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Arc-resistant low voltage switchgear

Arc-resistant low voltage switchgear

For years, electrical equipment has been designed to withstand and deal with the issue of bolted faults, where the current spikes to a dangerously high level but is safely interrupted by the protective devices contained in the equipment (breakers, fuses and relays). However, these devices typically do not detect and interrupt dangerous internal arcing faults, which have a lower current level, but can generate a far more dangerous scenario for operating personnel.

Arc faults can be caused by a breakdown of insulation materials, objects coming into close proximity with the energized bus assembly, even entry of rodents or other animals into the equipment. The thermal energy created by these events can get as high as 35,000ºF, melting materials and clothing from several feet away. Also consider that the arc blast produced by a lineup of 480 Vac switchgear rated at 85 kA can be equivalent to 20.7 lbs of TNT!


So, what is the solution?

Eaton’s solution: arc-resistant low voltage switchgear

Eaton introduces the addition of an ANSI Type 2 arc-resistant low voltage switchgear offering to its current product line. This is the latest release in arc-safe equipment from Eaton’s Electrical Sector. The arc-resistant low voltage switchgear protects operating and maintenance personnel from dangerous arcing faults
by redirecting or channeling the arc energy out the top of the switchgear, regardless of the origination location of the arc.

Eaton’s arc-resistant low voltage switchgear has been successfully tested to ANSI C37.20.7 at KEMA-Powertest, and has been ULT witnessed and certified.

Standard features
  • Ratings:
    • Up to 100 kA short circuit at 508 Vac maximum and up to 85 kA short circuit at 635 Vac maximum
    • Up to 10 kA horizontal main bus continuous current
    • Up to 5 kA vertical bus continuous current
    • MagnumE DS power circuit breaker frame ratings between 800A and 6000A
  • ANSI Type 2 arc-resistant design protects the operator around the entire perimeter of the equipment
  • Floor-to-ceiling height of 10 feet required whether exhausting into a room or through an arc plenum
  • Strengthened one-piece breaker door and latches
  • Dynamic flap system on rear ventilation openings that remain open under normal operating conditions, but close during an arcing event to prevent dangerous gasses from escaping
  • Patented bellows design allowing drawout of breaker into the disconnected position with the door closed, while simultaneously protecting the operator from any dangerous gasses during an arc event
  • Patented venting system that directs arc gasses out the top of the enclosure, regardless of the arc origination location
  • Up to four-high breaker configuration with no additional layout restrictions
  • Strengthened side and rear panels with standard split rear covers for cable access
  • NEMAT 1 enclosure, with either top or bottom cable or bus duct entry
  • Cable compartment floor plates


Optional features
  • Zone selective interlocking protection
  • ANSI Type 2B arc-resistant design protects the operator even with the low voltage instrument compartment door open
  • Arcflash Reduction Maintenance SystemE
  • Safety shutters
  • One-piece hinged and bolted rear panel
  • Insulated bus
  • Vented bus/cable compartment barrier
  • Cable compartment segregation barrier



  • Superior protection against arcs in breaker, bus or cable compartments
  • No increase in footprint over regular Magnum DS switchgear
  • Closed door racking
  • UL 1558 and UL 891
  • ANSI C37.20.1, ANSI C37.13, ANSI C37.51 and ANSI C37.20.7
  • CSAT standard—CSA C22.2 No. 31-04
  • Third-party (UL/CSA) witness tested
  • Seismic certification 2006-IBC

Testing procedures were completed per ANSI C37.20.7 standards with arcs initiated in:

  • Breaker compartment
  • Vertical and horizontal bus
  • Cable termination compartments

Additionally, the tested arc duration was up to the full 0.5 seconds recommended by ANSI C37.20.7, with no dependence on the tripping speed of an upstream breaker.

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Shielding Of Power Cables

Shielding Of Power Cables

Shielding of an electric power cable is accomplished by surrounding the assembly or insulation with a grounded, conducting medium.

This confines the dielectric field to the inside of this shield.

Two distinct types of shields are used:

- Metallic

- Nonmetallic



The purposes of the insulation shield are to:

  • Obtain symmetrical radial stress distribution withh the insulation.
  • Eliminate tangential and longitudinal stresses on the surface of the insulation.
  • Exclude from the dielectric field those materials such as braids, tapes, and fillers that are not intended as insulation.
  • Protect the cables from induced or direct aver-voltages. Shields do this by making the surge impedance uniform along the length of the cable and by helping to attenuate surge potentials.

