Page 674 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

As with previous tests, precautions such as safe isolation must be followed.
Also, all sensitive equipment and lamps will need to be removed as the test can
potentially cause damage.

Method
At one end of the disconnected, safely isolated circuit, the meter is connected
between the line and earth by crocodile clips. It is important not to touch the
conductors during the test as the meter generates large voltages that can harm.

The meter is set to 500 V DC and when the circuit is clear of lamps/sensitive
equipment the voltage can be applied by pressing the test button.

N P
C.P.C.
Edison screw
KEY POINT Single pole switch lamp holder
Short circuits may occur Safety
between live conductors. isolated
A neutral counts as a circuit
live conductor. Such
short circuits are likely
to result in a very high Set on 500 V d.c.
current and will therefore
quickly trigger an over-
current protection p Figure 11.33 Testing insulation resistance
device. Consider the
effect this would have on If the insulation around circuit cables are in good order no conductivity will
an old fuse box in which exist between the conductors and therefore a good insulation resistance value
a nail has been put in will be achieved, typically much greater than 200 MΩ. If the cable insulation
place instead of the
correct size of fuse – you has deteriorated, then conductivity between conductors may exist. This will be
can see that the risk of apparent when readings below 1 MΩ are recorded. If values below 1 MΩ are
melting cables and the recorded and there is no obvious reason then always call an electrician, do not
potential for a fire are reinstate the circuit. Note remember never apply a voltage of 500 V DC across
highly increased. the live conductors of a circuit unless the equipment has been removed.

If the test between line and earth is successful, continue and test between all
the other combinations of conductors following the same procedure. If a low
reading is found, this could be a short circuit.
Short circuit

Switch closed
Phase

Majority (large) Electrical
current flow load

Neutral

p Figure 11.34 Short circuit
662

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