A 25 Vdc Motor Reads 1 Meg Ohms Between One Power Terminal and Case

Insulation Resistance (IR) Values

Introduction:

The measurement of insulation resistance is a mutual routine test performed on all types of electrical wires and cables. Equally a production exam, this examination is often used as a customer acceptance examination, with minimum insulation resistance per unit length often specified by the customer. The results obtained from IR Examination are not intended to be useful in finding localized defects in the insulation as in a true HIPOT test, but rather requite data on the quality of the majority material used as the insulation.

Even when not required past the cease customer, many wire and cable manufacturers utilise the insulation resistance test to track their insulation manufacturing processes, and spot developing bug earlier procedure variables migrate outside of allowed limits.

Selection of IR Testers (Megger):

• Insulation testers with test voltage of 500, yard, 2500 and 5000 V are available.
• The recommended ratings of the insulation testers are given below:

Voltage Level	IR Tester
650V	500V DC
1.1KV	1KV DC
three.3KV	2.5KV DC
66Kv and To a higher place	5KV DC

Test Voltage for Meggering:

• When AC Voltage is used, The Rule of Pollex is Test Voltage (A.C) = (2X Proper name Plate Voltage) +g.
• When DC Voltage is used (Most used in All Megger), Exam 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:

Exam voltage	Measurement Range
250V DC	0MΩ to 250GΩ
500V DC	0MΩ to 500GΩ
1KV DC	0MΩ to 1TΩ
2.5KV DC	0MΩ to ii.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 non connected, and ZERO when the two terminals are continued 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 non actually the case.
• Make certain that the earth used when testing for world and open circuits is a expert i otherwise the test will requite wrong information
• Spare conductors should non be meggered when other working conductors of the same cable are continued to the respective circuits.

After completion of cable Meggering:

• Ensure that all conductors take been reconnected properly.
• Exam the functions of Points, Tracks & Signals connected through the cable for their correct response.
• In case of signals, attribute should be verified personally.
• In case of points, verify positions at site. Cheque 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 equally the test voltage was applied in order to be absolutely condom for the person conducting the test.
• Never employ Megger in an explosive temper.
• Make certain all switches are blocked out and cable ends marked properly for rubber.
• Cable ends to exist isolated shall be asunder from the supply and protected from contact to supply, or ground, or accidental contact.
• Erection of safety barriers with warning signs, and an open up communication aqueduct between testing personnel.
• Do non megger when humidity is more than 70 %.
• Good Insulation: Megger reading increases first then remain constant.
• Bad Insulation: Megger reading increases get-go and then decreases.
• Expected IR value gets on Temp. 20 to thirty decree centigrade.
• If above temperature reduces by 10 degree centigrade, IR values will increased by ii times.
• If in a higher place temperature increased past 70 degree centigrade IR values decreases by 700 times.

How to use Megger:

• Meggers is equipped with 3 connection Line Terminal (L), Globe Terminal (East) and Guard Concluding (G).

• Resistance is measured betwixt the Line and Earth terminals, where electric current will travel through scroll one. The "Guard" concluding is provided for special testing situations where 1 resistance must exist isolated from another. Let's u.s.a. check i situation where the insulation resistance is to exist 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 i of the conductors and connect the "Globe" lead of the megger to a wire wrapped around the sheath of the cable.

• In this configuration the Megger should read the resistance between ane conductor and the outside sheath.
• Nosotros want to mensurate Resistance between Conductor- 2To Sheaths but Actually Megger measure resistance in parallel with the series combination of usher-to-conductor resistance (R_c1-c2) and the first conductor to the sheath (R_c1-s).
• If nosotros don't care about this fact, we can go on with the test as configured. If nosotros desire to measure out only the resistance betwixt the second conductor and the sheath (R_c2-s), then we demand to use the megger's "Baby-sit" terminal.

