2.5 - Low voltage generating sets

A completely new Section of 24 Regulations was introduced by the 1997 amendments dealing with low voltage generating sets. This set of Regulations was numbered Section 551, the existing [551 - Transformers] being renumbered as 555 to accommodate it.

Where low voltage or extra-low voltage generating sets are used to power an installation, (a) as the sole means of supply, or (b) as a backup in case of failure of the supply, or (c) for use in parallel with the supply, these Regulations apply. Self-contained systems, operating at extra-low voltage, which include the source of energy (usually batteries) as well as the load are not covered.

Generating sets, for the purpose of these Regulations, include not only rotating machines powered by combustion engines, turbines and electric motors, but also photovoltaic cells (which convert energy from light into electricity) and electrochemical accumulators or batteries. Protection of circuits fed from generators must be no less effective than those applying to mains-fed systems. Voltage and frequency variations are much more likely with generators than with a mains supply, and it must be ensured that they do not cause danger or damage t6 the equipment.

Protection of persons and of equipment must be at least as effective in the case of an installation fed permanently or occasionally by a generating set as for a mains-fed installation. Special requirements for bonding apply where static invertors are used. A static invertor is an electronic system which produces an ac supply at a given voltage, frequency and waveform from a dc source (often from a battery). When two or more generating sets operate in parallel, circulating harmonic currents are a possibility. Such currents are at frequencies which are multiples of the normal supply frequency, and will possibly result in overloading of the connecting cables unless steps are taken to reduce or remove them. When not intended to run in parallel with the mains supply, for example when used as a standby system. interlocks and switching must be provided to ensure that parallel operation is not possible.

When a generating set is intended as a standby system for use in place of the mains supply in the event of failure, precautions must be taken to ensure that the generator cannot operate in parallel with the mains. Methods include an interlock between the operating systems of the changeover switches, a system of locks with a single transferable key, a three-position break-before-make changeover switch, or an automatic changeover switch with an interlock.

Where generators are intended to operate in parallel with the mains supply, the Supply Company must be consulted to ensure that the generator is compatible in all respects (including power factor, voltage changes, harmonic distortion, unbalance, starting, synchronising and voltage fluctuation) with the mains supply. In the event of any of the above parameters becoming incompatible with the mains supply, the generating set must be automatically disconnected. Such disconnection must also occur if its voltage or frequency stray outside the limits of protection. It must be possible to isolate the generating set from the mains supply, and that means of isolation must always be accessible to the supply company.