9
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 (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.
Operation Block Diagram
to ‘-’ve
terminal
Leakage Path
Tightly bound bare wire
to ‘+’ve
terminal
to ‘G’
terminal
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