Conductor Shielding

In cables rated over 2,000 volts, a conductor shield is required by indusby standards. The purpose of the semiconducting, also called screening, material over the conductor is to provide a smooth cylinder rather than the relatively rough surface of a stranded conductor in order to reduce the stress concentration at the interface with the insulation. Conductor shielding has been used for cables with both laminar and extruded insulations. The materials used are either semiconducting materials or ones that have a high dielectric constant and are known as stress control materials. Both serve the same function of stress reduction.

Conductor shields for paper insulated cables are either carbon black tapes or metallized paper tapes. The conductor shieldmg materials were originally made of semiconducting tapes that were helically wrapped over the conductor. Present standards still permit such a tape over the conductor. This is done, especially on large conductors, in order to hold the strands together firmly during the application of the extruded semiconducting material that is now required for medium voltage cables. Experience with cables that only had a semiconductingtape was not satisfactory, so the industry changed their requirements to call for an extruded layer over the conductor.

In extruded cables, this layer is now extruded directly over the conductor and is bonded to the insulation layer that is applied over this stress relief layer. It is extremely important that there be no voids or extraneous material between those two layers.

Presentday extruded layers are not only clean (free from undesirable impurities) but are very smooth and round. This has greatly reduced the formation of water tress that could originate from irregular surfaces. By extruding the two layers at the same time, the conductor shield and the insulation are cured at the same time. This provides the inseparable bond that minimizes the chances of the formation of a void at the critical interface. For compatibility reasons, the extruded shielding layer is usually made from the same or a similar polymer as the insulation. Special carbon black is used to make the layer over the conductor semiconducting to provide the necessary conductivity. Industry standards require that the conductor semiconducting material have a maximum resistivity of 1,000 meter-ohms. Those standards also require that this material pass a long-time stability test for resistivity at the emergency operating temperature level to insure that the layer remains conductive and hence provides a long cable life.

A water-impervious material can be incorporated as part of the conductor shield to prevent radial moisture transmission. This layer consists of a thin layer of aluminum or lead sandwiched between semiconducting material. A similar laminate may be used for an insulation shield for the same reason.

There is no definitive standard that describes the class of extrudable shielding materials known as “super smooth, super clean”. It is not usually practical to use a manufacturer’strade name or product number to describeany material. The term “super smooth, super clean” is the only way at this writing to describe a class of material that provides a higher quality cable thanan earlier version. This is only an academic issue since the older type of materials are no longer used for medium voltage cable construction by known suppliers. The point is that these newer materials have tremendously improved cable performance in laboratory evaluations.

Insulation Shielding For Medium-Voltage Cables

The insulation shield for a medium voltage cable is made up of two components:

  • Semiconducting or stress relief layer
  • Metallic layer of tape or tap , drain wires, concentric neutral wires, or a metal tube.

They must function as a unit for a cable to achieve a long service life

Stress Relief Layer

The polymer layer used with exbuded cables has replaced the tapes shields that were used many years ago. This extruded layer is called the extruded insulation shield or screen. Its properties and compatibility requirements are similar to the conductor shield previously described except that standards require that the volume resistivity of thisexternal layer be limited to 500 meter-ohms.

The nonmetallic layer is directly Over the insulation and the voltage stress at that interface is lower than at the conductor shield interface.. This outer layer is not required to be bonded for cables rated up to 35 kV. At voltages above that, it is strongly recommendedt that this layer be bonded to the insulation .
Since most users want this layer to be easily removable, the Association of Edison Illuminating Companies (AEIC) has established strip tension limits. Presently these limits are that a 1/2 inch wide strip cut parallel to the conductor peel offwith a minimum of 6 pounds and a minimum of 24 pounds of force that is at a 90º angle to the insulation surface.

Metallic Shield

The metallic portion of the insulation shield or screen is necessary to provide a low resistance path for charging current to flow to ground. It is important to realize that the extruded shield materials will not survive a sustained current flow of more than a few milliamperes. These materials are capable of handing the small amounts of charging current, but cannot tolerate unbalanced or fault currents.

The metallic component of the insulation shield system must be able to accommodate these higher currents. On the other hand, an excessive amount of metal in the shield of a single-conductor cable is costly in two ways. First, additional metal over the amount that is actually required increases the initial cost of the cable. Secondly, the greater the metal component of the insulation shield, the higher the shield losses that result h m the flow of current in the central conductor.

A sufficient amount of metal must be provided in the cable design to ensure that the cable will activate the back-up protection in the event of any cable fault over the life of that cable. There is also the concern for shield losses.

