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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
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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.
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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
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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Transformator shot with thermovision camera
Substation ventilation is generally required to dissipate the heat produced by transformers and to allow drying after particularly wet or humid periods. However, a number of studies have shown that excessive ventilation can drastically increase condensation. Ventilation should therefore be kept to the minimum level required. Furthermore, ventilation should never generate sudden temperature variations that can cause the dew point to be reached. For this reason: Natural ventilation should be used whenever possible. If forced ventilation is necessary, the fans should operate continuously to avoid temperature fluctuations. Guidelines for sizing the air entry and exit openings of substations are presented hereafter.
Calculation methods
Natural ventilation
A number of calculation methods are available to estimate the required size of substation ventilation openings, either for the design of new substations or the adaptation of existing substations for which condensation problems have occurred.
The basic method is based on transformer dissipation. The required ventilation opening surface areas S and S’ can be estimated using the following formulas:
where:
S = Lower (air entry) ventilation opening area [m2] (grid surface deducted)
S’= Upper (air exit) ventilation opening area [m2] (grid surface deducted)
P = Total dissipated power [W]
P is the sum of the power dissipated by:
- The transformer (dissipation at no load and due to load)
- The LV switchgear
- The MV switchgear
H = Height between ventilation opening mid-points [m]
Note:
This formula is valid for a yearly average temperature of 20 °C and a maximum altitude of 1,000 m.
It must be noted that these formulas are able to determine only one order of magnitude of the sections S and S’, which are qualified as thermal section, i.e. fully open and just necessary to evacuate the thermal energy generated inside the MV/LV substation. The pratical sections are of course larger according ot the adopted technological solution.
Indeed, the real air flow is strongly dependant:
- on the openings shape and solutions adopted to ensure the cubicle protection index (IP): metal grid, stamped holes, chevron louvers,…
- on internal components size and their position compared to the openings: transformer and/or retention oil box position and dimensions, flow channel between the components, …
- and on some physical and environmental parameters: outside ambient temperature, altitude, magnitude of the resulting temperature rise.
The understanding and the optimization of the attached physical phenomena are subject to precise flow studies, based on the fluid dynamics laws, and realized with specific analytic software.
Example:
Transformer dissipation = 7,970 W LV switchgear dissipation = 750 W MV switchgear dissipation = 300 W The height between ventilation opening mid-points is 1.5 m.
Calculation:
Dissipated Power P = 7,970 + 750 + 300 = 9,020 W
Ventilation opening locations
To favour evacuation of the heat produced by the transformer via natural convection, ventilation openings should be located at the top and bottom of the wall near the transformer. The heat dissipated by the MV switchboard is negligible. To avoid condensation problems, the substation ventilation openings should be located as far as possible from the switchboard.
 «Over» ventilated MV/LV Substation. The MV cubicle is subjected to sudden temperature variations. |  Substation with adapted ventilation. The MV cubicle is no longer subjected to sudden temperature variations. |
If the MV switchboard is separated from the transformer, the room containing the switchboard requires only minimal ventilation to allow drying of any humidity that may enter the room.
Type of ventilation openings
To reduce the entry of dust, pollution, mist, etc., the substation ventilation openings should be equipped with chevron-blade baffles. Always make sure the baffles are oriented in the right direction.
MV cubicle ventilation
Any need for natural ventilation is taken into account by the manufacturer in the design of MV cubicles. Ventilation openings should never be added to the original design.
Source:
Instruction: Medium Voltage equipment on sites exposed to high humidity and/or heavy pollution by Schneider Electric
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