SEFELEC1500M Instructions
150
V1.06
16.4.
INSULATION RESISTANCE MEASUREMENT
The insulation resistance is measured to ensure that the various components and sub-
assemblies in an electrical assembly have sufficiently high insulation resistance to ensure
that the leakage currents do not reach unacceptable values.
The principle is to apply a given stable direct voltage (there are various standard voltages)
between predefined points, for a time that is usually specified, and then measuring the cur-
rent passing through the material
under test. Ohm’s law (Resistance = Voltage / Current) is
used to calculate the insulation resistance. This value is tested against the minimum thresh-
old specified in the standard used for the test.
16.4.1.
PRECAUTIONS
The equipment under test must be connected in such a way as to minimise the parasitic
leakage caused by the measurement technique.
The accessories supplied have shielding that is connected to a guard voltage to provide a
high immunity to parasitic leakage currents and mains fields.
If extensions are used for the basic accessories, care should be taken to not introduce any
measurement error (short leads, leads kept clear of metal parts or even insulating parts,
etc.).
For very high resistance measurements (>
100 GΩ
), an operator putting his hand close to
the equipment under test could reduce the measurement accuracy or make the measure-
ment unstable. Care should be taken not to wear nylon lab coats or insulating objects
which may create high static electricity fields which could affect the measurement of high
resistances. (
100 GΩ
at
100 V = 1 nA
measured current).
16.4.2.
CAPACITANCE MEASUREMENT
Recall that many pieces of modern electrical equipment have mains filters including capac-
itors to meet EMC standards. During measurement on capacitors it is recommended to
use the
CAPACITOR
measuring mode in order to stabilise the values measured.
With these capacitors, even the smallest fluctuation in or interference on the measure-
ment voltage will pass directly into the current measurement system which has a very high
gain and will amplify these fluctuations. The
CAPACITOR
mode implements circuits which
will limit the instability of the values measured.
Always carry out insulation resistance measurements on capacitive circuits at increasing
test voltages. Hysteresis and polarisation effects in the dielectrics will give false measure-
ments if reducing test voltages are used between each test. If reducing test voltages are
used, the device will over-estimate the insulation resistance and will take a long time to fall
to the real value.