Voltage Measurement; Insulation Testing; Choice Of Test Leads - Megger BM15 Mode D'emploi

OPERATION

Voltage measurement

When switched to the 'V' position, the instrument measures up to 600 V a.c.
to the specified accuracy of the instrument. DC voltage is also indicated on the
display, but not to the specified accuracy. When not testing (i.e. in standby
mode) and connected to a live circuit, the instruments default to voltmeter (0
to 600 Volts a.c. or d.c.) irrespective of the rotary switch position. Any voltage
present will immediately be shown. Thus indication is given that the item has
not been completely de-energized. The instrument also monitors circuit
discharge when the 'Test' button is released following an insulation test on a
capacitive item, e.g. a long cable.

Insulation Testing

After connecting the test leads to the instrument and carrying out the
Preliminary Performance Checks:
1) Set the selector switch to the required test voltage. Connect the test
leads to the isolated circuit to be tested, as follows:-
(a) For insulation tests to earth (ground):- Connect either test
lead to earth (ground) or the frame of the equipment,and
the other lead to that part of the circuit to be tested.
b) For insulation tests between wires:- Connect a lead to the
core of each of the wires.
2) Press the 'Test' button (or turn the generator handle).
3) The meter pointer will indicate the value of insulation resistance on
the Ω scale.
If a capacitive circuit is tested, the pointer will initially deflect towards 0 Ω and
then gradually rise to its final steady value as the capacitance is charged up to
the output voltage of the tester.
If several successive readings of ∞ (infinity) are obtained, connect the two
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farthest ends of the test leads together and carry out a check on the leads. A
0Ω reading should result which double checks that the leads are not
disconnected or broken and therefore, the insulation resistance readings are
correct.
Capacitive circuits automatically discharge through the tester when the 'Test'
button is released. Decaying discharge voltage will be indicated on the voltage
scale. Wait a few moments for the voltage to decay to below 60 V before
disconnecting the test leads.
Do not forget to release a locked Test button, and discharge
any capacitance before touching the item under test, or
disconnecting the test leads.

Choice of Test Leads

BM15/MJ15 are supplied with three unfused leads terminated in crocodile
clips. These are the best leads to use for insulation tests on non-live circuits.
The crocodile clips ensure that any capacitive load remains connected until it
is automatically discharged at the end of a test.
GS 38 (UK Safety Authority Guidance Note) advises the use of fused test prods
if the instrument is to be used for making voltage measurements on live, high
energy circuits. These leads are available as an optional extra. See
'Accessories'.
Fused prods must not be used for insulation testing. If the fuse should
rupture, or the prods lose contact during a test, the system under test would
remain charged without any apparent evidence of danger!