It therefore becomes essential that:

  • The type of circuitinterruptingequipmentto be analyzed.What is the design and operational setting of the hse, recloser, or circuit breaker?
  • What fault current will the cable encounter over its life?
  • What shield losses can be tolerated? How many times is the shield to be grounded? Will there be shield breaks to prevent circulating currents?
Concentric Neutral Cables

When concentric neutral cables are specified, the concentric neutrals must be manufactured in accordance with ICEA standards. These wires must meet ASTM B3 for uncoated wires or B33 for coated wires. These wires are applied directly over the nonmetallic insulation shield with a lay of not less than six or more than ten times the diameter over the concentric wires.

Shielding Of Low Voltage Cables

Shielding of low voltage cables is generally required where inductive interference can be a problem. In numerous communication, instrumentation, and control cable applications, small electrical signals may be transmitted on the cable conductor and amplified at the receiving end. Unwanted signals (noise) due to inductive interference can beaslargeasthedesiredsignal. This can result in false signals or audible noise that can effect voice communications.

Across the entire frequency spectrum, it is necessary to separate disturbances into electric field ef€ects and magnetic field effects.

Electric Fields

Electric field effects are those which are a function of the capacitive coupling or mutual capacitance between the circuits. Shielding can be effected by a continuous metal shield to isolate the disturbed circuit fiom the disturbing circuit. Even semiconducting extrusions or tapes supplemented by a grounded dmin wirecan serve some shielding function for electric field effects.

Magnetic Fields

Magnetic field effects are the result of a magnetic field coupling between circuits. This is a bit more complex thanfor electrical effects.

At relatively low frequencies, the energy emitted from the source is treated as radiation. This increases with the square of the frequency. This electromagnetic radiation can cause dislxrbancesat considerable distance and will penetrate any “openings” in the shielding. This can occur with braid shields or tapes that are not overlapped. The type of metal used in the shield also can effect the amount of disturbance. Any metallic shield material, as opposed to magnetic metals, will provide some shield due to the eddy currents that are set up in the metallic shield by the impinging field. These eddy currents tend to neutralize the disturbing field. Non-metallic, semiconducting shielding is not effective for magnetic effects. In general, the most effective shielding is a complete steel conduit, but thisis not always practical.

The effectiveness of a shield is called the “shielding factor” and is given as:

SF = Induced voltage in shield circuit / Inducted voltage in unshielded circuit

Test circuits to measwe the effectiveness of various shielding designs against electrical field effects and magnetic field effects have been reported by Gooding and Slade.

AUTHORS: Lawrence J. Kelly and Carl C. Landinger


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Acti 9 - The Fifth Generation Of Modular Systems

Acti 9 - The Fifth Generation Of Modular Systems

Acti 9 represents the fifth generation of Schneider’s low voltage modular systems. His older brother Multi 9 has finally evolved to much better and smarter system. Multi 9 was the famous and most known product of Schneider’s ex brand Merlin Gerin (now is incorporated into Schneider global brand), and now new Acti 9 is ready to inherit it.

Before Acti 9 – iC60 and Multi 9 – C60 modular systems, there was also Multi 9 – C32, F32 and F70 at the beginning of development.

Acti 9 covers all applications, especially in polluted environments and networks, for absolute safety and improved continuity of service.

Acti 9 exclusivities

For absolute safety and improved continuity of service.

  • VISI-SAFE – Guaranteed safe intervention on site
  • VISI-TRIP – Fast location of the faulty outgoer to minimize dowtime
  • The super immunization “Si” on RCD – Improved continuity of service, especially in polluted environments and networks
  • Front face class 2 - Continuous safety for operators and non-qualified personnel

Acti 9 exclusivities

VISI-SAFE concept is combining:

  • Contact position indication with the green strip
  • Impulse voltage withstand: Uimp 6 kV
  • Insulation voltage: Ui 500 V
  • Pollution degree: level 3 (conductive pollution, dust,etc.)

Easy to choose

  • Compliance with both IEC/EN 60898 & IEC/EN 60947-2 - Suitable for commercial and industrial applications
  • RCDs fully coordinated up to the MCB’s breaking capacity – Peace of mind, easy to select

Easy to install

  • Quick and ergonomic wiring, safe connections
    - IP20 insulated flap terminals
    - Distribloc system
  • Twice the standard terminal tightening torque

Easy to operate

  • Great readability:
    - large circuit labelling area
    - specific colour code system.
  • Upgradeability with Multiclip system
  • Load rebalancing and addition of new outgoers.
  • Device removable with comb busbar in place
  • Double locking.

