224-225 / 319
Guide to commercial installations | Part 2 Panelboard: Standards and Regulations
8
Isolation and switching
Isolation aims to make dead, for safety reasons, all or a
discrete section of the electrical installation by separating it
from every source of electrical energy.
This is often achieved by switching an isolation device in the
panelboard. Regulation 537.2.2.1 requires that the device
must isolate all live conductors.
The neutral conductor is a live conductor, but in a TN-S or
TN-C-S system, regulation 537.4.1.2 does not require
isolation of the neutral conductor where it is reliably
connected to earth.
If the supply complies with the Electrical Safety, Quality and
Continuity Regulations 2002, a 3 pole isolating device is
sufficient for a 3-phase supply; although it is not a
requirement to isolate or switch the neutral conductor in
installations supplied from a TN supply system, regulation
537.2.1.7 requires the ability to disconnect the neutral. This
requirement is normally met by the provision of a suitable
terminal or bolted link, which must be in an accessible
position, can only be disconnected by means of a tool, is
mechanically strong and will reliably maintain electrical
continuity.
A three-phase TT system will require disconnection of the
neutral, so you will need a 4-pole isolation device.
Table 53.4 of BS 7671 gives guidance on the selection of
isolating and switching devices. The main switch for a
panelboard is typically BS EN 60947 part 2 or 3, while you
can use outgoing circuit breakers to BS EN 60947-2 for
isolating individual circuits, provided they are marked with the
appropriate symbol (see below).
A device to BS EN 60947 part 2 or 3 which is suitable for
isolation must be marked with one of the following symbols.
BS EN 60947 part 3 symbol indicating suitability for isolation
and BS EN 60947 part 2 for a CBI (circuit-breaker without
overcurrent protection. They are capable of being tripped by
an auxiliary device, e.g. shunt or undervoltage release).
BS EN 60947 part 2 symbol indicating suitability for isolation
for a circuit-breaker