7.7.2 - Special requirements

The person in the restrictive location is protected by the use of the separated extra-low voltage (SELV) system. A SELV supply must be fed from a safety source, such as a special step-down isolating transformer. Protection against direct contact by obstacles or placing out of reach is not permitted.

Protection against indirect contact (touching parts which have become live as the result of a fault) for fixed equipment is achieved by one of the following methods:

1. - if the supply is SELV, then protection from indirect contact is automatically prevented because the system is totally isolated and has no earthing connections,

2. - for SELV protection can be by automatic disconnection, the protective device opening the circuit within 0.4 s in the event of a fault. To ensure low earth-fault loop impedance, the conductive location must again be solidly earthed,

3. - when SELV is used, protection by electrical isolation may be used. This means that equipment in use is fed from the unearthed secondary winding of an isolating transformer so that there is no path for earth currents. When more than one supply to equipment is required, each must be from a separate transformer secondary winding,

4. - equipment to Class II may be used, with additional protection from an RCD with an operating current not exceeding 30 mA.

Separated and isolating sources, other than those specified in 3) above, must be situated outside the restrictive conductive location unless they are part of an installation in a permanent restrictive conductive location. By its very nature an enclosed space is likely to be dark, and handlamps will be needed by the operator. Socket outlets intended to feed handlamps must be SELV, and these outlets can also he used to feed hand tools. However, the limitation of 25 V on SELV means that the range of hand tools available is very limited, and where higher voltages are used, sockets for this purpose can be fed using electrical separation.