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AKD-20 low-voltage switchgear continues the tradition of the AKD switchgear line while delivering enhanced arc flash protection. Built to ANSI standards, its protection features include non-vented panels plus insulated and isolated bus, and it integrates our new state-of-the-art EntelliGuard® breaker-trip unit system. It also features an optimized footprint so that it now fits into a smaller area for the most common configurations.

EntelliGuard® G circuit breakers are the newest line of GE low-voltage circuit breakers, the next step in the evolution of a line known for its exceptional designs and performance. They are available from 800A to 5000A, with fault interruption ratings up to 150kAIC – without fuses.

Integral to the EntelliGuard G line are the new, state-of-the-art EntelliGuard TU Trip Units, which provide superior system protection, system reliability, monitoring and communications. The breaker-trip unit system delivers superior circuit protection without compromising either selectivity or arc flash protection. The EntelliGuard breaker-trip unit system demonstrates yet again GE’s core competencies in reliable electric power distribution, circuit protection and personnel protection. AKD-20 includes many features that address the needs of system reliability, arc flash protection and reduced footprint size.

Features and Benefits

  • The optimized footprint uses smaller section sizes when possible. Sections are provided in 22″, 30″ or 38″ widths.
  • Breaker compartment doors have no ventilation openings, thus protecting operators from hot ionized gases vented by the breaker during circuit interruption.
  • A superior bus system offers different levels of protection.  Insulated and isolated bus makes maintenance procedures touch friendly to reduce the risk of arc flash.
  • True closed-door drawout construction is standard with all AKD-20 equipment. The breaker compartment doors remain stationary and closed while the breaker is racked out from the connect position, through test, to the disconnect position. Doors are secured with rugged 1/4-turn latches.
  • An easy-to-read metal instrument panel above each circuit breaker holds a variety of control circuit devices, including the RELT switch.
  • Each circuit breaker is located in a completely enclosed ventilated compartment with grounded steel barriers to minimize the possibility of fault communication between compartments.
  • Optional safety shutters protect operators from accidental contact with live conductors when the breaker is withdrawn.
  • Easy access to equipment compartments simplifies maintenance of the breaker cubicle and control circuit elements as well as inspection of the bolted bus connections.
  • The conduit entrance area meets NEC requirements.  Extended depth frame options are available in 7″ and 14“ sizes for applications requiring additional cable space. The section width also can be increased for additional cable space.
  • A rail-mounted hoist on top of the switchgear provides the means for installing and removing breakers from the equipment. This is a standard feature on NEMA 3R outdoor walk-in construction and optional on indoor construction.
  • Control wires run between compartments in steel riser channels. Customer terminal blocks are located in metal-enclosed wire troughs in the rear cable area. Intercubicle wiring is run in a wireway on top of the switchgear, where interconnection terminal blocks are located.
  • All EntelliGuard G circuit breakers are equipped with rollers and a guidebar to provide easy and accurate drawout operation.
  • An optional remote racking device reduces the risk of the arc flash hazard by allowing the operator or electrician to move the breaker anywhere between the DISCONNECT and CONNECT positions from outside the arc flash boundary.
  • Optional infrared (IR) scanning windows can be installed in the switchgear rear covers to facilitate the use of IR cameras for thermally scanning cable terminations.
  • AKD-20 switchgear can be expanded easily to handle increased loading and system changes. Specify a requirement for a fully equipped future breaker to obtain a cubicle that has been set up for additional breaker installation, or add vertical sections without modifications or the use of transition sections.
  • An array of safety interlock and padlocking features are available to accommodate any type of lockout-tagout procedure a customer may have.
  • Optional Power Management:  With the proper devices and GE Enervista Power Management Control System (PMCS), facility power can be tracked and controlled.
  • Optional Metering and Power Quality:  The latest high technology EPM devices are available for the AKD-20 with broad capabilities for usage monitoring, cost allocation, load monitoring, demand tracking, common couplings with utilities, load and process control, and power quality monitoring.


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Guide To Low Voltage Busbar Trunking Systems

Guide To Low Voltage Busbar Trunking Systems

Modern electrical desdign and installations are often placing increasing demands on all products of the electrical equipment manufacturer.