• Connecting the "Guard" terminal to the first conductor places the ii conductors at virtually equal potential. With little or no voltage between them, the insulation resistance is nigh space, and thus there will be no current between the two conductors. Consequently, the Megger's resistance indication volition be based exclusively on the current through the second conductor's insulation, through the cable sheath, and to the wire wrapped around, non 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 y'all to be selective in evaluating certain specific components in a large slice of electrical equipment. For example consider a two core cable with a sheath. As the diagram below shows in that location are iii resistances to be considered.

• If we mensurate between core B and sheath without a connexion to the guard final some electric current volition pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard concluding to A the 2 cablevision cores will be at very virtually the aforementioned potential and thus the shunting effect is eliminated.

(one) IR Values For Electric Apparatus & Systems :

(PEARL Standard / NETA MTS-1997 Table 10.i)

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	xv,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 g V practical between each live usher and earth for a flow of one minute the insulation resistance of HV installations shall be at least 1 Mega ohm or every bit specified by the Agency of Indian Standards.
• Medium and Low Voltage Installations- At a force per unit area of 500 V applied between each alive conductor and earth for a flow of one minute, the insulation resistance of medium and low voltage installations shall be at least ane Mega ohm or every bit specified by the Agency of Indian Standards] from time to time.

As per CBIP specifications the acceptable values are two 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 straight in megohms or may exist calculated from measurements of practical voltage and leakage current.
• The recommended practice in measuring insulation resistance is to always basis 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 basis removed in order to measure the insulation resistance of the winding grounded. If the footing cannot exist 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 department of the circuit.
• We demand to test winding to winding and winding to ground ( Eastward ).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. South.D Myer)
Transformer	Formula
i Phase Transformer	IR Value (MΩ) = C X E / (√KVA)
3 Stage Transformer (Star)	IR Value (MΩ) = C Ten Due east (P-north) / (√KVA)
3 Stage Transformer (Delta)	IR Value (MΩ) = C X E (P-P) / (√KVA)
Where C= one.v 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 of operations 20°C):

Temperature correction Factor
OC	OF	Correction Factor
0	32	0.25
5	41	0.36
10	l	0.50
xv	59	0.720
20	68	1.00
30	86	1.98
forty	104	3.95
50	122	seven.85

• Example: For 1600KVA, 20KV/400V,Iii Stage Transformer
• IR Value at HV Side= (ane.5 ten 20000) / √ 1600 =16000 / 40 = 750 MΩ at 20⁰C
• IR Value at LV Side = (ane.5 x 400 ) / √ 1600= 320 / 40 = 15 MΩ at xx⁰C
• IR Value at 30⁰C =15X1.98= 29.7 MΩ

Insulation Resistance of Transformer Gyre

Transformer Curl  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 Five To 5KV	2.5KV	1,000 MΩ	five,000 MΩ
5KV To 15KV	5KV	5,000 MΩ	25,000 MΩ
15KV To 69KV	5KV	ten,000 MΩ	50,000 MΩ

 IR Value of Transformers:

Voltage	Test Voltage (DC)  LV side	Examination  Voltage (DC) HV side	Min IR Value
415V	500V	two.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 excursion the windings.
• Guard the terminals to eliminate surface leakage over terminal bushings.
• Tape the temperature.
• Connect the test leads (avert joints).
• Apply the test voltage and note the reading. The IR. Value at threescore 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 exam.
• 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 nether vacuum.

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

• Two winding transformer:

1. (HV + LV) – GND
2. HV – (LV + GND)
3. LV – (HV + GND)

• Three winding transformer:

1. HV – (LV + Tv set + GND)
2. LV – (HV + TV + GND)
3. (HV + LV + TV) – GND
4. TV – (HV + LV + GND)

• Motorcar transformer (two winding):

1. (HV + LV) – GND

• Machine Transformer (three winding):

1. (HV + LV) – (TV + GND)
2. (HV + LV + TV) – GND
3. TV – (HV + LV + GND)

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

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

 Factors affecting on IR value of Transformer

The IR value of transformers are influenced by

• surface status of the last bushing
• quality of oil
• quality of winding insulation
• temperature of oil
• duration of application and value of test voltage

(three) IR Value for Tap Changer:

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

 (4) IR Value for Electrical motor:

