Explaining the need for higher short-circuit ratings

Canary Wharf
The high-density electrical loading in high-rise buildings such as Canary Wharf inevitably increase short-circuit current ratings.
The changing nature of electrical installations is reducing their natural impedance, so that higher short-circuit currents occur. DAVID PITT and CHRIS SMITH explain the consequences.There 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. Short supply length 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 — so supply lengths are short. Substations may be in the basement of buildings or even at intermediate levels or in roof-top plant rooms. Consequently the distance between the transformer and switchboard is short. Further, the use of low-impedance busbar risers and distribution systems means that impedances are even lower than with traditional cable distribution. Prospective fault-currents at the main low-voltage switchboard in a major office or factory used to be around 50 kA. Today, fault-current levels up to 100 kA have to be considered. This is reflected down through the installation to panelboards and even final distribution boards. For example a 6 to10kA short-circuit rating for miniature circuit-breakers and RCBOs may no longer be adequate; 15 kA 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. Table 1 shows typical fault-current levels at the main switchboard, tap-off units and final distribution boards in a building at Canary Wharf. The prospective fault level at the main switchboard is 54 kA 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 47 kA. 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 15 kA short-circuit rating. Choosing the right equipment Selecting 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 medium-voltage supply and main low-voltage distribution, and the downstream systems — panelboards and distribution boards, fused combination switches, MCCBs, MCBs and RCBOs. 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 10 000 A and short-circuit withstand of 100 kA for one second (220 kA peak); the maximum values were previously 4000 A and 80 kA, 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 100 kA or higher. Modular panelboard systems for assembling on site by a contractor, are available in ratings up to 2000 A with a short-circuit rating of 50 kA for one second. Distribution boards are being developed with higher short-circuit ratings.
Table 1. Prospective fault levels in a modern high-rise building are much higher than they used to be.
Meanwhile, miniature circuit-breakers are sometimes specified instead of MCCBs to save space. MCCBs with ratings of 16 to 63 A typically occupy 25 to 30 mm per pole. In contrast, MCBs are available up to 63 A occupying only 18 mm per pole. Specifiers are thus tending to call for MCBs rather than MCCBs in this band, which means that the MCBs must have higher short-circuit ratings. The pattern spreads 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 Terminal 5 at Heathrow Airport, miniature circuit-breakers and RCBOs with a 15 kA short-circuit rating have been specified. This is because the distribution boards are relatively close to the substations. Conclusion 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. David Pitt and Chris Smith are with Eaton Electric Ltd, Reddings Lane, Tyseley, Birmingham B7 5TQ.
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