Faulty thinking

The construction of modern, high-rise, commercial buildings with on-site substations and low impedance busbar riser systems has led to demand for an increase in short-circuit ratings at the main switchboards, sub-distribution boards and final distribution boards. This pattern is likely to spread.

A few years ago, prospective fault current levels at the main low voltage switchboard in a major office or factory were around 50 kA. This was reflected in the short-circuit rating of the switchboards and associated switchgear. Today fault current levels up to 100 kA must be considered.

The principal reason for this is the trend to use transformers with higher ratings, typically up to 3-4 MVA, or two transformers configured for connection in parallel. These transformers are frequently sited within the building, sometimes at different floor levels to serve separate load centres. This means that the distance between the switchboards and the point of supply is relatively short, leading to low circuit impedances and high prospective fault currents.

The use of modern, low impedance busbar trunking systems for low voltage (lv) distribution between the transformers and switchboards, and between switchboards and sub-distribution boards, further reduces circuit impedances through the system compared with traditional cable distribution. All of this makes for a more efficient distribution, with a low voltage drop.

This means that systems must be designed to withstand the stresses associated with higher prospective fault currents. This applies to the busbar distribution systems, the switchboards and all the current-carrying devices installed in them.

Choosing the right equipment
Selection of the appropriate high fault capacity device to protect the sub-circuit is a critical factor in ensuring safe operation of a 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. It should also consider the downstream systems – panelboards, distribution boards, fused combination switches, moulded-case circuit-breakers, miniature circuit-breakers and residual circuit-breaker overloads (rcbo).

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, ie the separation of different sections within a switchboard to prevent contact with the live parts of adjacent functional parts to limit the probability of initiating arc faults and the passage of solid objects from one section to another.

Meanwhile, modular construction can speed up design, manufacture and installation, ensuring consistent electrical performance and, frequently, more compact panels.

Cubicle switchboards to BS EN 60 439-1 are now available with a busbar rating of 10 kA and short-circuit withstand of 100 kA for one second (220 kA peak). Previously the maximum ratings were 4 kA 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. Even moulded-case circuit-breakers are now available with fault ratings of 100 kA or higher.

Modular panelboard systems, capable of being assembled on site by a competent contractor, are available in ratings up to 2 kA, with a fault current rating of 50 kA for one second. Distribution boards are being developed with increasing levels of short-circuit rating.

The trend to higher fault current levels is also reflected in the demand from consultants for miniature circuit-breakers – and even rcbos – with a 15 kA short-circuit rating to BS EN 60 947-2, where 6-10 kA is currently the norm. This is driven by the trend to higher fault current levels and the need to save space.

Moulded-case circuit-breakers are readily available with short-circuit ratings of 15 kA or higher. These typically occupy 25-30 mm per pole at the lower end (16-63 A). Miniature circuit-breakers are available up to 63 A, occupying only 18 mm per pole.

The pattern spreads
The main impetus for these developments in the UK has been recent projects in London's Canary Wharf where the demands of computers and air conditioning systems have led to the provision of several substations in each of the high-rise buildings. Typical ratings are 3·5 and 4 MVA and distribution is generally by low impedance busbar trunking.

This pattern is already beginning to spread as consultants apply the same standards to specifications elsewhere. One example of this change in design specifications is the new Terminal 5 building at Heathrow Airport, where a 15 kA short-circuit rating has been specified for miniature circuit-breakers and rcbos.