8.4.3 - Correct polarity
If a single-pole switch or fuse is connected
in the neutral of the system rather than in the phase, a
very dangerous situation may result as illustrated in {Fig
8.8}.
Fig 8.8 - The danger
of breaking the neutral of a circuit
Fig 8.9 - Polarity test
of an installation
It is thus of the greatest importance that
single-pole switches, fuses and circuit breakers are connected
in the phase (non-earthed) conductor, and verification of
this connection is the purpose of the polarity test. Also
of importance is to test that the outer (screw) connection
of E S lampholders is connected to the earthed (neutral)
conductor, as well as the outer contact of single contact
bayonet cap (BC) lampholders. The test may be carried out
with a long wander lead connected to the phase conductors
at the distribution hoard and to one terminal of an ohmmeter
or a continuity tester on its low resistance scale. The
other connection of the device is equipped with a shorter
lead which is connected in turn to switches, centre lampholder
contacts, phase sockets of socket outlets and so on. A very
low resistance reading indicates correct polarity (see {Fig
8.9}).
To
avoid the use of a long test lead, a temporary connection
of phase to protective systems may be made at the mains
position. A simple resistance test between phase and protective
connections at each outlet will then verify polarity. In
the unlikely event of the phase and protective conductor
connections having been transposed at the outlet, correct
polarity will still be shown by this method; this error
must he overcome by visual verification -
Special
care in checking polarity is necessary with periodic tests
of installations already connected to the supply, which
must be switched off before polarity testing. It is also
necessary to confirm correct connection of supply phase
and neutral. Should they be transposed, all correctly-connected
single-pole devices will be in the neutral, and not in three
phase conductor.
One practical method of checking polarity and continuity
of ring or radial circuits for socket outlets is to connect
two low power lamps to a 13 A plug. One is connected between
phase and neutral, and the other between phase and earth.
Plugging in at each socket tests correct polarity and the
continuity of live and protective conductors when both indicators
light. It is important where RCD protection is employed
to use very low power indicator lamps such as neon or LED
devices (with suitable current limiting resistors where
necessary). The smallest filament lamp will take sufficient
current from phase to earth to trip most RCDs.
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