Products must have:

• Reliable service life
• Adaptability to new requirements
• Low installation costs
• Low maintenance costs
• Inherent safety features
• Minimal purchase cost
• Energy efficiency

In today’s market one of the most important elements is cost effectiveness. In an electrical installation, one area where savings can be made and provide the features listed above is in the use of busbar trunking systems. Busbar trunking installations can be categorised into two basic types:

  • Distribution
  • Feeder

Distribution Feeder

This is the most common use of busbar trunking and is applied to distribute power over a predetermined area.    Busbar trunking can be run vertically or horizontally, or a combination of both. Typical applications would be:

  • Supply to large numbers of light fittings
  • Power distribution around factories and offices
  • Rising main in office blocks or apartment blocks to supply distribution boards serving individual floors.

Power is taken from busbar trunking by the use of tap off units which connect at defined positions along the busbar trunking, and allow power to be taken from the system, usually via a suitable protective device.

Advantages over cable:

  • The contractor can achieve savings with respect to material i.e. cable trays and multiple fixings and also labour costs associated with multiple runs of cable.
  • Reduced installation time since busbar trunking requires less fixings per metre run than cable.
  • Multiple tap-off outlets allow flexibility to accommodate changes in power requirements subsequent to the initial installation (subject to the rating of the busbar trunking).
  • Repositioning of distribution outlets is simpler
  • System is easily extendable.
  • Engineered product with proven performance.
  • Type tested to recognised international and national standards.
  • Aesthetically pleasing in areas of high visibility.

Feeder Trunking

When used for the interconnection between switchboards or switchboard and transformer, busbar trunking systems are more economical to use, particularly for the higher current ratings, where multiple single core cables are used to achieve the current rating and compliance with voltage drop and voltage dip requirements.

Beside this, bunch of cables are increasing possiblity of heating between cables and eventually short circuit.

Advantages over cable:

  • Greater mechanical strength over long runs with minimal fixings resulting in shorter installation times.
  • Replaces multiple runs of cable with their associated supporting metalwork.
  • Easier to install compared to multiples of large cables with all of the associated handling problems.
  • Less termination space required in switchboards.
  • Type tested short circuit fault ratings.
  • Takes up less overall space, bends and offsets can be installed in a much smaller area than the equivalent cable space.
  • Cable jointer not required.
  • Busbar trunking systems may be dismantled and re-used in other areas
  • Busbar trunking systems provide a better resistance to the spread of fire.
  • Voltage drop and voltage dip in the majority of cases is lower than the equivalent cable arrangement.

Typical Busbar Layout
Typical Busbar Layout

Tap-Off Units

Tap-off units are of two types, either plug-in or fixed. Plug-in units are designed to be accommodated at tap-off outlets at intervals along the distribution busbar trunking. Fixed tap-off outlets are engineered and positioned during manufacture to suit the specified installation. The tap-off unit usually contains the device providing protection to the outgoing circuit terminated at the unit to distribute power to the required load.

There are various types of protective devices, for example:

1. HRC fuses to BS EN 60269-1 (BS88)
2. Miniature Circuit Breakers to BS EN 60898
3. Moulded Case Circuit Breakers to BS EN 60947-2

HRC fuses may be incorporated into fuse combination units to BS EN 60947-3. The degree of enclosure protection of the tap-off unit is defined by BS EN 60529.

Each tap-off unit contains the necessary safety features for systems and personnel protection, as follows:

  • Plug-in units are arranged to be non-reversible to ensure that they can only be connected to give the correct phase rotation.
  • Plug-in units are arranged to connect the protective circuit before the live conductors during installation and disconnect the protective circuit after the live conductors while being removed.
  • Where units are provided with a switch disconnector or circuit-breaker these are capable of being locked in the OFF position.
  • Covers permitting access to live parts can only be removed by the use of a tool and will have any internally exposed live parts shielded to a minimum of IP2X or IPXXB in accordance with BS EN 60529.
  • Outgoing connection is achieved by cable terminations in the unit or by socket outlets to BS EN 60309-2 or BS 1363.

Fire Stops

Recommendations for the construction of fire-stops and barriers where trunking penetrates walls and floors classified as fire barriers. Internally the trunking may or may not require fire-stop measures according to the construction; where they are required these will generally be factory-fitted by the manufacturer and positioned according to a schematic drawing for the installation. Compact or sandwich-type trunking does not require internal fire-barriers, as suitability as a fire-barrier is inherent in the design.

However in all cases verification of the performance of the trunking under fire conditions needs to be provided by the manufacturer.

The following information is provided for guidance, and the method used should be agreed with the trunking manufacturer. It is not the responsibility of the trunking manufacturer to provide the specification or detail the rating or construction of the fire-stop external to the trunking.

