Operation
8
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 scale.
If a capacitive circuit is tested, the pointer will initially deflect towards
0 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 farthest ends of the test leads together and carry out a check
on the leads. A 0 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.
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 !
Do not forget to release a locked Test button, and
discharge any capacitance before touching the item
under test, or disconnecting the test leads.
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