Insulation Resistance (IR) Values


The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. As a production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often specified by the customer. The results obtained from IR Test are not intended to be useful in finding localized defects in the insulation as in a true HIPOT test, but rather give information on the quality of the bulk material used as the insulation.

Even when not required by the end customer, many wire and cable manufacturers use the insulation resistance test to track their insulation manufacturing processes, and spot developing problems before process variables drift outside of allowed limits.

 Selection of IR Testers (Megger):

  • Insulation testers with test voltage of 500, 1000, 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
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 Temp. 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:

  • Meggers is equipped with three connection 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 (Rc1-c2) and the first conductor to the sheath (Rc1-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 (Rc2-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



Correction Factor

























  • 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


Coil  Voltage

Megger Size


Min.IR Value Liquid Filled T/C


Min.IR Value Dry Type T/C

0 – 600 V


100 MΩ

500 MΩ

600 V To 5KV


1,000 MΩ

5,000 MΩ

5KV To 15KV


5,000 MΩ

25,000 MΩ

15KV To 69KV


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:
  1. (HV + LV) – GND
  2. HV – (LV + GND)
  3. LV – (HV + GND)
  • Three winding transformer:
  1. HV – (LV + TV + GND)
  2. LV – (HV + TV + GND)
  3. (HV + LV + TV) – GND
  4. TV – (HV + LV + GND)
  • Auto transformer (two winding):
  1. (HV + LV) – GND
  • Auto 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 – 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 /km at buried temperature, the subject cable should be programmed for replacement.
  • If insulation resistance of the cable is found between 1000 and 100 /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


5KV,10 KV

1000 MΩ


5KV,10 KV

1000 MΩ

Control Circuit


50 MΩ



5KV,10 KV

1000 MΩ


5KV,10 KV

50 MΩ

Control Circuit


50 MΩ



5KV,10 KV

1000 MΩ


5KV,10 KV

1000 MΩ

Control Circuit


50 MΩ



5KV,10 KV

1000 MΩ

Electrical Motor



50 MΩ

LT Switchgear



100 MΩ

LT Transformer



100 MΩ

IR Value of S/S Equipments As per DEP Standard



IR Value at Commissioning Time (MΩ)

IR Value at Maintenance Time(MΩ)


HV Bus

200 MΩ

100 MΩ

LV Bus

20 MΩ

10 MΩ

LV wiring

5 MΩ

0.5 MΩ

Cable(min 100 Meter)


(10XKV) / KM

(KV) / KM

Motor & Generator




Transformer Oil immersed


75 MΩ

30 MΩ

Transformer Dry Type


100 MΩ

25 MΩ


10 MΩ

2 MΩ

Fixed Equipments/Tools


5KΩ / Volt

1KΩ / Volt

Movable Equipments


5 MΩ


Distribution Equipments


5 MΩ


Circuit Breaker

Main Circuit

2 MΩ / KV


Control Circuit




D.C Circuit-Earth



LT Circuit-Earth



LT-D.C Circuit






 (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.

Star-Delta Starter


Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. Two methods are used for reduction of starting voltage are star delta starting and auto transformer stating.


Working Principal of Star-Delta Starter:

  • This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs normally.
  • Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.
  • Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltage starters that can be applied.
  • The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.
  • There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.

Star-delta Starter Consists following units:

1)     Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).

2)     Time relay (pull-in delayed) 1 No.

3)     Three-pole thermal over current release 1No.

4)      Fuse elements or automatic cut-outs for the main circuit 3 Nos.

5)     Fuse element or automatic cut-out for the control circuit 1No.

Power Circuit of Star Delta Starter:

  • The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.
  • The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1, V1, W1.
  • In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period of time, the star contactor is opened, and then the delta contactor (KM2) is closed. The control of the contactors is by the timer (K1T) built into the starter. The Star and Delta are electrically interlocked and preferably mechanically interlocked as well. In effect, there are four states:


  • The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup.
  • Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of a star delta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and a timer.

Control Circuit of Star-Delta Starter (Open Transition):


  • The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit.
  • When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.
  • When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.
  • After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch.
  • When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO.
  • Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.
  • A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active.
  • The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective device which automatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay.
  • At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.

