7
Insulation Testing
After connecting the test leads to the instrument and carrying
out the Preliminary 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
the
‘+’
test lead to earth (ground) or the frame of the
equipment,and the
‘-’
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 and whilst keeping it pressed
turn the generator handle (hand cranked models) at
>180 rev/min.
3)
The meter needle will indicate the value of insulation
resistance on the
‘M
Ω
’
scale.
If a capacitive circuit is tested the needle will initially deflect
towards zero 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 ‘
∞
’
are obtained, connect
the two farthest ends of the test leads together and carry out
a check on the leads. A
zero
reading should result which
double checks that the leads are not disconnected or broken
and therefore, the insulation resistance readings are correct.
Note:—
With non-hand cranked models, the readings are
taken after pressing the ‘
Test’
button, there is no other control
to operate.
Capacitive circuits automatically discharge through the tester
when the
‘Test’
button is released. The approximate
discharge voltage will be indicated on the voltage scale. Wait
a few moments for the voltage to decay to zero before
disconnecting the test leads.
Using the Guard terminal (G)
For basic insulation tests and where there is little possibility of
surface leakage affecting the measurement, it is unnecessary
to use the guard terminal. i.e. if the insulator is clean and
there are unlikely to be any adverse current paths. However
in cable testing, there may be surface leakage paths across
the insulation between the bare cable and the external
sheathing due to the presence of moisture or dirt. Where it is
required to remove the effect of this leakage, particularly at
high testing voltages, a bare wire may be bound tightly
around the insulation and connected via the third test lead to
the guard terminal
‘G
’.
The guard terminal is at the same potential as the negative
terminal. Since the leakage resistance is effectively in parallel
with the resistance to be measured, the use of the guard
causes the current flowing through surface leakage to be
diverted from the measuring circuit. The instrument therefore
reads the leakage of the insulator, ignoring leakage across its
surface.
Leakage Path
to ‘
-
’ve terminal
to ‘
G
’ terminal
to ‘
+
’ve terminal