Protective Earth Condustor Sizes

The sealing external to the busbar trunking (with or without an internal fire barrier) will need to conform to applicable building regulations. This may require filling the aperture around the busbar trunking with material to maintain the same fire proofing as the wall or floor.
Careful consideration needs to be given to the access required to complete the fire- stop. It may be necessary to install sections of fire-stop at the stage of installation of the trunking if access afterwards is impossible e.g. trunking runs in close proximity.

The protective earth connection(s) to the busbar trunking system shall conform to Section 543-01 of BS 7671 (IEE Wiring Regulations Sixteenth Edition).

Low-Noise Earth Systems

A low-noise earth, commonly referred to as a ‘clean earth’, is typically specified when electronic apparatus supplied from the system is sensitive to spurious voltages arising on the system earth. This is particularly true with IT equipment, found in all commercial premises these days, where data processing functions can be corrupted.

The low-noise earth is provided by a conductor separated from the protective earth (PE) and from all extraneous earth paths throughout the distribution system.
Many busbar trunking systems provide a ‘clean earth’ conductor in addition to the three phase conductors plus neutral, using the case or an external conductor as PE.

Tap-off units must be specified as ‘clean earth’ for the circuits concerned since the separation of the earths must be maintained and an additional termination will be provided for the load circuit ‘clean earth’ conductor. Sizing of the ‘clean earth’ conductor is not specified in BS 7671 (IEE Wiring Regulations Sixteenth Edition) but the usual practice is to calculate the size in the same way as for the protective earth conductor.

Neutral Sizes/Harmonics

The designer of the electrical network specifies the size of the neutral conductor depending upon the network loading. Typically this tends to be a neutral conductor the same size as the phase conductors (i.e.100% neutral).    As a minimum a 50% neutral may be specified.

The BS 7671 (IEE Wiring Regulations Sixteenth Edition) states “In a discharge lighting circuit and polyphase circuits where the harmonic content of the phase currents is greater than 10% of the fundamental current, the neutral conductor shall have a cross-sectional area not less than that of the phase conductor(s).”

With the increase of non-linear (almost anything electronic) single phase loads connected to a network, for example electronic ballasts in lighting fittings, or switch-mode power supplies (the type found in personal computers and servers) the total harmonic distortion is increased.


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Dry-Type disc wound transformers in MV applications

Dry-Type disc wound transformers in MV applications

Medium voltage, dry-type transformers may have their high voltage windings constructed using either the layer winding technique or the disc winding technique.

Both winding techniques provide the same result in terms of electrical performance parameters, i.e. turns ratio, impedance etc.

However, the use of transformers employing disc wound high voltage windings can result in increased reliability and therefore reduced downtime.


The basic purpose of a transformer is to convert electricity at one voltage to electricity at another voltage, either of higher or lower value. In order to achieve this voltage conversion, coils are wound on a laminated silicon steel core which provides a path for the magnetic flux. The coils comprise a number of turns of conductor, either copper or aluminum, wound as two electrically separate windings, called the primary winding and the secondary winding. The primary winding is connected to the source of voltage while the secondary winding is connected to the load. The ratio of primary to secondary turns is the same as the required ratio of primary to secondary voltages.

The turns of conductor forming the primary and secondary windings must be insulated from one another, while the primary winding must be insulated from the secondary winding and both the primary and secondary windings must be insulated from ground. The insulation of turns and windings is collectively called the insulation system of the transformer. The insulation system must be designed to withstand the effects of lightning strikes and switching surges to which the transformer is subjected, in addition to the normal operating voltages. A further requirement of the insulation system is that it must withstand the environmental conditions to which it is exposed, such as moisture, dust etc. A variety of techniques and materials are employed to achieve the necessary performance characteristics of the insulation system.

Layer winding

Fig.1 Layer winding

Fig.1 Layer winding

For low voltage, i.e. 600 Volt class windings, the winding technique used almost exclusively is the layer winding technique, also sometimes called helical winding or barrel winding. In this technique, the turns required for the winding are wound in one or more concentric layers connected in series, with the turns of each layer being wound side by side along the axial length of the coil until the layer is full. The conductors of the winding are insulated and so between turns there will be a minimum of two thicknesses of insulation. Between each pair of layers there will be layers of insulation material and/or an air duct.

Low voltage windings will generally be wound top to bottom, bottom to top etc. using a continuous conductor, until all layers are complete. High voltage windings, i.e. above 600 Volt class, may be wound in the same way, provided the voltage between layers is not too great.

