Highrise buildings in London’s
Canary Wharf.
T here is a significant move to specify higher short-circuit ratings for the main switchboards, sub-distribution boards and final distribution boards in modern commercial buildings, particularly high-rise buildings for banks, building societies and insurance companies.
The short-circuit rating of equipment must be higher than the prospective fault current at that point. This is the maximum overcurrent that could be expected to flow in the event of a fault of negligible impedance between live conductors, or between a live conductor and earth, under normal operating conditions.
This current will depend on the impedance in the conductors between the point of supply (normally a substation transformer) and the equipment in question.
In many modern installations, with heavy electrical loads such as IT equipment and air conditioning plant, substations are frequently located close to the load centre. Consequently, supply lengths are short. Substations may be located in the basement of buildings or even at intermediate levels or in

rooftop plant rooms. Consequently, the distance between the transformer and switchboard is short. Furthermore the use of low impedance busbar risers and distribution systems means that impedances are even lower than with traditional cable distribution.
Prospective fault-current levels at the main low voltage switchboard in a major office or factory used to be of the order 50kA but today fault-current levels up to 100kA have to be considered. This is reflected down through the installation to panelboards and even final distribution boards. For example a
6-10kA short circuit rating for miniature circuit breakers and RCBOs may no longer be adequate; 15kA is sometimes specified.
All this means that systems have to be designed to withstand the stresses associated with higher prospective fault currents. This applies to the busbar distribution systems and tap-off units, switchboards, panelboards, distribution boards and all the current-carrying devices installed in them.
Transformers and packaged substations located on site, close to the switchboard, result in high fault-current levels.
Table 1 shows typical fault current levels at the main switchboard, tap-off units and final distribution boards in one of the Canary Wharf buildings. The prospective fault level at the main switchboard is 54kA but the use of low impedance busbar trunking means that the fault rating at tap-off units at the different floors is still high, calling for MCCB short-circuit ratings in excess of
46 or 47kA. The use of cable runs from here to the distribution boards means that there is considerable attenuation but fault-current levels are still high for
MCBs, calling for a 15kA short-circuit rating.
Table 1. Prospective fault levels in a modern high-rise building
Ref
Busbar 1
Busbar 2
Busbar 3
Ip fault level at main switchboard
54kA
54kA
54kA
Icu
Fault level at tap-off
46.91kA
46.91kA
45.62kA
Icn
Fault level at distribution board
9.77kA
14.94kA
13.88kA

Selection of the appropriate high fault capacity switchboards, busbar trunking and sub-circuit protective devices is a critical factor in ensuring safe operation of the distribution system. The specifier is advised to consult the switchgear manufacturer to ensure that equipment is appropriate to the needs of the installation.
The system design must take account of the MV supply and main LV distribution, and the downstream systems – panelboards and distribution boards, fused combination switches, MCCBs, MCBs and RCBOs.
Eaton ’s MEM Memform Form 4 cubicle switchboards are now available with a short-circuit capability of 100kA and busbar ratings up to 10,000A.
Other requirements of the modern marketplace also impact upon the design of this equipment. For example the cost of space means that more compact designs are demanded. The need for maximum safety of users is leading to the specification of higher Forms of Separation. Meanwhile modular construction can speed up design, manufacture and installation, ensuring consistent electrical performance and, frequently, more compact panels.
Cubicle switchboards to BS EN 60439-1 are now available with a busbar rating as high as 10,000A and short-circuit withstand of 100kA for 1 second
(220kA peak) where previously the maximum was 4000A and 80kA respectively. Cubicles are available with Form 4, Type 6 or 7 separation.
Compatible air circuit breakers and fused combination switches are readily available and even MCCBs are now available with short-circuit ratings of
100kA or higher.
Modular panelboard systems, capable of being assembled on site by a competent contractor, are available in ratings up to 2000A with a short-circuit

rating of 50kA for one second while distribution boards are being developed with increasing levels of short-circuit rating.
Meanwhile, miniature circuit breakers are sometimes specified instead of
MCCBs in order to save space. MCCBs with ratings of 16-63A typically occupy 25-30mm per pole. In contrast, MCBs are available up to 63A occupying only 18mm per pole. Consequently specifiers are tending to call for
MCBs rather than MCCBs in this band; but this in turn means that the MCBs must have higher short-circuit ratings. MCBs and RCBOs are now available with a 15kA short-circuit rating to meet this need.
Eaton MCBs and RCBOs are now available with a
15kA shortcircuit rating. Here, the unique MEM ‘pod’
RCBO comprising MCB with fieldfittable RCD ‘pod’.
The new Series G moulded-case circuit-breakers from Eaton are available with short-circuit ratings up to 100kA on the 250A frame size.

While the needs of developments such as Canary Wharf provided the initial impetus for these developments, the trend is inevitably reflected elsewhere, wherever large financial institutions, or multi-national companies move into new buildings with extensive IT installations and air conditioning loads.
In the new Heathrow Airport Terminal 5, miniature circuitbreakers and RCBOs with a 15kA short-circuit rating have been specified. This is because the distribution boards are relatively close to the substations.
Wherever switchboards, panelboards and distribution boards are located close to packaged substations in commercial, industrial or public buildings, close attention must be paid to prospective fault-current levels and the need for panels and devices with appropriate short-circuit performance.
This article was originally published in’Modern Building Systems’.
Contact: Karen Shaw
Eaton Electrical Ltd.,
Grimshaw Lane, Middleton,
Manchester M24 1GQ
Tel: +44 (0) 161 655 8900
Fax: +44 (0) 8700 507 525
Email: KarenAShaw@Eaton.com
Web: www.eatonelectrical.co.uk