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

• For rated voltage below 1KV, measured with a 500VDC Megger.
• For rated voltage in a higher place 1KV, measured with a 1000VDC Megger.
• In accord with IEEE 43, clause nine.3, the following formula should be practical.
• Min IR Value (For Rotating Machine) =(Rated voltage (v) /g) + 1

As per IEEE 43 Standard 1974,2000
IR Value in MΩ
IR (Min) = kV+i	For most windings fabricated before about 1970, all field windings, and others non 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

• Instance-1: For 11KV, Three Phase Motor.
• IR Value =11+1=12 MΩ but every bit per IEEE43 It should be 100 MΩ
• Example-2: For 415V,Iii 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/(one thousand+2XKW))

IR Value of Motor every bit per NETA ATS 2007. Department 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 Cablevision)	IR Value
New Motor	20 MΩ
A used motor which tin be reinstalled	10 MΩ
Motor  Installed in Well (With Cablevision)
New Motor	2 MΩ
A used motor which can be reinstalled	0.five MΩ

(v) IR Value for Electrical cable and wiring:

• For insulation testing, we need to disconnect from panel or equipment and go on them isolated from power supply. The wiring and cables demand 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 = 1000 x Log x (D/d)

• R =IR Value in MΩs per one thousand feet (305 meters) of cablevision.
• M =Insulation material constant.( Varnished Cambric=2460, Thermoplastic Polyethlene=50000,Composite Polyethylene=30000)
  D =Outside diameter of usher insulation for single conductor wire and cable
• ( D = d + 2c + 2b bore 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
Afterward repairs – Sheath	1KV DC	x MΩ
Later repairs – Insulation	5KV DC	1000MΩ

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

Application	Examination Voltage	Min IR Value
11KV New cables – Sheath	5KV DC	grand MΩ
11KV Later on repairs – Sheath	5KV DC	100 MΩ
33KV no TF's connected	5KV DC	1000 MΩ
33KV with TF'south connected.	5KV DC	15MΩ

IR Value Measurement (Conductors to conductor (Cross Insulation))

• The first conductor for which cantankerous insulation is being measured shall be continued to Line last of the megger. The remaining conductors looped together (with the assistance of crocodile clips) i. e. Conductor two and onwards, are connected to Earth terminal of megger. Conductors at the other terminate are left free.
• Now rotate the handle of megger or press push button of megger. The reading of meter will prove the cross Insulation between conductor ane and residue of the conductors. Insulation reading shall be recorded.
• At present connect next usher to Line terminal of the megger & connect the remaining conductors to earth terminal of the megger and have measurements.

IR Value Measurement ( Conductor to Globe Insulation)

• Connect conductor under exam to the Line terminal of the megger.
• Connect globe concluding of the megger to the earth.
• Rotate the handle of megger or printing push button of megger. The reading of meter volition show the insulation resistance of the conductors. Insulation reading shall be recorded after applying the exam voltage for virtually a infinitesimal till a steady reading is obtained.

 IR Value Measurements:

• If during periodical testing, insulation resistance of cable is plant between five and 1 MΩ /km at buried temperature, the subject field cable should be programmed for replacement.
• If insulation resistance of the cable is found betwixt 1000 and 100 KΩ /km, at cached temperature, the subject field cablevision should be replaced urgently within a twelvemonth.
• If the insulation resistance of the cable is institute less than 100 kilo ohm/km., the bailiwick cablevision must be replaced immediately on emergency basis.

 (half-dozen) IR Value for Transmission / Distribution Line:

Equipment.	Megger Size	Min IR Value
S/Due south .Equipments	v KV	5000MΩ
EHVLines.	5 KV	10MΩ
H.T. Lines.	ane KV	5MΩ
LT / Service Lines.	0.five KV	5MΩ

(7) IR Value for Panel Bus:

• IR Value for Panel = 2 x KV rating of the console.
• Instance, for a v KV console, the minimum insulation is 2 x 5 = 10 MΩ.

 (8) IR Value for Substation Equipment:

By and large meggering Values of Substation Equipments are.

