6.6.1 - Segregating circuits

The 15th and 16th editions of the IEE Wiring Regulations applied segregation of circuits which need to be kept apart by classifying them in four separate categories (the Regs themselves only mentioned three categories, but since two types of circuit both categorised as 3 had to be separated from each other, there were effectively four categories).

The second amendments to BS 7671 (published in 1997) have simplified the situation. There are now only two categories, which are known as voltage bands.

Voltage Band I is defined as levels of voltage which are too low to provide serious electric shocks; effectively this limits the band to extra-low voltage (ELV), including telecommunications, signalling, bell, control and alarm circuits.

Voltage Band II covers all voltages used in electrical installations not included in Band I. This means that all 230/400 V (240/415 V) supplies are included in Band II.

As expected, BS 7671 prohibits Band I and Band II cables sharing the same cable enclosure or multicore cable unless: every cable is insulated for the highest voltage present,
or each conductor in a multicore cable is insulated for the highest voltage present, unless conductors of the two bands are separated by an earthed metal screen,
or they are installed in separate compartments of a trunking or ducting system, or they are installed on a tray with a partition providing separation,
or a separate conduit or ducting system is provided for each band.

This does mean that BS 7671 allows circuits such as those for fire alarm Systems, emergency lighting, telephones, data transmission, intruder alarms, sound systems, bell and call systems, etc., may now be run together without segregation. BS 5838: 1988, on the other hand. makes it clear that fire alarm cables must be separated from all others, and IEE Guidance Note 4 requires that escape lighting cables should be mineral insulated or separated from all others by at least 300 mm.  Care must be taken to ensure that circuits are not affected by electrical interference, both electrostatic (due to electric fields) or electro-magnetic (due to electro-magnetic fields). In some ways this makes the circuit designer's task more difficult, because he must now ensure that there will be no interference, whereas before, he simply had to ensure that the required segregation was employed.

In some instances it will be necessary for circuit outlets for both voltage bands to share a common box; switchplate or block. In such a case, the connections of circuits of differing bands must be segregated by a partition, which must be earthed if of metal.