Power Engineering: Insulation Resistance

(IR) Insulation Resistance

Introduction:

The insulation resistance is defined as the resistance (in mega-ohm) offered by the insulation to an impressed direct voltage. The resulting current is called insulation current and consists of two main component-

(a)The current which flows within the volume of insulation and is composed of capacitance current dielectric absorption current and irreversible conduction current.

(b)the current which flows in leakage paths over the surface of insulation , is termed as leakage current

Selection of IR Testers (Megger):

Voltage Level	IR Tester
650V	500V DC
1.1KV	1KV DC
3.3KV	2.5KV DC
66Kv and Above	5KV DC

Test Voltage for Meggering:

When AC Voltage is used, The Rule of Thumb is Test Voltage (A.C) = (2X Name Plate Voltage) +1000.
When DC Voltage is used (Most used in All Megger), Test Voltage (D.C) = (2X Name Plate Voltage).

Equipment / Cable Rating	DC Test Voltage
24V To 50V	50V To 100V
50V To 100V	100V To 250V
100V To 240V	250V To 500V
440V To 550V	500V To 1000V
2400V	1000V To 2500V
4100V	1000V To 5000V

Measurement Range of Megger:

Test voltage	Measurement Range
250V DC	0MΩ to 250GΩ
500V DC	0MΩ to 500GΩ
1KV DC	0MΩ to 1TΩ
2.5KV DC	0MΩ to 2.5TΩ
5KV DC	0MΩ to 5TΩ

Precaution while Meggering:

Before Meggering:

Make sure that all connections in the test circuit are tight.
Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminals are connected together and the handle is rotated.

During Meggering:

Make sure when testing for earth, that the far end of the conductor is not touching, otherwise the test will show faulty insulation when such is not actually the case.
Make sure that the earth used when testing for earth and open circuits is a good one otherwise the test will give wrong information
Spare conductors should not be meggered when other working conductors of the same cable are connected to the respective circuits.

After completion of cable Meggering:

Ensure that all conductors have been reconnected properly.
Test the functions of Points, Tracks & Signals connected through the cable for their correct response.
In case of signals, aspect should be verified personally.
In case of points, verify positions at site. Check whether any polarity of any feed taken through the cable has got earthed inadvertently.

Safety Requirements for Meggering:

All equipment under test MUST be disconnected and isolated.
Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.
Never use Megger in an explosive atmosphere.
Make sure all switches are blocked out and cable ends marked properly for safety.
Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or accidental contact.
Erection of safety barriers with warning signs, and an open communication channel between testing personnel.
Do not megger when humidity is more than 70 %.
Good Insulation: Megger reading increases first then remain constant.
Bad Insulation: Megger reading increases first and then decreases.
Expected IR value gets on temperature 20 to 30 decree centigrade.
If above temperature reduces by 10 degree centigrade, IR values will increased by two times.
If above temperature increased by 70 degree centigrade IR values decreases by 700 times.

How to use Megger:

Megger is equipped with three connections Line Terminal (L), Earth Terminal (E) and Guard Terminal (G).

Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another. Let’s us check one situation where the insulation resistance is to be tested in a two-wire cable.

To measure insulation resistance from a conductor to the outside of the cable, we need to connect the “Line” lead of the megger to one of the conductors and connect the “Earth” lead of the megger to a wire wrapped around the sheath of the cable.

In this configuration the Megger should read the resistance between one conductor and the outside sheath.
We want to measure Resistance between Conductor- 2To Sheaths but Actually Megger measure resistance in parallel with the series combination of conductor-to-conductor resistance (R_c1-c2) and the first conductor to the sheath (R_c1-s).
If we don’t care about this fact, we can proceed with the test as configured. If we desire to measure only the resistance between the second conductor and the sheath (R_c2-s), then we need to use the megger’s “Guard” terminal.

Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential. With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the Megger’s resistance indication will be based exclusively on the current through the second conductor’s insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor’s insulation.
The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath. As the diagram below shows there are three resistances to be considered.