What is Open or Closed Transition Starting

(1)   Open Transition Starters.

  • Discuss mention above is called open transition switching because there is an open state between the star state and the delta state.
  • In open transition the power is disconnected from the motor while the winding are reconfigured via external switching.
  • When a motor is driven by the supply, either at full speed or at part speed, there is a rotating magnetic field in the stator. This field is rotating at line frequency. The flux from the stator field induces a current in the rotor and this in turn results in a rotor magnetic field.
  • When the motor is disconnected from the supply (open transition) there is a spinning rotor within the stator and the rotor has a magnetic field. Due to the low impedance of the rotor circuit, the time constant is quite long and the action of the spinning rotor field within the stator is that of a generator which generates voltage at a frequency determined by the speed of the rotor. When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generator and this result in a very high current and torque transient. The magnitude of the transient is dependent on the phase relationship between the generated voltage and the line voltage at the point of closure can be much higher than DOL current and torque and can result in electrical and mechanical damage.
  • Open transition starting is the easiest to implement in terms or cost and circuitry and if the timing of the changeover is good, this method can work well. In practice though it is difficult to set the necessary timing to operate correctly and disconnection/reconnection of the supply can cause significant voltage/current transients.
  • In Open transition there are Four states:
  1. OFF State: All Contactors are open.
  2. Star State: The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
  3. Open State: This type of operation is called open transition switching because there is an open state between the star state and the delta state. The Main contractor is closed and the Delta and Star contactors are open. There is voltage on one end of the motor windings, but the other end is open so no current can flow. The motor has a spinning rotor and behaves like a generator.
  4. Delta State: The Main and the Delta contactors are closed. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available

(2)   Closed Transition Star/Delta Starter.

  • There is a technique to reduce the magnitude of the switching transients. This requires the use of a fourth contactor and a set of three resistors. The resistors must be sized such that considerable current is able to flow in the motor windings while they are in circuit.
  • The auxiliary contactor and resistors are connected across the delta contactor. In operation, just before the star contactor opens, the auxiliary contactor closes resulting in current flow via the resistors into the star connection. Once the star contactor opens, current is able to flow round through the motor windings to the supply via the resistors. These resistors are then shorted by the delta contactor. If the resistance of the resistors is too high, they will not swamp the voltage generated by the motor and will serve no purpose.
  • In closed transition the power is maintained to the motor at all time. This is achieved by introducing resistors to take up the current flow during the winding changeover. A fourth contractor is required to place the resistor in circuit before opening the star contactor and then removing the resistors once the delta contactor is closed. These resistors need to be sized to carry the motor current. In addition to requiring more switching devices, the control circuit is more complicated due to the need to carry out resistor switching
  • In Close transition there are Four states:
  1. OFF State. All Contactors are open
  2. Star State. The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
  3. Star Transition State. The motor is connected in star and the resistors are connected across the delta contactor via the aux [KM4] contactor.
  4. Closed Transition State. The Main [KM3] contactor is closed and the Delta [KM2] and Star [KM1] contactors are open. Current flows through the motor windings and the transition resistors via KM4.
  5. Delta State. The Main and the Delta contactors are closed. The transition resistors are shorted out. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available.

Effect of Transient in Starter (Open Transient starter)

  • It is Important the pause between star contactor switch off and Delta contactor switch is on correct. This is because Star contactor must be reliably disconnected before Delta contactor is activated. It is also important that the switch over pause is not too long.
  • For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained with Direct Online (DOL) starting.
  • If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.
  • When connected to Delta configuration the phase voltage increases by a ratio of V3 or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection.
  • During transition period of switchover the motor must be free running with little deceleration. While this is happening “Coasting” it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causes voltage surges and spikes. Known as a changeover transient.

 Size of each part of Star-Delta starter

(1)   Size of Over Load Relay:

  • For a star-delta starter there is a possibility to place the overload protection in two positions, in the line or in the windings.
  • Overload Relay in Line:
  • In the line is the same as just putting the overload before the motor as with a DOL starter.
  • The rating of Overload (In Line) = FLC of Motor.
  • Disadvantage: If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting is x1.732 too high).
  • Overload Relay in Winding:
  • In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then always measures the current inside the windings.
  • The setting of Overload Relay (In Winding) =0.58 X FLC (line current).
  • Disadvantage: We must use separate short circuit and overload protections.

