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Suitable circuit protection devices must be fitted to protect installed electrical equipment and people. However, it is important that only the protection device immediately upstream of a fault trips if a fault occurs, so that the rest of the electricity supply still works. This is called discrimination (or 'selectivity'), and it is crucial in ensuring, for example, that lights don't go out, which is both an inconvenience and an important safety consideration in its own right in homes, offices, hotels and factories. ‘Selectivity’ means that only the protection device immediately upstream of a fault trips if a fault occurs, so that the rest of the electricity supply still works. Therefore, in low voltage (LV) electrical installations, correct discrimination will minimise the cost and disruption of electrical faults occurring. James Hunt takes a look at this sometimes misunderstood topic:
Fitting the electricity supply chain with circuit protection devices having adjustable tripping characteristics at various points allows discrimination such that any fault disruption is minimised. Discrimination, therefore, can involve circuits having air circuit breakers (ACBs), moulded case circuit breakers (MCCBs), miniature circuit breakers (MCBs) and fuses, as well as residual current devices (RCDs/RCBOs etc) - or a mixture of them.
Determining discrimination:
In BS 7671: 2008, Regulation 531-2-9 states: 'Where, for compliance with the requirements of the regulations for fault protection or otherwise to prevent danger, two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger, the characteristics of the devices shall be such that the intended discrimination is achieved'.
Regulations 531 - 537 provide information on what is required for the discrimination and coordination of other protection devices. But how is discrimination actually achieved?
Note these initial points:
- Short-circuit discrimination - To be considered if there are high prospective fault levels (a short circuit near the protective device will be of exceptionally high energy according to the square law I2t).
- Overload discrimination - This relates to fault current magnitude - the upstream device must always have a higher continuous current rating (and a higher instantaneous pick-up value) than the next device downstream.
- Time discrimination - This concerns the period during which the circuit breaker 'sees' the fault current. Adjustable time delays in upstream device(s) are required, and these must withstand the thermal and electrodynamic effects of the full prospective fault current during the delay period.
In deciding what discrimination approach to take, this also depends on the protective device type(s) used. For example, Category A MCCBs are energy-limiting devices not intended for discrimination under short-circuit conditions, although discrimination is possible using peak let-through current curves with care. Category B MCCBs, though, are intended for discrimination under short-circuit conditions with respect to other load side short-circuit protective devices (Category A or B MCCBs, MCBs or fuses).
Looking at RCDs, now a virtual requirement in domestic installations under the 17th Edition, things are different again. When two (or more) RCDs are used in series, a time-delayed upstream device is required to achieve discrimination, remembering that RCD characteristics are different from those of fuses and circuit breakers.
To simplify things, protection device manufacturers publish tables listing prospective fault levels to which discrimination can be achieved. Just look up the device and rating for the upstream device against the type and rating for the downstream device. The maximum prospective fault level to which discrimination is possible is at the intersection of the two curves.
Fuses and breakers together:
Because of their higher short-circuit breaking capacities, fuses are often used in conjunction with circuit breakers, though the two have very different characteristics. Fuse manufacturers produce time/current curves for breakers and fuses used together (see also Appendix 3 to BS7671: 2008).
For example, Figure 1 (click below to download) compares the time/current characteristics of a 16A MCB with those of a 32A fuse. The two curves do not touch, so discrimination can be obtained with the fuse upstream of the MCB. In Figure 2 (again, click on the link below to see it), however, where the curve is for a 25A fuse, discrimination is only achieved up to 95A. Beyond this the fuse would blow before the MCB opens. These figures are courtesy of Eaton.
For short circuit discrimination, this is achieved if the circuit breaker's total let-through energy is less than the fuse's pre-arcing energy at all fault levels - tables listing pre-arcing energies are also available, while circuit breaker manufacturers produce curves presenting the total let-through energy at different prospective fault current values.
Circuit breaker tripping mechanisms react to fault current size and duration, so a high fault current can magnetically repel the contacts, reducing surface contact, increasing impedance and reducing the current flow. This limits the current flowing to the next device in the chain. The problem is that such phenomena do not show on the time-current charts. Note too, that very fast acting devices may also limit the current. For these reasons, full discrimination is not always possible.
Essential back-up protection:
In addition to achieving correct discrimination, electrical designers involved with large, complex LV projects must also maximise fault protection whilst minimising the cost and space taken up by protection devices. For example, an oversized switch room can be expensive over a building's lifetime.
Back-up protection saves money by using the current-limiting effect of upstream circuit breakers to allow fitting of downstream devices that have a smaller rated breaking capacity than would normally be needed. The fault current that the downstream breaker 'sees' is restrained by the upstream device - the only one required to have a breaking capacity that matches the highest possible fault current.
There is, however, a discrimination disadvantage - with back-up protection, there is a threshold fault current ('takeover current'), above which both circuit breakers will trip. Total discrimination is only possible for fault currents below this level.
And finally…
Discrimination, and especially back-up protection, must be right. If it isn't, the consequences could be serious, yet the considerations can be complex.
When designing an LV electrical installation, it is the electrical designer's job to carry out a proper electrical protection study to ensure that everything is safely protected. Yet in practice, this task is often left to electrical contractors, who have to be sure that they can achieve what is required. If they are unsure, it is safer for them to ask advice from electrical consultants, or protective device manufacturers such as ABB, Eaton, Legrand and Schneider Electric.