Using The Guard Terminal - Megger BM15 Gebrauchsanleitung

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
Leakage Path Tightly bound bare wire
to '+'ve to 'G'
terminal terminal
to '-'ve
terminal
9