Operation
6
Performance Checks
The instrument will operate in any position, but the specified
accuracies assume that the instrument is face up, on a firm
level surface. This is particularly true for hand cranked units
to obtain a smooth constant crank speed.
1)
Without the test leads being connected to the instrument,
but with the rotary selector switch set to the 1 kV range
(
MJ160
to 500 V range) press and hold down the
‘Test’
button, whilst turning the generator handle,(
MJ159
,
MJ160
and
MJ359
) at >180 rev/min.The meter pointer should
remain over the
‘
∞
’ (infinity) position on the scale. This
establishes that there is no leakage through the instrument
itself.
2)
Check that the test leads, probes and crocodile clips are
in good order, clean and with no broken or cracked insulation.
Connect two of the test leads to the ‘
+
’ and ‘
-
’ terminals on the
side of the instrument case and ensure that their clips are not
touching anything.
3)
Press the
‘Test’
button again and keep it pressed whilst
turning the generator handle (Hand cranked models) at >180
rev/min and observe the meter needle. The needle should
rest over the ‘
∞
’ (infinity) position on the scale. If it does not,
the test leads may be faulty and should be inspected more
closely for damage. Replace them if necessary with
calibrated leads
available as optional accessories
4)
Connect the test lead clips together, press the
‘Test’
button and turn the generator handle (hand cranked models)
again. The meter should read zero. If it indicates infinity or a
high resistance value the leads may be open circuit and
should be inspected further. Replace them if necessary.
(Shorting the leads together and obtaining a zero reading also
shows that the instrument is working).
Note:
—To avoid creating leakage paths when insulation
testing, it is advisable not to allow the leads to twist together
nor trail across metalwork etc. more than is really necessary.
Voltage measurement
When not testing (i.e. in standby mode) the instruments act as
a voltmeter (0 to 600 Volts a.c.) Therefore, as soon as the test
leads are connected to the item under test, any a.c. voltage
present will be immediately 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. In this case it is important
to realize that the actual voltage (d.c. in nature) is not given,
but the meter does indicate when the voltage has decayed to
zero and therefore when it is safe to remove the test leads.
Note, however, that the instrument does not indicate the
presence of negative d.c.voltage.
Resistance Measurement
With the leads connected to the instrument, and having
completed the Preliminary Checks:
1)
Set the selector switch to the ‘
Ω
’
position.
2)
Connect the leads across the isolated circuit.
3)
Press and hold the
‘Test’
button and turn the generator
handle (hand cranked models) at >180 rev/min.
4)
The resistance will be indicated on the ‘
Ω
’
scale.
Note:-
If necessary repeat the continuity test with the leads
reversed. The effects of any stray e.m.fs. in the sample under
test may then be negated by taking the average of the two
readings.
