3.3.3
Isolator positions
Every circuit must have its means of isolation,
which must be lockable in the OFF position where remote
from the equipment protected. The OFF position must be clearly
marked in all cases so that there is no doubt in the mind
of the operator as to whether his circuit is isolated and
thus safe to work on. Adequate notices and labels must be
displayed to ensure safe and proper isolation and to supplement
locks to prevent inadvertent re-closing of isolators when
this could cause danger. For single phase systems, both
live conductors (phase and neutral) must be broken by the
isolator (TT system). On three phase supplies, only the
three phases (R, Y, and B or LI, L2 and L3) need to be broken,
the neutral being left solidly connected (TN-S and TNC-S
systems).
This neutral connection is usually through
a link which can be removed for testing. Clearly, it is
of great importance that the link is not removed during
normal operation, so it must comply with one or both of
the following requirements:
1. - it can only be removed by using
tools, and/or
2. - it is accessible only to skilled
persons.
In many circuits capacitors are connected
across the load. There are many reasons for this, the most
usual in industrial circuits being power factor correction.
When the supply to such a circuit is switched off, the capacitor
will often remain charged for a significant period, so that
the isolated circuit may be able to deliver a severe shock
to anyone touching it.
The Regulations require that a means of
discharging such capacitors should be provided. This usually
takes the form of discharge resistors {Fig 3.4}, which provide
a path for discharge current. These resistors are connected
directly across the supply, so give rise to a leakage current
between live conductors. This current is reduced by using
larger resistance values, but this increases the time taken
for the capacitors to discharge to a safe potential difference.
In practice, a happy medium is struck between these conflicting
requirements, resistors with values of about 100K Ohms being
common.
Fig 3.4 Discharge
resistors connected to a three-phase capacitor bank
When isolating
a circuit so that it can be worked on, care must be taken
to ensure that there is no uninterruptible power supply
(UPS) or other standby system which may make it dangerous
although isolated from the mains supply.
