the cityscape.

A different perspective is brought to bear when looking from the inside out. In the daylight, high contrasts occur in rooms between the high luminances in the window area and the opposite facing walls. In deep rooms that have low ceilings, this can give rise to a dark impression. In this case, lighting the walls will help to create a balance between the light and dark areas. Although, as a supplement to the daylight, it might not do much to aid vision at the workplace, it does considerably improve the way the room is perceived. At night, windows appear as dark surfaces from the inside and the perspective seems to end where the indoor area ends. The task of extending spatial perception beyond the window falls to the outdoor lighting, and is achieved by illuminating architectural features in the outdoor area. The role of an illuminated outdoor wall in creating a single, holistic spatial composition combining indoor and outdoor areas can also be performed by illuminating vegetation. Reducing the lighting level internally will improve the view of the outdoor world, because there will be less reflections on the window surface. The internal wallwashing is visible from outside. Combined with transparent facades, the flowing transition from outside to inside gives great spatial depth. To give a holistic appearance to the spatial concept, the uniform wallwashing inside the building is continued externally. Whatever the time of day, vertical lighting will ensure the wall remains the dominant feature of the internal space.

Colour and scenography
The colour contrast of cold and warm hues

The colour contrast of cold and warm hues gives a festival in the ruins a most attractive setting. Many luminaires allow the light colour to be changed using colour filters

In architectural lighting, different light colours ranging from warm white and neutral white through to daylight white are used to differentiate spatial areas or to produce a different lighting atmosphere by day and by night. Styling with coloured light has a more intensive effect than white light. The bandwidth of colours ranges from pastel-shades through to strong primary and secondary colours. Coloured lighting can be used to give rooms and spaces dramatic scenic light and make them more eye-catching. White walls provide a neutral background capable of transmitting every nuanced hue of the light colour. The impression of a coloured wall can be further strengthened using coloured light of the same colour, lending the room or area an impressive colour intensity. One way of creating coloured light is to use colour filters mounted on the luminaires. Much more flexible in terms of scenographic light, however, are light sources that can dynamically change their colour. The principle behind this is the additive colour mixing of separately controllable light sources in the colours red, green and blue (RGB technology), e.g. using coloured fluorescent lamps or LEDs. A lighting control system that controls the dimmer settings of the individual colour light sources and, in so doing, makes specific colours of light reproducible and integrates them in useful lighting scenes and sequences is indispensable for the effective use of Varychrome luminaires or other installations with RGB technology. The colour contrast of cold and warm hues gives a festival in the ruins a most attractive setting.

Many luminaires allow the light colour to be changed using colour filters. The intensity of coloured surfaces can be significantly increased with coloured light of the same hue and can produce an impressive atmosphere. The possibilities of styling rooms with light are almost unlimited. One of the most fascinating properties of light is its ability to continually reinterpret architecture. Staging and controlling such metamorphoses with the inclusion of the time dimension is what we refer to as ‚“scenographic light“. By using innovative lighting tools for internal and external environments and systems for intelligently linking the luminaires, light can be formed into a coherent scenography in terms of its interaction with space, time and atmosphere.

Scenographic light enormously increases the ability to experience quality in architecture, it magically attracts attention (as a concept for shop window lighting for instance) and it interprets the themes and concepts of exhibitions and events.

Purple scenographic light

Purple scenographic light

Green scenographic light

Green scenographic light

Blue scenographic light

Blue scenographic light



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Short Circuit Parameters in Low Voltage AC Circuits

Low-voltage equipment standards IEC60947 and IEC60439 currently include short-circuit ratings for products and assemblies respectively, defined in terms of the ability of the equipment to operate at a level of peak current, an RMS current for a specified time and/or a level of current conditional upon a short-circuit protective device in series. In practice the correct application of the various short-circuit ratings needs to be fully understood by the circuit designer to avoid leaving a circuit or equipment with inadequate short-circuit protection. It is also useful to take full advantage of the capability of devices and systems to avoid over-engineering, with the consequent unnecessary additional cost. This guide does not concern itself with the issue of selectivity between devices in series, which must be considered separately.

