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b. Any surge protection devices or other
electronic devices that may influence test
results or suffer damage from the test
voltage must be disconnected before
performing an insulation resistance test. If
it is not practical to disconnect these
devices, a test voltage of 250V dc can be
used for the particular circuit but the
insulation resistance must be at least 1MΩ.
i) Insulation resistance to earth
Carry out the steps a) and b) above and then
measure the insulation resistance between live
conductors (line and neutral) and the CPC.
ii) Insulation resistance between live
conductors
Carry out steps a) and b) above and then
measure the insulation resistance between L and
N conductors.
iii)SELV and PELV circuits
Carry out steps a) and b) above and then:
- measure the insulation resistance between
SELV and PELV circuits and live parts of other
circuits at 500V dc.
- measure the insulation resistance between
SELV or PELV conductors at 250V dc and
between PELV conductors and protective
conductors of the PELV circuit at 250V dc.
Circuit nominal voltage
SELV and PELV
Up to and including 500V with the exception of the above systems
Above 500V
8
The test voltage and minimum insulation
resistance values are shown below. In most
cases, the measured value will be considerably
greater than 1M
and the test instrument will
indicate over range. See table below.
EQUIPMENT DISCONNECTED
SWITCHES
CLOSED
LAMPS
REMOVED
N
L
E
INCOMING TAILS
LINE and NEUTRAL ARE TESTED SEPERATELY TO EARTH
FOLLOWED BY A TEST BETWEEN LINE AND NEUTRAL.
A TEST VALUE IN EXCESS OF 1MΩ SHOULD BE
ACHIEVED FOR A FULL INSTALLATION.
A TEST VALUE IN EXCESS OF 2MΩ SHOULD BE ACHIEVED
FOR INDIVIDUAL CIRCUITS (HSE GUIDELINES).
Diagram 7: Insulation Resistance Test
4. Polarity
The polarity test is carried out to verify:
a) That all single-pole switches in lighting circuits
are connected into the line conductor.
b) That the centre pins of an Edison Screw (ES)
lamp holder are connected to the line
conductor.
Test voltage (V d.c.
Min. insulation resistance (mΩ)
250
0.5
500
1.0
1000
1.0
www.seaward.co.uk
c) That the correct pin of socket outlets is
connected to the line conductor.
The polarity test is carried out as follows :
i) Link the line and the CPC in the Consumer Unit.
ii) Test between the line and CPC at each
accessory point.
A satisfactory reading will be indicated by a low
resistance value.
The resistance measured at the extremity of the
circuit is the sum of the resistances of the line
conductor and protective conductor (R
+ R
) and
1
2
should be equal to the value obtained by using
Tables 9A, 9B, and 9C in the IEE On-site Guide.
See Diagram 8.
E
ES LAMP
HOLDER
SWITCH
E
CLOSED
NOTE:
NEUTRAL
CONDUCTOR
OMITTED FOR
CLARITY
EARTH
BAR
LINK
L
CONSUMER UNIT
Diagram 8: Polarity Test
Tried. Tested. Trusted.
5. Earth electrode resistance
Method 1 Three pole measurement
The earth electrode resistance test is used to
confirm that the earth electrode provides a
satisfactory path to earth. The resistance
measurement is made between the earth
electrode and test spikes inserted into the
ground. The use of a four terminal test instrument
such as the Seaward ERT 1557 eliminates the
resistance of the test leads.
The E and ES terminals of a four terminal earth
resistance tester are connected to the earth
electrode, the S terminal to a potential test spike
and the H terminal to a second test spike.
15 m to 25 m
15 m to 25 m
3 m
3 m
Earth
Electrode
P2
C2
Water
Gas
Pipe
Pipe
Diagram 9: Three pole earth electrode resistance measurement
The position of the test spikes is important. For
best results the distance between the earth
1
electrode and the current spike C2 should be 10
times the maximum dimension of the earth
9
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