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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 electric energy.
This is commonly achieved by switching off an isolation device
within the distribution board. Regulation 537.2.2.1 requires
that the device shall isolate all live conductors, subject to the
provisions of regulation 537.1.2.
The neutral conductor is also a live conductor. In a TN-S or
TN-C-S installation, however, regulation 537.1.2 allows the
neutral conductor to not be isolated where it is reliably connected
to earth.
If the supply complies with the Electrical Safety, Quality and
Continuity Regulations 2002, a three-pole isolating device is
sufficient for a three-phase supply.
Regulation 537.2.1.7, however, says that there should be some
provision for disconnecting the neutral, for example by using a
bolted link.
Three-phase TT supplies will require disconnection of the neutral,
so a four-pole isolation device is needed.
For a single-phase supply where the main switch will be used by
‘ordinary persons’, the isolating switch must interrupt both live
conductors.
Symbol indicating suitability for isolation