NEPTUNE
EN - 39
9.2. MEASUREMENT OF INSULATION RESISTANCE
Purpose of the test
Check that the insulation resistance of the installation complies with the requirements of
the applicable standard. This test has to be performed with the circuit being tested not
powered and with the possible loads it supplies disconnected.
Description
Test voltage [V]
Minimum allowable value [M
]
SELV or PELV systems
Systems up to 500V (private inst.)
Systems over 500V
250VDC
500VDC
1000VDC
> 0.250 M
> 1.00 M
> 1.00 M
Table 1: Most common test types, test voltages and relevant limit values.
Parts of the system to be checked
Check the insulation resistance between:
Each active conductor and the earth (the neutral conductor is considered as an active
conductor except in TN-C power supply systems, where it is considered as part of the
earthing (PEN)). During this measurement, all active conductors may be connected to
each other. Should the measured result not be within the limits prescribed by the
standards, the test must be repeated separately for each single conductor.
Active conductors. Guidelines recommends also checking the insulation between
active conductors when it is possible.
Allowable values
The values of the measured voltage and of the minimum insulation resistance can be
taken from the following table:
Rated voltage of circuit [V]
Test voltage
[V]
Insulation resistance
[M
]
SELV and PELV *
250
0.250
Up to/equal to 500 V, except for the above-
mentioned circuits
500
0.500
Over 500 V
1000
1,000
* The terms SELV and PELV replace, in the standard
’
s new wording, the old definitions of
“
very
low safety voltage
”
or
“
very low functional voltage
”
.
Table 2: Most common test types, insulation resistance measurement
If the system includes electronic devices, it is necessary to disconnect them from the
system. Should this not be possible, only perform the test between active conductors
(which, in this case, must be connected to each other) and the earth connection.
