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Circuit Breaker Discrimination

Circuit breaker discrimination (also known as selectivity) is the ability of the local circuit breaker to operate before any upstream circuit breakers under fault conditions. This applies to HV and LV, the illustration below is for a simple single-feed LV system.

With correct discrimination, a single fault on one rack power supply will trip the local MCB in the power distribution unit, but not the distribution circuit breaker or worse still, the main circuit breaker.

In the same way, a fault on the power distribution unit will trip the distribution breaker feeding it, not the main circuit breaker. The intention is to limit the service interruption to the minimum amount for any fault, whether this fault is overload or short-circuit.

This all sounds very obvious and straightforward but as always the devil is in the detail. To correctly design a system the designer has to know the likely fault current at every point in the system and the time-current characteristics of each circuit breaker (or fuse). The graphs of these time-current (and time-energy) characteristics are then correlated to the calculated fault currents so that trip timings of the breakers can be assessed. The IEE Regulations stipulate that the final sub-circuit breakers open on earth fault in 0.4 secs, whereas the distribution circuit breakers must open in a maximum of 5 secs. The fault currents will be much less for faults at the final sub-circuits than faults at the power source or distribution switchboard due to the circuit impedances. These days all of these fault currents are worked out in software models rather than by hand, many factors such as cable type and size affect the fault current and hence the breaker trip opening times.

The designer has a choice of circuit breaker trip characteristics, the basic being thermal-magnetic as in miniature circuit breakers, then moving up to various types of electronic relay. These relays cost considerably more, and can add what amounts to a delay in the tripping time to allow downstream breakers to open first. To be able to hold in during a fault event the breaker has to be physically robust, again adding to the cost. The task of the designer is to find a compromise between necessary tripping characteristics and installation costs.

An essential process often overlooked is the setting of the circuit breaker relays during commissioning. Circuit breakers “out of the box” usually come set to 40% of their maximum ratings so if they are not correctly set up they could trip on overload sometime in the future as the critical load builds up.

Special considerations for UPS circuits

In the sketch above the “power source” is not specified but in data centres it is usually an electronic battery backed uninterruptible power supply (UPS). These present particular difficulties to the designer as they have severely restricted fault current capabilities – especially the modern high-frequency transformer-less types. In many cases it is not possible to meet the 0.4 secs and 5 secs criterion on battery power alone, however the IEE Regulations do recognise the strategy of relying on the operation of the UPS static bypass to clear downstream faults. If this is utilised special conditions for final sub-circuit earthing are imposed.

Most UPS systems these days are 2N, meaning that each rack power supply has an “A” and a “B” supply. The principles of discrimination still apply meaning that a fault on a particular rack power supply will trip either the “A” or “B” circuit breakers, or both in some circumstances, but not the circuit breakers feeding the “A” and “B” PDU’s.

Future-tech Conclusion

Correct circuit breaker discrimination is essential for the reliable operation of a data centre power system. It has to be designed in at the outset, not as an add-on after all the plant selection and cable sizing has taken place. The designer has to ensure that the design settings are transferred to the actual circuit breakers before the facility goes live. The designer has to take into account the type of power source under consideration – UPS, generator or utility transformer.