11kV and 33kV Cables between Cores and Earth
IR Value Measurement (Conductors to conductor (Cross
Insulation))
·
The
first conductor for which cross insulation is being measured shall be connected
to Line terminal of the megger. The remaining conductors looped together (with
the help of crocodile clips) i. e. Conductor 2 and onwards, are connected to
Earth terminal of megger. Conductors at the other end are left free.
·
Now
rotate the handle of megger or press push button of megger. The reading of
meter will show the cross Insulation between conductor 1 and rest of the
conductors. Insulation reading shall be recorded.
·
Now
connect next conductor to Line terminal of the megger & connect the
remaining conductors to earth terminal of the megger and take measurements.
IR Value Measurement (Conductor to Earth Insulation)
·
Connect
conductor under test to the Line terminal of the megger.
·
Connect
earth terminal of the megger to the earth.
·
Rotate
the handle of megger or press push button of megger. The reading of meter will
show the insulation resistance of the conductors. Insulation reading shall be
recorded after applying the test voltage for about a minute till a steady
reading is obtained.
IR Value Measurements:
·
If
during periodical testing, insulation resistance of cable is found
between 5 and 1 MΩ /km at buried temperature, the subject
cable should be programmed for replacement.
·
If
insulation resistance of the cable is found between 1000 and
100 KΩ /km, at buried temperature, the subject cable should be
replaced urgently within a year.
·
If
the insulation resistance of the cable is found less than 100 kilo ohm/km., the
subject cable must be replaced immediately on emergency basis.
1.
IR Value for Transmission /
Distribution Line
Equipment |
Megger Size |
Min.
IR Value |
S/S Equipment’s |
5
KV DC |
1000
MΩ |
11kV After repairs – Sheath |
5
KV DC |
100
MΩ |
33kV no TF’s connected |
5
KV DC |
1000
MΩ |
33kV with TF’s connected |
5
KV DC |
15 MΩ |
IR Value for Panel Bus = 2 x
KV rating of the Panel
Example: for
a 5 KV panel, the minimum insulation is 2 x 5 = 10 MΩ
Generally meggering Values of substation Equipment’s are.
Typical
IR Value of S/S Equipment’s |
|||
Equipment |
|
Megger
size |
IR
Value (Min) |
Circuit Breaker |
(Phase – Earth) |
5 KV, 10 KV |
1000 MΩ |
(Phase – Phase) |
5 KV, 10 KV |
1000 MΩ |
|
Control Circuit |
0.5 KV |
50 MΩ |
|
CT/PT |
(Pri – Earth) |
5 KV, 10 KV |
1000MΩ |
(Sec – Phase) |
5 KV, 10 KV |
50 MΩ |
|
Control Circuit |
0.5 KV |
50 MΩ |
|
Isolator |
(Phase – Earth) |
5 KV, 10 KV |
1000 MΩ |
(Phase – Phase) |
5 KV, 10 KV |
1000 MΩ |
|
Control Circuit |
0.5 KV |
50 MΩ |
|
L.A |
(Phase – Earth) |
5 KV, 10 KV |
1000 MΩ |
Electrical Motor |
(Phase – Earth) |
0.5 KV |
50 MΩ |
LT Switchgear |
(Phase – Earth) |
0.5 KV |
100 MΩ |
LT Transformer |
(Phase – Earth) |
0.5 KV |
100 MΩ |
IR
Value of S/S Equipment’s As per DEP Standard |
|||
Equipment |
Meggering |
IR
Value at Commissioning Time (MΩ) |
IR
Value at Maintenance Time |
Switchgear |
HV Bus |
200
MΩ |
100
MΩ |
LV Bus |
20
MΩ |
10
MΩ |
|
LV Wiring |
5
MΩ |
0.5
MΩ |
|
Cable (min 100 Meter) |
HV & LV |
(10XKV)
/ KM |
(KV)
/ KM |
Motor & Generator |
Phase – Earth |
10
(KV+1) |
2
(KV+1) |
Transformer Oil Immersed |
HV + LV |
75
MΩ |
30
MΩ |
Transformer Dry Type |
HV |
100
MΩ |
25
MΩ |
LV |
10
Ω |
2
MΩ |
|
Fixed Equipment’s /
Tools |
Phase – Earth
|
5
KΩ / Volt |
1
KΩ / Volt |
Movable Equipment’s |
Phase – Earth
|
5
MΩ |
1
MΩ |
Distribution Equipment’s |
Phase – Earth
|
5
MΩ |
1
MΩ |
Circuit Breaker |
Main Circuit |
2
MΩ / KV |
- |
Control Circuit |
5
MΩ |
- |
|
Relay
|
D.C Circuit - Earth |
40
MΩ |
- |
LT Circuit -
Earth |
50
MΩ |
- |
|
LT – D.C Circuit |
40
MΩ |
- |
|
LT – LT |
70 MΩ |
- |
A
low resistance between phase and neutral conductors, or from live conductors to
earth, will result in a leakage current. This cause deterioration of the
insulation, as well as involving a waste of energy which would increase the
running costs of the installation.
The
resistance between Phase-Phase-Neutral-Earth must never be less than
0.5 M Ohms for the usual supply voltages.
In
addition to the leakage current due to insulation resistance, there is a
further current leakage in the reactance of the insulation, because it acts as
the dielectric of a capacitor. This current dissipates no energy and is not
harmful, but we wish to measure the resistance of the insulation, so DC
Voltage is used to prevent reactance from being included in the measurement.
Single Phase Wiring
>The
IR test between Phase-Natural to earth must be carried out on the complete
installation with the main switch off, with phase and neutral connected
together, with lamps and other equipment disconnected, but with fuses in,
circuit breakers closed and all circuit switches closed.
Where
two-way switching is wired, only one of the two stripper wires will be tested.
To test the other, both two-way switches should be operated and the system
retested. If desired, the installation can be tested as a whole, when a value
of at least 0.5 M Ohms should be achieved.
Three Phase Wiring
In
the case of a very large installation where there are many earth paths in
parallel, the reading would be expected to be lower. If this happens, the
installation should be subdivided and retested, when each part must meet the
minimum requirement.
Three Phase Wiring
The
IR tests must be carried out between Phase-Phase-Neutral-Earth with a minimum
acceptable value for each test of 0.5 M Ohms.
IR Testing for Low voltage |
||
Circuit voltage |
Test voltage |
IR Value (Min) |
Extra Low Voltage |
250V DC |
0.25MΩ |
Up to 500 V except for
above |
500 V DC |
0.5MΩ |
500 V To 1KV |
1000 V DC |
1.0MΩ |
Min
IR Value = 50 MΩ / No of Electrical outlet. (All
Electrical Points with fitting & Plugs)
Min IR Value = 100 MΩ / No of Electrical outlet. (All Electrical
Points without fitting & Plugs).
Required Precautions
Electronic
equipment like electronic fluorescent starter switches, touch switches, dimmer
switches, power controllers, delay timers could be damaged by the application
of the high-test voltage should be disconnected.
Capacitors
and indicator or pilot lamps must be disconnected or an inaccurate test reading
will result.
Where
any equipment is disconnected for testing purposes, it must be subjected to its
own insulation test, using a voltage which is not likely to result in damage.
The result must conform with that specified in the British Standard concerned,
or be at least 0.5 M Ohms if there is no Standard
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