Principles of Application

The Installation

In order to ensure the capability of equipment under short-circuits conditions the circuit designer must firstly have available the prospective fault level at the point of installation of each item of equipment. This is produced by a system protection study. IEC60781 provides an application guide for calculation of short-circuit currents in lowvoltage radial systems. Short-circuit parameters are defined by this guide in terms, which include the following:

  • Prospective (available) short-circuit current:
    The current that would flow if the short-circuit were replaced by an ideal connection of negligible impedance without any change of the supply.
  • Peak short-circuit current Ip
    The maximum possible instantaneous value of the prospective (available) short-circuit current.
  • Symmetrical short-circuit breaking current Ib
    The r.m.s. value of an integral cycle of the symmetrical a.c. component of the prospective (available) shortcircuit current at the instant of contact separation of the first pole of a switching device.
  • Steady-state short-circuit current Ik
    The r.m.s. value of the short-circuit current which remains after the decay of the transient phenomena.
    - unlimited
    - limited by an SCPD (short-circuit protective device)
LV Assemblies (switchboard, distribution board etc.)

An assembly will have a short-circuit rating, assigned by the manufacturer, defined in terms of the maximum prospective fault level applicable at the point it is connected into the system.

This will have been determined by test and/or design calculations as specified in the assembly standard, IEC60439-1, or applicable part thereof.

The terminology to define the short-circuit rating of an assembly is given in the standard as follows:

  • Rated short-time current (Icw) (of a circuit of an assembly)
    Summarised as: The r.m.s value of short-time current that a circuit of an assembly can carry without damage under specified test conditions, defined in terms of a current and time e.g. 20kA, 0,2s.
  • Rated peak withstand current (Ipk) (of a circuit of an assembly)
    Summarised as: The value of peak current that a circuit can withstand satisfactorily under specified test conditions.
  • Rated conditional short-circuit current (Icc) (of a circuit of an assembly)
    Summarised as: The value of prospective short-circuit current that a circuit, protected by a specified shortcircuit protective device (SCPD), can withstand satisfactorily for the operating time of that device, under specified test conditions. Note: the short-circuit protective device may form an integral part of the assembly or may be a separate unit.An assembly may be assigned a value of Icc alone.

- An assembly may be assigned values of Icw and Ipk (but cannot be assigned a value of Icw or Ipk alone).
- An assembly may be assigned values of Icw, Ipk and Icc.
- An assembly may be assigned different values of Icc for different circuit protective devices and/or system voltages.
- An assembly may be assigned different values of Icw for different short-time periods e.g. 0.2s, 1s, 3 s.


In terms of short-circuit capability switchgear must be considered in respect of it’s function in the particular application. A switching device is considered in two respects, self-protection and use as a short-circuit protective device (SCPD) where applicable.

Switchgear – Self Protection Against Short Circuit

Two cases are considered:

  • Load and overload switching alone, without any short-circuit switching capability:
    In this case the switching device will be short-circuit rated on a similar basis to a circuit of an assembly (see above), with a rating of Icw and/or a conditional short-circuit rating, but will in addition have a rated short-circuit making capacity Icm.
  • Load, overload and short-circuit switching capability:
    • Fused switchgear – in this case the short-circuit breaking function is provided by the integral fuses and the device will have a conditional short-circuit rating
    • Circuit breakers – the circuit-breaker will be self-protecting up to its breaking capacity rating (see later). At fault levels above the breaking capacity rating a circuit-breaker may be capable of operating with ‘back-up’ protection by an SCPD (this is in effect a conditional rating, but the term is not generally used in this context).
Switchgear – Application as SCPD
  • Fused Switchgear and Fuses as SCPD
    Since the short-circuit breaking function in fused switchgear is provided by the fuses it is the fuse characteristics that are considered. These are given in IEC60269-1 as follows:

    • Breaking capacity of a fuselink
      - value (for a.c. the r.m.s. value of the a.c. component) of prospective current that a fuselink is capable of breaking at a stated voltage under prescribed conditions.
    • Cut-off current
      Summarised as: maximum instantaneous value reached by the current during the breaking operation of a fuselink when it operates to prevent the current reaching the prospective peak.
    • Operating I²t (Joule integral)
      Summarised as: Integral of the square of the current over the operating time of the fuse.
      Sometimes referred to as ‘energy let-through’. When expressed in A²t gives the energy dissipated per ohm and thus represents the thermal effect on the circuit.
  • Circuit-breakers as SCPD
    • Moulded-case circuit-breakers (MCCBs) and air circuit-breakers (ACBs) are rated according to IEC60947-2 as follows
      • Rated short-circuit making capacity (Icm)
        Summarised as: The maximum peak prospective current that the circuit-breaker can make on to satisfactorily.
    • Rated short-circuit breaking capacities:
      • Rated ultimate short-circuit breaking capacity (Icu)
        Summarised as: The r.m.s prospective current that the circuit breaker is capable of breaking at a specified voltage under defined test conditions, which include one break and one make/break operations.
      • Rated service short-circuit breaking capacity (Ics)
        Summarised as: The r.m.s prospective current that the circuit breaker is capable of breaking at a specified voltage under defined test conditions, which include one break and two make/break operations. The standard specifies fixed relationships to Icu of 25, 50, 75 or 100%.
      • Rated short-time withstand current (Icw)
        Summarised as: The r.m.s value of short-time current assigned by the manufacturer based on specified test conditions. Minimum values are given in the standard.

