22
Insulation measurements above 100 G
Ω
Measurements up to 100 G
Ω
can be made without any special
precautions, assuming that the test leads are reasonably clean
and dry. The guard lead can be used to remove the effects of
surface leakage if necessary. When measuring resistances
above 100 G
Ω
, the test leads should not be allowed to touch
each other, or any other object since this will introduce
leakage paths. Sharp points at the test lead connections
should also be avoided since this will encourage corona
discharge.
The output is isolated, and so will float relative to ground such
that the positive terminal is at plus half of the test voltage, and
the negative terminal is at minus half of the test voltage with
respect to ground.
Leakages therefore occur between the positive terminal and
ground, between the negative terminal and ground, and
directly between the positive and negative terminals. These
leakages have a significant effect and can occur through air.
If the guard lead is grounded, and since the negative terminal
is at the same voltage as the guard terminal, the leakage into
the negative terminal will be considerably reduced. This will
improve accuracy because the current flowing into the
negative terminal is measured by the instrument and used to
calculate resistance. This technique is only permissible if the
item under test is isolated from ground. In this context
isolated
means insulated by a resistance of at least 5 M
Ω
for the
positive terminal, or at least 10 k
Ω
for the negative terminal.
Conversely, if the positive terminal is grounded, then the
negative terminal will be at a voltage equal to the test voltage
relative to ground, which will result in an increase in leakage
current, and worsening of measurement accuracy.
When making measurements above 100 G
Ω
therefore, the
user should ground the Guard lead where possible, otherwise
parallel leakage paths may occur.
Alternatively, screened leads are available as an optional
accessory from Megger. When using a screened lead the
screen is plugged into the Guard terminal, diverting any
leakage currents. This considerably improves measurements
made with a floating output, where the leads might touch each
other or another object other than the test piece.
