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
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
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
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.