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High buildings; high fault currents01 July 2004By David Pitt and Chris Smith, product managers, Eaton MEMConsulting engineers working on tall buildings for banks and financial companies are specifying higher
short-circuit ratings for the main switchboards, sub-distribution boards and final distribution boards. This is because the design of the electrical distribution systems, with large on-site transformers and low impedance busbar distribution systems, results in higher prospective fault current levels.
Where, just a few years ago, prospective fault-current levels at the main low voltage switchboard were around 50kA, today fault-current levels up to 100kA must be considered. This is reflected down through the installation to panelboards and final distribution boards to the point where a 6-10kA short-circuit rating for miniature circuit-breakers and RCBOs is no longer adequate and 15kAis sometimes specified.
This trend began with developments such as London’s Canary Wharf and the telecommunications installations associated with the Internet. Manufacturers such as Eaton MEM are supplying equipment for high fault level in a range of installations.
Packaged substations are therefore installed within a building and house transformers with ratings of 3-4MVA, or two transformers configured for parallel connection. Substations are located in basements and at intermediate levels or in roof-top plant-rooms. The distances between the switchboards, sub-distribution
boards and load centres are short and of relatively low impedance, resulting in high prospective fault current levels.
The use of modern low-impedance busbar trunking systems, for low voltage distribution between transformers and switchboards, and between switchboards and panelboards, reduces circuit impedances still further compared with traditional cable distribution. This offers benefits by reducing heat loss and voltage drop, but also has the effect of increasing prospective fault current levels.
Systems must 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, and distribution boards.
The prospective fault level at the main switchboard (in one of the Canary Wharf buildings) 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, calling for MCBs with a 15kA rating.
System design must take account of the MV supply and main LV distribution, and the downstream systems
such as panelboards and distribution boards, fused combination switches, and moulded-case circuit-breakers.
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.
Cubicle switchboards complying with BS EN 60439-1 are now available with a busbar rating of 10,000A and shortcircuit withstand of 100kA for 1s (220kA peak) where previously the maximum was 4000A and 80kA respectively.
Compatible air circuit-breakers and fused combination switches are readily available and even moulded-case circuit-breakers are available with fault 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 fault current rating of 50kA for 1s while distribution boards are being developed with increasing levels of short-circuit rating.
The trend to higher fault current levels is also reflected in demand from consultants for miniature circuit-breakers, and even RCBOs, with a 15kA short-circuit rating (to BS EN 60947-2) where 6-10kA is currently the
norm. This is driven, not only by the trend to higher fault current levels but also by the need to use MCBs rather than MCCBs to save space.
Although moulded-case circuit-breakers are readily available with short-circuit ratings of 15kA or higher, ratings of 16-63A typically occupy 25-30mm per pole. In contrast, miniature circuit-breakers 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 ratings.
While the needs of developments such as Canary Wharf provided the initial impetus for these developments, the trend is reflected elsewhere wherever large financial institutions, or multi-national companies move into new buildings.
The MCB trend is not restricted to high-rise commercial developments. For example, in the new Heathrow Airport Terminal 5 project, miniature circuit-breakers and RCBOs with a 15kA shortcircuit rating have been specified because the distribution boards are relatively close to the substations. Contact Details and Archive...Most Viewed Articles... |
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