(2)   Size of Main and Delta Contractor:

  • There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor.
  • Size of Main Contactor= IFL x 0.58

(3)   Size of Star Contractor:

  • The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is 1/ √3= (58%) of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.
  • Size of Star Contactor= IFL x 0.33

Motor Starting Characteristics of Star-Delta Starter:

  • Available starting current: 33% Full Load Current.
  • Peak starting current: 1.3 to 2.6 Full Load Current.
  • Peak starting torque: 33% Full Load Torque.

Advantages of Star-Delta starter:

  • The operation of the star-delta method is simple and rugged
  • It is relatively cheap compared to other reduced voltage methods.
  • Good Torque/Current Performance.
  • It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter:

  • Low Starting Torque (Torque = (Square of Voltage) is also reduce).
  • Break In Supply – Possible Transients
  • Six Terminal Motor Required (Delta Connected).
  • It requires 2 set of cables from starter to motor.
  • It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.
  • In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. The delta of motor is formed in starter and not on motor terminals.
  • High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks. In some cases the current peak can reach a value that is even bigger than for a D.O.L start.
  • Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
  • Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque .
  • The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which make transient Sparks.

Features of star-delta starting

  • For low- to high-power three-phase motors.
  • Reduced starting current
  • Six connection cables
  • Reduced starting torque
  • Current peak on changeover from star to delta
  • Mechanical load on changeover from star to delta

 Application of Star-Delta Starter:

  •  The star-delta method is usually only applied to low to medium voltage and light starting Torque motors.
  • The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start. This starting method only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.

Direct On Line Starter


  • Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.
  • The simplest form of motor starter for the induction motor is the Direct On Line starter. The DOL starter consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.
  • Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.

Principle of DOL:

  •  To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.
  • As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.
  • The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.
  • Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.
  • The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia.
  • DOL starting have a maximum start current and maximum start torque. This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always experience a voltage drop when starting a motor. But if this motor is not a high power one it does not affect much.

Parts of DOL Starters:

(1)   Contactors & Coil.

  • Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits.
  • Magnetic motor controllers use electromagnetic energy for closing switches. The electromagnet consists of a coil of wire placed on an iron core. When a current flow through the coil, the iron of the magnet becomes magnetized, attracting an iron bar called the armature. An interruption of the current flow through the coil of wire causes the armature to drop out due to the presence of an air gap in the magnetic circuit.

  • Line-voltage magnetic motor starters are electromechanical devices that provide a safe, convenient, and economical means of starting and stopping motors, and have the advantage of being controlled remotely. The great bulk of motor controllers sold are of this type.
  • Contactors are mainly used to control machinery which uses electric motors. It consists of a coil which connects to a voltage source. Very often for Single phase Motors, 230V coils are used and for three phase motors, 415V coils are used. The contactor has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the control circuit. A contact is conducting metal parts which completes or interrupt an electrical circuit.
  • NO-normally open
  • NC-normally closed

(2)   Over Load Relay (Overload protection).

  • Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.
  • Under any condition of overload, a motor draws excessive current that causes overheating. Since motor winding insulation deteriorates due to overheating, there are established limits on motor operating temperatures to protect a motor from overheating. Overload relays are employed on a motor control to limit the amount of current drawn.
  • The overload relay does not provide short circuit protection. This is the function of over current protective equipment like fuses and circuit breakers, generally located in the disconnecting switch enclosure.
  • The ideal and easiest way for overload protection for a motor is an element with current-sensing properties very similar to the heating curve of the motor which would act to open the motor circuit when full-load current is exceeded. The operation of the protective device should be such that the motor is allowed to carry harmless over-loads but is quickly removed from the line when an overload has persisted too long.
  • Normally fuses are not designed to provide overload protection. Fuse is protecting against short circuits (over current protection). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless inrush current and a damaging overload. Selection of Fuse is depend on motor full-load current, would “blow” every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.
  • The overload relay is the heart of motor protection. It has inverse-trip-time characteristics, permitting it to hold in during the accelerating period (when inrush current is drawn), yet providing protection on small overloads above the full-load current when the motor is running. Overload relays are renewable and can withstand repeated trip and reset cycles without need of replacement. Overload relays cannot, however, take the place of over current protection equipment.