A circuit-breaker can only be assigned a rated short-time withstand current Icw if it is equipped with a time-delay overcurrent release.

All circuit-breakers to IEC60947-2 will have values of Icu and Ics.

Characteristics of circuit-breakers not mandated in IEC60947-2 but having application to short-circuit protection:

  • Cut-off current
    The maximum instantaneous value reached by the current during the breaking operation of a circuit-breaker when it operates to prevent the current reaching the prospective peak.
  • Operating I²t (Joule integral)
    Integral of the square of the current over the operating time of the circuit-breaker on a short-circuit. Sometimes referred to as ‘energy let-through’. When expressed in A²t gives the energy dissipated per ohm and thus represents the thermal effect on the circuit.

Examples of the Practical Application of the Product Characteristics

In simple studies only the r.m.s value of steady-state short-circuit current (Ik) is quoted. The peak current is assumed to be in a standard relationship to the r.m.s current, determined by the overall power factor, and taken into account in the rating of SCPDs to the respective IEC standards.

Circuit Protection

The application of short-circuit protective devices (SCPD) to circuit protection i.e. the protection of cables, is detailed in the installation rules, IEC364. In general it is accepted that selection of the protective device on the basis of thermal protection of a cable automatically provides short-circuit protection up to the breaking capacity of the SCPD, in the case of non-time-delayed devices.

Short-Circuit Protection for LV assemblies
Switchboard/Motor-Control Centre

The prospective short-circuit current at the input to the switchboard is obtained from a system protection study.
This will be given as an r.m.s value.

  • If the switchboard has an Icw current value higher than the prospective current level then the only requirement is to limit the time for which a short-circuit could persist to within the short-time value. This is achieved by the setting of releases upstream or at the incomer to the switchboard.
  • If the switchboard has an Icc rating higher than the prospective current level then the only requirement is to include the specified SCPD in the circuit. This may be added in the circuit upstream or may already be included as an incomer to the switchboard.
Busbar Trunking (BBT)

The prospective short-circuit current at the input to the switchboard is obtained from a system protection study.
This will be given as an r.m.s value.

  • If the BBT has an Icw current value higher than the prospective short circuit current level then the only requirement is to limit the time for which a short-circuit could persist to within the short-time value. This is achieved by the time-delay setting of overcurrent releases upstream.
  • If the BBT has an Icw lower than the prospective short circuit current level Ik but has an Icc rating higher than Ik then the only requirement is to include the specified SCPD in the circuit upstream or in the end-feed unit. The suitability of any given SCPD may be derived from the cut-off current and Joule-integral characteristics by comparison with proof-test parameters.
Motor Control Gear (MCG)

Motor starters and contactors are not generally self-protecting against the effects of short-circuit and therefore need to be associated with an SCPD. In this particular case test procedures to IEC60947-4-1 recognise the difficulty of protecting sensitive devices from damage under heavy short-circuit conditions. Thus a special case of conditional rating is obtained which allows two classes of co-ordination with an SCPD:

Type 1 – in which a certain amount of damage to the MCG is accepted.
Type 2 – in which the MCG is capable of further use.

These ratings can only be obtained by type-testing and thus the data must be obtained from the manufacturer of the SCPD or the MCG.

Miniature Circuit Breakers (MCBs)

When applied in other than domestic (household) situations the short-circuit capability of MCBs to IEC60898 is often inadequate and they need to be ‘backed-up’ by another SCPD. Details of how the appropriate SCPD is determined are given, for circuit-breakers, in Appendix A of IEC60947-2. Basically this shows that only testing of the required combination is satisfactory and thus the data must be obtained from the manufacturer of the SCPD or the MCB. The same applies to fuses used as SCPD.



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Works with many PLCs: Allen-Bradley, Schneider, Siemens, Omron...

Works with many PLCs: Allen-Bradley, Schneider, Siemens, Omron...

This software helps you troubleshoot problems in your PLC logic or equipment through easy data acquistation and analysis of time dependent behaviour. It is espcially well suited for Siemens Simatic S5 and S7 PLC applications.