If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cable cores will be at very nearly the same potential and thus the shunting effect is eliminated.

(1) IR Values for Electrical Apparatus & Systems:

(PEARL Standard / NETA MTS-1997 Table 10.1)

Max.Voltage Rating Of Equipment	Megger Size	Min.IR Value
250 Volts	500 Volts	25 MΩ
600 Volts	1,000 Volts	100 MΩ
5 KV	2,500 Volts	1,000 MΩ
8 KV	2,500 Volts	2,000 MΩ
15 KV	2,500 Volts	5,000 MΩ
25 KV	5,000 Volts	20,000 MΩ
35 KV	15,000 Volts	100,000 MΩ
46 KV	15,000 Volts	100,000 MΩ
69 KV	15,000 Volts	100,000 MΩ

One Meg ohm Rule for IR Value for Equipment:

Based upon equipment rating:
< 1K V = 1 MΩ minimum
>1KV = 1 MΩ /1KV

As per IE Rules-1956:

At a pressure of 1000 V applied between each live conductor and earth for a period of one minute the insulation resistance of HV installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards.
Medium and Low Voltage Installations- At a pressure of 500 V applied between each live conductor and earth for a period of one minute, the insulation resistance of medium and low voltage installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards] from time to time.

As per CBIP specifications the acceptable values are 2 Mega ohms per KV

(2) IR Value for Transformer:

Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual windings. Insulation resistance tests are commonly measured directly in megohms or may be calculated from measurements of applied voltage and leakage current.

The recommended practice in measuring insulation resistance is to always ground the tank (and the core). Short circuit each winding of the transformer at the bushing terminals. Resistance measurements are then made between each winding and all other windings grounded.

Windings are never left floating for insulation resistance measurements. Solidly grounded winding must have the ground removed in order to measure the insulation resistance of the winding grounded. If the ground cannot be removed, as in the case of some windings with solidly grounded neutrals, the insulation resistance of the winding cannot be measured. Treat it as part of the grounded section of the circuit.
We need to test winding to winding and winding to ground ( E ).For three phase transformers, We need to test winding ( L1,L2,L3 ) with substitute Earthing for Delta transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N ) for wye transformers.

IR Value for Transformer (Ref: A Guide to Transformer Maintenance by. JJ. Kelly. S.D Myer)
Transformer	Formula
1 Phase Transformer	IR Value (MΩ) = C X E / (√KVA)
3 Phase Transformer (Star)	IR Value (MΩ) = C X E (P-n) / (√KVA)
3 Phase Transformer (Delta)	IR Value (MΩ) = C X E (P-P) / (√KVA)
Where C= 1.5 for Oil filled T/C with Oil Tank, 30 for Oil filled T/C without Oil Tank or Dry Type T/C.

Temperature correction Factor (Base 20°C):

Temperature correction Factor
^OC	^OF	Correction Factor
0	32	0.25
5	41	0.36
10	50	0.50
15	59	0.720
20	68	1.00
30	86	1.98
40	104	3.95
50	122	7.85

Example: For 1600KVA, 20KV/400V,Three Phase Transformer
IR Value at HV Side= (1.5 x 20000) / √ 1600 =16000 / 40 = 750 MΩ at 20⁰C
IR Value at LV Side = (1.5 x 400 ) / √ 1600= 320 / 40 = 15 MΩ at 20⁰C
IR Value at 30⁰C =15X1.98= 29.7 MΩ

Insulation Resistance of Transformer Coil

Transformer Coil Voltage	Megger Size	Min.IR Value Liquid Filled T/C	Min.IR Value Dry Type T/C
0 – 600 V	1KV	100 MΩ	500 MΩ
600 V To 5KV	2.5KV	1,000 MΩ	5,000 MΩ
5KV To 15KV	5KV	5,000 MΩ	25,000 MΩ
15KV To 69KV	5KV	10,000 MΩ	50,000 MΩ

IR Value of Transformers:

Voltage	Test Voltage (DC) LV side	Test Voltage (DC) HV side	Min IR Value
415V	500V	2.5KV	100MΩ
Up to 6.6KV	500V	2.5KV	200MΩ
6.6KV to 11KV	500V	2.5KV	400MΩ
11KV to 33KV	1000V	5KV	500MΩ
33KV to 66KV	1000V	5KV	600MΩ
66KV to 132KV	1000V	5KV	600MΩ
132KV to 220KV	1000V	5KV	650MΩ

Steps for measuring the IR of Transformer:

Shut down the transformer and disconnect the jumpers and lightning arrestors.
Discharge the winding capacitance.
Thoroughly clean all bushings
Short circuit the windings.
Guard the terminals to eliminate surface leakage over terminal bushings.
Record the temperature.
Connect the test leads (avoid joints).
Apply the test voltage and note the reading. The IR. Value at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature.
The transformer Neutral bushing is to be disconnected from earth during the test.
All LV surge diverter earth connections are to be disconnected during the test.
Due to the inductive characteristics of transformers, the insulation resistance reading shall not be taken until the test current stabilizes.
Avoid meggering when the transformer is under vacuum.

Test Connections of Transformer for IR Test (Not Less than 200 MΩ):

Two winding transformer:

(HV + LV) – GND
HV – (LV + GND)
LV – (HV + GND)

Three winding transformer:

HV – (LV + TV + GND)
LV – (HV + TV + GND)
(HV + LV + TV) – GND
TV – (HV + LV + GND)

Auto transformer (two winding):

(HV + LV) – GND

Auto Transformer (three winding):

(HV + LV) – (TV + GND)
(HV + LV + TV) – GND
TV – (HV + LV + GND)

For any installation, the insulation resistance measured shall not be less than:

HV – Earth 200 M Ω
LV – Earth 100 M Ω
HV – LV 200 M Ω

Factors affecting on IR value of Transformer

The IR value of transformers are influenced by

surface condition of the terminal bushing
quality of oil
quality of winding insulation
temperature of oil
duration of application and value of test voltage

(3) IR Value for Tap Changer:

IR between HV and LV as well as windings to earth.
Minimum IR value for Tap changer is 1000 ohm per volt service voltage

(4) IR Value for Electric motor:

For electric motor, we used a insulation tester to measure the resistance of motor winding with earthing ( E ).

For rated voltage below 1KV, measured with a 500VDC Megger.
For rated voltage above 1KV, measured with a 1000VDC Megger.
In accordance with IEEE 43, clause 9.3, the following formula should be applied.
Min IR Value (For Rotating Machine) =(Rated voltage (v) /1000) + 1

As per IEEE 43 Standard 1974,2000
IR Value in MΩ
IR (Min) = kV+1	For most windings made before about 1970, all field windings, and others not described below
IR (Min) = 100 MΩ	For most dc armature and ac windings built after about 1970 (form wound coils)
IR (Min) = 5 MΩ	For most machines with random -wound stator coils and form-wound coils rated below 1kV

Example-1: For 11KV, Three Phase Motor.
IR Value =11+1=12 MΩ but as per IEEE43 It should be 100 MΩ
Example-2: For 415V,Three Phase Motor
IR Value =0.415+1=1.41 MΩ but as per IEEE43 It should be 5 MΩ.
As per IS 732 Min IR Value of Motor=(20XVoltage(p-p/(1000+2XKW))

IR Value of Motor as per NETA ATS 2007. Section 7.15.1

Motor Name Plate (V)	Test Voltage	Min IR Value
250V	500V DC	25 MΩ
600V	1000V DC	100MΩ
1000V	1000V DC	100MΩ
2500V	1000V DC	500MΩ
5000V	2500V DC	1000MΩ
8000V	2500V DC	2000MΩ
15000V	2500V DC	5000MΩ
25000V	5000V DC	20000MΩ
34500V	15000V DC	100000MΩ

IR Value of Submersible Motor:

IR Value of Submersible Motor
Motor Out off Well (Without Cable)	IR Value
New Motor	20 MΩ
A used motor which can be reinstalled	10 MΩ
Motor Installed in Well (With Cable)
New Motor	2 MΩ
A used motor which can be reinstalled	0.5 MΩ

(5) IR Value for Electrical cable and wiring:

For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply. The wiring and cables need to test for each other ( phase to phase ) with a ground ( E ) cable. The Insulated Power Cable Engineers Association (IPCEA) provides the formula to determine minimum insulation resistance values.
R = K x Log 10 (D/d)
R =IR Value in MΩs per 1000 feet (305 meters) of cable.
K =Insulation material constant.( Varnished Cambric=2460, Thermoplastic Polyethlene=50000,Composite Polyethylene=30000)
D =Outside diameter of conductor insulation for single conductor wire and cable
( D = d + 2c + 2b diameter of single conductor cable )
d – Diameter of conductor
c – Thickness of conductor insulation
b – Thickness of jacket insulation

HV test on new XLPE cable (As per ETSA Standard)

Application	Test Voltage	Min IR Value
New cables – Sheath	1KV DC	100 MΩ
New cables – Insulation	10KV DC	1000 MΩ
After repairs – Sheath	1KV DC	10 MΩ
After repairs – Insulation	5KV DC	1000MΩ

11kV and 33kV Cables between Cores and Earth (As per ETSA Standard)

Application	Test Voltage	Min IR Value
11KV New cables – Sheath	5KV DC	1000 MΩ
11KV After repairs – Sheath	5KV DC	100 MΩ
33KV no TF’s connected	5KV DC	1000 MΩ
33KV with TF’s connected.	5KV DC	15MΩ

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.

(6) IR Value for Transmission / Distribution Line:

Equipment.	Megger Size	Min IR Value
S/S .Equipments	5 KV	5000MΩ
EHVLines.	5 KV	10MΩ
H.T. Lines.	1 KV	5MΩ
LT / Service Lines.	0.5 KV	5MΩ

(7) IR Value for Panel Bus:

IR Value for Panel = 2 x KV rating of the panel.
Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 MΩ.

(8) IR Value for Substation Equipment:

Generally Meggering Values of Substation Equipments are.

.Typical IR Value of S/S Equipments
Equipment		Megger Size	IR Value(Min)
Circuit Breaker	(Phase-Earth)	5KV,10 KV	1000 MΩ
	(Phase-Phase)	5KV,10 KV	1000 MΩ
	Control Circuit	0.5KV	50 MΩ
CT/PT	(Pri-Earth)	5KV,10 KV	1000 MΩ
	(Sec-Phase)	5KV,10 KV	50 MΩ
	Control Circuit	0.5KV	50 MΩ
Isolator	(Phase-Earth)	5KV,10 KV	1000 MΩ
	(Phase-Phase)	5KV,10 KV	1000 MΩ
	Control Circuit	0.5KV	50 MΩ
L.A	(Phase-Earth)	5KV,10 KV	1000 MΩ
Electrical Motor	(Phase-Earth)	0.5KV	50 MΩ
LT Switchgear	(Phase-Earth)	0.5KV	100 MΩ
LT Transformer	(Phase-Earth)	0.5KV	100 MΩ

IR Value of S/S Equipments As per DEP Standard
Equipment	Meggering	IR Value at Commissioning Time (MΩ)	IR Value at Maintenance Time(MΩ)
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
Transformer Dry Type	LV	10 MΩ	2 MΩ
Fixed Equipments/Tools	Phase-Earth	5KΩ / Volt	1KΩ / Volt
Movable Equipments	Phase-Earth	5 MΩ	1MΩ
Distribution Equipments	Phase-Earth	5 MΩ	1MΩ
Circuit Breaker	Main Circuit	2 MΩ / KV
Circuit Breaker	Control Circuit	5MΩ
Relay	D.C Circuit-Earth	40MΩ
	LT Circuit-Earth	50MΩ
	LT-D.C Circuit	40MΩ
	LT-LT	70MΩ

(9) 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.

1 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.

3 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.

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.