  • The overload relay consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves, directly or indirectly, to break the circuit.
  • Overload relays can be classified as being thermal, magnetic, or electronic.
  1. Thermal Relay: As the name implies, thermal overload relays rely on the rising temperatures caused by the overload current to trip the overload mechanism. Thermal overload relays can be further subdivided into two types: melting alloy and bimetallic.
  2. Magnetic Relay: Magnetic overload relays react only to current excesses and are not affected by temperature.
  3. Electronic Relay: Electronic or solid-state overload relays, provide the combination of high-speed trip, adjustability, and ease of installation. They can be ideal in many precise applications.

Wiring of DOL Starter:

(1)   Main Contact:

  • Contactor is connecting among Supply Voltage, Relay Coil and Thermal Overload Relay.
  • L1 of Contactor Connect (NO) to R Phase through MCCB
  • L2 of Contactor Connect (NO) to Y Phase through MCCB
  • L3 of Contactor Connect (NO) to B Phase through MCCB.
  • NO Contact (-||-):
  • (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)
  • Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is connected to Common wire of Start/Stop Button.
  • NC Contact (-|/|-):
  • (95-96) is a normally closed NC contact (opens when the thermal overloads trip if associated with the overload block)

(2)   Relay Coil Connection:

  • A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermal over Load Relay’s NC Connection (95).

(3)   Thermal Overload Relay Connection:

  • T1,T2,T3 are connect to Thermal Overload Relay
  • Overload Relay is Connecting between Main Contactor and Motor
  • NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and Common Connection of Start/Stop Button.

Wiring Diagram of DOL Starter:

Working of DOL Starter:

  • The main heart of DOL starter is Relay Coil. Normally it gets one phase constant from incoming supply Voltage (A1).when Coil gets second Phase relay coil energizes and Magnet of Contactor produce electromagnetic field and due to this Plunger of Contactor will move and Main Contactor of starter will closed and Auxiliary will change its position NO become NC and NC become (shown Red Line in Diagram)  .
  • Pushing Start Button:
  • When We Push the start Button Relay Coil will get second phase from Supply Phase-Main contactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2).Now Coil energizes and Magnetic field produce by Magnet and Plunger of Contactor move. Main Contactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) from NO to NC .
  • Release Start Button:
  • Relay coil gets supply even though we release Start button. When We release Start Push Button Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) – Auxiliary contactor (54)-Stop Button-96-95-Relay coil (shown Red / Blue Lines in Diagram).
  • In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95.
  • Pushing Stop Button:
  • When we push Stop Button Control circuit of Starter will be break at stop button and Supply of Relay coil is broken, Plunger moves and close contact of Main Contactor becomes Open, Supply of Motor is disconnected.

 Motor Starting Characteristics on DOL Starter:

  • Available starting current:    100%.
  • Peak starting current:           6 to 8 Full Load Current.
  • Peak starting torque:            100%

Advantages of DOL Starter:

  1. Most Economical and Cheapest Starter
  2. Simple to establish, operate and maintain
  3. Simple Control Circuitry
  4. Easy to understand and trouble‐shoot.
  5. It provides 100% torque at the time of starting.
  6. Only one set of cable is required from starter to motor.
  7. Motor is connected in delta at motor terminals.

Disadvantages of DOL Starter:

  1.  It does not reduce the starting current of the motor.
  2. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).
  3. Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.
  4. Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same lines and therefore not suitable for higher size squirrel cage motors
  5. High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes.

Features of DOL starting

  • For low- and medium-power three-phase motors
  • Three connection lines (circuit layout: star or delta)
  • High starting torque
  • Very high mechanical load
  • High current peaks
  • Voltage dips
  • Simple switching devices

DOL is Suitable for:

  • A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW.
  • DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts.

DOL is not suitable for:

  • The peak starting current would result in a serious voltage drop on the supply system
  • The equipment being driven cannot tolerate the effects of very high peak torque loadings
  • The safety or comfort of those using the equipment may be compromised by sudden starting as, for example, with escalators and lifts.
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