To reduce the voltage stress between layers, high voltage windings are often wound in only one direction, for example, top to bottom. When the first layer of winding is complete, the winding conductor is laid across the completed layer from bottom to top and then the next layer is wound, again from top to bottom. In this way, the voltage stress between layers is halved.

The conductor must, of course, have additional insulation where it crosses the winding from bottom to top.

Fig.2 Transformer with layer wound coils

Fig.2 Transformer with layer wound coils

Disc winding

In the disc winding, the required number of turns are wound in a number of horizontal discs spaced along the axial length of the coil. The conductor is usually rectangular in cross-section and the turns are wound in a radial direction, one on top of the other i.e. one turn per layer, until the required number of turns per disc has been wound.

Fig.3 Disc winding

Fig.3 Disc winding

The conductor is then moved to the next disc and the process repeated until all turns have been wound. There is an air space, or duct, between each pair of discs. The disc winding requires insulation only on the conductor itself, no additional insulation is required between layers, as in the layer winding.

The disc wound high voltage winding is usually wound in two halves, in order that the required voltage adjustment taps may be positioned at the electrical center of the winding. In this way the magnetic, or effective length of the winding is maintained, irrespective of which tap is used, and therefore the magnetic balance between primary and secondary windings is always close to its optimum.

This is essential to maintain the short circuit strength of the winding, and reduces the axial electromagnetic forces which arise when the windings are not perfectly balanced.

Fig.4 Transformer with disc wound coils

Fig.4 Transformer with disc wound coils

Characteristics of Layer wound coils

As stated previously, the layer wound coil requires insulation between layers, in addition to the conductor insulation. The thickness of insulation required will depend upon the voltage stress between layers, and comprises one or more thicknesses of the appropriate insulation material. In practice, due to the nature of the construction of a layer wound coil, the finished coil will have several unavoidable small air pockets between turns and between layers. Many of these air pockets will become filled with resin during vacuum pressure impregnation of the coil.

Fig. 5 Equivalent circuit for Impulse voltage distribution

Fig. 5 Equivalent circuit for Impulse voltage distribution

However, it sometimes happens that some air pockets remain and it is in these air pockets that partial discharges can occur, greatly increasing the possibility of premature aging of the insulation and eventual failure.

Catastrophic failure can occur within a few months of energization. Under short circuit conditions, the electromagnetic forces developed cause transformer windings to attempt to telescope. At the same time the coil end blocking is trying to prevent movement. The result is often that the turns of the winding have a tendency to slip over one another, causing turn-to turn failure, due to abrasion of the insulation as the turns rub together. A further disadvantage of the layer wound coil is its poor impulse voltage distribution between the first few turns of the winding, due to the high ground capacitance and the low series capacitance.

A transformer winding forms a complex network of resistance, inductance and capacitance. As far as the impulse voltage distribution is concerned, the resistance can be ignored and at the instant of application of the impulse wave, when very high frequencies are predominant, the inductive elements become effectively infinite impedances. The whole structure therefore reduces to a capacitive network (see fig.5). Each turn of a transformer winding is insulated with a dielectric material and can be thought of as one plate of a multiple plate capacitor. In addition, the combination of dielectric material and air between each turn and ground forms further capacitive elements.

Characteristics of Disc wound coils

The major advantage of the disc wound coil lies in its open construction and relative lack of insulation. For a 15kV class transformer employing a disc wound primary winding, the number of discs will typically be in the range 36 to 48, resulting in a relatively low voltage per disc. Since each disc is separated from the next by an air space, the voltage stress between discs can easily be handled by the combination of conductor insulation and air, no additional insulation being necessary.

Each disc comprises a number of turns with each turn occupying one layer, i.e. one turn per layer: the voltage stress between layers is therefore the same as the voltage stress between turns and again, can easily be handled by the conductor insulation. The turns of each disc, being wound tightly together provide almost no possibility of air pockets being present within the disc.

Due to the open construction of the discs, any small air pockets which may be present are readily filled with resin during vacuum pressure impregnation of the coil. A properly designed and manufactured dry-type transformer disc winding therefore displays very low values of partial discharge, typically in the range 10 to 20 picocoulombs.

Unlike the layer wound coil, the disc wound coil provides good impulse voltage distribution, due to its inherently low value of ground capacitance and high series capacitance. The disc wound coil also displays excellent short circuit strength. Each disc by itself is mechanically very strong and the complete assembly of discs are held very securely in place. While the electromagnetic forces resulting from a short circuit result in a tendency, for the windings to telescope, the high voltage turns usually remain intact relative to each other. Instead, the complete disc has a tendency to distort as an assembly, with all the turns distorting by the same amount. The transformer can often continue to function, despite the distortion, until a convenient time arises for repair.