PLC-ANALYZER pro 5 is a software system for logic analysis and acquisition of recorded data on PLC-controlled facilities. Acquisition, representation, and evaluation of PLC signals such as inputs, outputs, flags, data words, etc. is now very easy.

Online display makes possible observation of the signal waveform in real time. In addition to long-term recording, trigger conditions can be specified for the acquisition of particular events. This allows rarely occurring sporadic errors to be recorded for later analysis.

In contrast to traditional logic analyzers, the PLC-ANALYZER pro 5 has the decisive advantage of recording process data through standardized PLC interfaces.

The program e. g. supports MPI/PPI, PROFIBUS and TCP/IP Ethernet for SIMATIC S7 or the programming unit interface for SIMATIC S5.

A computer that is connected for the purpose of programming the PLC can be used for recording process data without hardware modifications. The tiresome process of hooking up monitoring cables is now a thing of the past.

Cycle-precise recording is attractive because of the complete acquisition of measured values in each PLC cycle. By using the measurement interface AD_USB-Box external voltage and current signals, which are not available in the PLC, can also be recorded. Project files make it possible to automate frequently recurring acquisition sessions for various facilities.

The PLC-Analyzer pro 5 is well suited for the following applications:

* Failure diagnosis for PLC systems
* Finding and localizing sporadic errors
* Analysis and optimization, cycle time reduction
* Long-term recording of measured values
* Documentation and support of QA, + TPM/OEE
* Installation, maintenance, construction and education

The software works with many PLCs including Allen-Bradley, Schneider Electric, Siemens and others. Its capabilities can also be expanded by an optional box for recording data variables external to the PLC, and also an additonal solution for longer term historical recording.

List of available PLC drivers for PLC-ANALYZER pro 5:

Siemens SIMATIC S7MPI/PPI, PROFIBUScycle precise (also suitable for
Download 3 MB*
Siemens SIMATIC S7Ethernet TCP/IP, PROFINETcycle precise (also suitable for
Download 3 MB*
Siemens SIMATIC S5programming interfacecycle preciseDownload 3 MB*
Ethernet TCP/IPDownload 674 KB
Siemens LOGO!programming interfaceDownload 650 KB
Siemens SINUMERIK (S5)programming interfacecycle preciseDownload 3 MB*
Siemens SIMOTION C/P/DMPI / PROFIBUS / Ethernet TCP/IPservo-cycle preciseDownload 629 KB
BOSCH CLprogramming interface (BUEP 19E)Download 574 KB
CoDeSysEthernet TCP/IPfor CoDeSys based systemsDownload 759 KB
PILZ PSSprogramming interfaceDownload 569 KB
PILZ PSSEthernet TCP/IPDownload 598 KB
PHOENIX ILCEthernet TCP/IPDownload 603 KB
Jetter JetControl / Delta / Nanoserial / Jetway / PC-PPLCDownload 716 KB
Jetter JetControlEthernet TCP/IPDownload 716 KB
B&REthernet TCP/IP / serialDownload 689 KB
Allen-Bradley ControlLogix / PLC / SLCRS232 / DH+ / DH-485Download 605 KB
Allen-Bradley ControlLogix / PLC / SLCEthernet TCP/IPDownload 605 KB
GE Fanuc Series 90 / VersaMax / Nano / Microprogramming interface (SNP)Download 573 KB
GE Fanuc CNC/PMCEthernet TCP/IP / HSSB
HITACHI H / EH-150 / Micro-EHprogramming interfaceDownload 588 KB
HITACHI H / EH-150 / Micro-EHEthernet TCP/IPDownload 588 KB
MITSUBISHI MELSEC Q / A / FXprogramming interfaceDownload 616 KB
Schneider Modicon TSX Quantum / Momentum / CompactModbus PlusDownload 580 KB
Modbus IDownload 565 KB
Schneider Modicon TSX Quantum / Momentum / CompactModbus TCP/IPDownload 569 KB
Schneider Modicon TSX Premium / Atrium / Micro / NanoTCP/IP / Uni-TelwayDownload 569 KB
Schneider AEG TSX A250 / A120 / Microprogramming interface (KS)Download 562 KB
OMRON C / CV / CS1programming interface (Host Link)Download 571 KB
Beckhoff TwinCAT I/Orecording of TwinCAT I/O-variablesDownload 656 KB
AUTEM AD_USB-BoxUSB-Portrecording of external voltage and current signalsDownload 296 KB

Take a look for details on AUTEM website.


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