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 

 

 

2.   IR Value for Panel bus

 

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

 

3.   IR Value for substation equipment

 

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

-

 

 

4.   IR Value for domestic / industrial wiring

 

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.

 



                                                        Single Phase Wiring



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