The flow of electric current through the turns of a transformer winding causes power losses which manifest themselves in the form of heat. These losses are called ‘’load losses’’ and are proportional to the square of the current. Obviously, it is necessary to dissipate this heat, to prevent overheating of the transformer, and in a dry-type transformer, this is achieved by the use of air spaces, or ducts, within the winding. The layer wound coil relies on vertical air ducts between layers and between windings, for cooling. Cool air enters the air ducts at the bottom of the coil and by natural convection, rises through the ducts, collecting heat on its way, then exits the coil at the top. It is essential for proper operation of the transformer that these air ducts are kept clear at all times.

The insulation required between the layers of a layer wound coil has a tendency to thermally lag the winding, impeding the dissipation of heat. The greater the operating voltage of the winding, the greater is the amount of insulation required and the greater is the lagging effect of the insulation. Some radiation also takes place from the outer surfaces of the coils. The open nature of the disc wound coil greatly improves the transfer of heat from the winding to the surrounding air. The thermal lagging effect of insulation is removed and the multiple horizontal air spaces between discs provide a large surface area for cooling by both radiation and convection.


The combination of layer wound low voltage winding, disc wound high voltage winding, NOMEX insulation and vacuum pressure impregnation of the windings with a solventless epoxy resin, results in a very reliable transformer with a long life expectancy. Transformers constructed in this way will be relatively free from partial discharge and will provide excellent impulse strength and short circuit strength, vital requirements for reliable operation in the most demanding of applications.


Author: Derek Foster, Olsun Electrics Corporation


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Maintenance Of Low Voltage Circuit Breakers

Maintenance Of Low Voltage Circuit Breakers

The deterioration of low voltage circuit breaker is normal and  this process begins as soon as the circuit breaker is installed. If  deterioration is not checked, it can cause failures and malfunctions. The purpose of an electrical preventive maintenance and testing program should be to recognize these factors and provide means for correcting them.

A good organized maintenance program can minimize accidents, reduce unplanned shutdowns and lenghten the mean time between failures of electrical equipment.

Benefits of good electrical equipment maintenance can be reduced cost of process shutdown (caused by circuit breaker failure), reduced cost of repairs, reduced downtime of equipment, improved safety of personnel and property.

Frequency Of Maintenance

Low-voltage circuit breakers operating at 600 volts alternating current and below should be inspected and maintained very 1 to 3 years, depending on their service and operating conditions. Conditions that make frequency maintenance and inspection necessary are:

  1. High humidity and high ambient temperature.
  2. Dusty or dirty atmosphere.
  3. Corrosive atmosphere.
  4. Frequent switching operations.
  5. Frequent fault operations.
  6. Older equipment.

A breaker should be inspected and maintained if necessary whenever it has interrupted current at or near its rated capacity.

Maintenance Procedures

Manufacturer’s instructions for each cir­ cuit breaker should be carefully read and followed. The following are general pro­ cedures that should be followed in the maintenance of low-voltage air circuit breakers:

  1. An initial check of the breaker should be made in the TEST position prior to withdrawing it from to enclo­sure.
  2. Insulating parts, including bushings, should be wiped clean of dust and smoke.
  3. The alignment and condition of the movable and stationary contacts should be checked and adjusted ac­cording to the manufacturer’s instruction book.
  4. Check arc chutes and replaces any damaged parts.
  5. Inspect breaker operating mechanism for loose hardware and missing or broken cotter pins, etc. Examine cam, latch, and roller surfaces for damage or wear.
  6. Clean and relubricate operating mechanism with a light machine oil (SAE-20 or 30) for pins and bearings and with a nonhardening grease for the wearing surfaces of cams, rollers, etc.
  7. Set breaker operating mechanism adjustments as described in the manufacturer’s instruction book. If these adjustments cannot be made within the specified tolerances, it may indicate excessive wear and the need for a complete overhaul.
  8. Replace contacts if badly worn or burned and check control device for freedom of operation.
  9. Inspect wiring connections for tightness.
  10. Check after servicing circuit breaker to verify the contacts move to the fully opened and fully closed positions, that there is an absence of friction or binding, and that electrical operation is functional.

Much of the essence of effective electrical equipment preventive maintenance can be sumarrized by four rules:

  • Keep it DRY
  • Keep it CLEAN
  • Keep it COOL
  • Keep it TIGHT




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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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