.Typical IR Value of S/Due south Equipments
Equipment		Megger Size	IR Value(Min)
Circuit Breaker	(Phase-Earth)	5KV,10 KV	1000 MΩ
	(Phase-Phase)	5KV,10 KV	one thousand MΩ
	Control Circuit	0.5KV	50 MΩ
CT/PT	(Pri-Earth)	5KV,10 KV	1000 MΩ
	(Sec-Stage)	5KV,10 KV	50 MΩ
	Control Excursion	0.5KV	l MΩ
Isolator	(Phase-Earth)	5KV,10 KV	chiliad MΩ
	(Phase-Phase)	5KV,10 KV	k MΩ
	Control Excursion	0.5KV	50 MΩ
Fifty.A	(Stage-Earth)	5KV,10 KV	1000 MΩ
Electrical Motor	(Phase-Globe)	0.5KV	l MΩ
LT Switchgear	(Phase-Earth)	0.5KV	100 MΩ
LT Transformer	(Stage-Globe)	0.5KV	100 MΩ

IR Value of Southward/S Equipments As per DEP Standard
Equipment	Meggering	IR Value at Commissioning Fourth dimension ( M Ω)	IR Value at Maintenance Fourth dimension( Yard Ω)
Switchgear	HV Charabanc	200 MΩ	100 MΩ
	LV Bus	xx MΩ	10 MΩ
	LV wiring	5 MΩ	0.v MΩ
Cable(min 100 Meter)	HV & LV	(10XKV) / KM	(KV) / KM
Motor & Generator	Phase-World	10(KV+1)	2(KV+1)
Transformer Oil immersed	HV & LV	75 MΩ	30 MΩ
Transformer Dry out Type	HV	100 MΩ	25 MΩ
	LV	10 MΩ	2 MΩ
Fixed Equipments/Tools	Phase-World	5KΩ / Volt	1KΩ / Volt
Movable Equipments	Phase-Earth	five MΩ	1MΩ
Distribution Equipments	Phase-Earth	5 MΩ	1MΩ
Circuit Breaker	Principal Circuit	ii MΩ / KV
	Command Excursion	5MΩ
Relay	D.C Circuit-World	40MΩ
	LT Circuit-Earth	50MΩ
	LT-D.C Circuit	40MΩ
	LT-LT	70MΩ

 (ix) IR Value for Domestic /Industrial Wiring:

• A low resistance betwixt stage and neutral conductors, or from live conductors to earth, will issue in a leakage current. This cause deterioration of the insulation, every bit well as involving a waste material of energy which would increase the running costs of the installation.
• The resistance betwixt Phase-Phase-Neutral-Earth must never be less than 0.5 M Ohms for the usual supply voltages.
• In add-on to the leakage current due to insulation resistance, in that location is a farther current leakage in the reactance of the insulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is non harmful, but nosotros wish to mensurate the resistance of the insulation, and then DC Voltage is used to prevent reactance from being included in the measurement.

 i Phase Wiring:

• The IR test betwixt Phase-Natural to world must be carried out on the consummate installation with the main switch off, with phase and neutral continued together, with lamps and other equipment disconnected, merely with fuses in, circuit breakers airtight and all excursion switches closed.
• Where ii-manner switching is wired, only one of the two stripper wires will exist tested. To test the other, both 2-way switches should exist operated and the organization 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 Stage Wiring:

• In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to exist lower. If this happens, the installation should be subdivided and retested, when each part must run into the minimum requirement.

• The IR tests must be carried out betwixt Phase-Stage-Neutral-Globe with a minimum acceptable value for each test of 0.five 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 Five DC	0.5MΩ
500 V To 1KV	1000 V DC	1.0MΩ

• Min IR Value = 50 M Ω / No of Electric outlet. (All Electric Points with  fitting & Plugs).

• Min IR Value = 100 1000 Ω / No of Electric 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 exist damaged by the awarding of the loftier test voltage should be disconnected.
• Capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will effect.
• Where whatever equipment is asunder 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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Source: https://electricalnotes.wordpress.com/2012/03/23/insulation-resistance-ir-values/

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