Understanding Insulation Resistance Testing
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1. D yA current Represents volumetric leakage current dielectric loss CAPACITANCE 60 CHARGING CURRENT TOTAL CURRENT CURRENT MICROAMPERES CONDUCTION OR LEAKAGE CURRENT ABSORPTION CURRENT a 45 2 253 456789 10 15 2 25 3 45678910 Gy AEMC INSTRUMENTS Technical Assistance 800 343 1391 SECONDS Insulation Resistance Readings Readings are time dependent at the start capacitance is what you see first at or about one minute absorption at 10 minutes reading is mainly leakage current These changing readings are best seen with analog bargraphs on digital instruments or needle movement on analog instruments Spot Reading Test Method For this test the megohmmeter is connected across the insulation of the windings of the machine being tested A test voltage is applied for a fixed period of time usually 60 seconds and a reading is taken The spot reading test should only be carried out when the winding temperature is above the dew point The operator should make a note of the winding temperature so that it will be possible to correct the reading to a base temperature of 20 C Test Duration To obtain comparable results tests must be of the same duration Usually the reading is taken after 60 seconds Interpretation of Re2. d Low voltage winding to ground and high voltage winding to guard e High voltage winding to low voltage winding Understanding Insulation Resistance Testing pdf 02 06 Rev 02 3 of 6 Single Phase Transformer a High voltage winding to low voltage winding and ground Mope 1060 b Low voltage winding to high voltage winding and ground c High voltage winding to low voltage winding d High voltage winding to ground e Low voltage winding to ground Three Phase Transformer a High voltage winding to low voltage winding and ground b High voltage winding to ground with low voltage winding to guard Moner 1080 c High voltage winding to low voltage winding d Low voltage winding to ground and high voltage winding to guard Moner 1080 e High voltage winding to low voltage winding Moot 1080 Earth Terminal Line Terminal G Guard Terminal AEMC INSTRUMENTS Technical Assistance 800 343 1391 Understanding Insulation Resistance Testing pdf 02 06 Rev 02 4 of 6 Cable Testing Motor and Generator Testing Transformers are tested at or above the rated voltage to Before testing the above lift the rotor brushes ground the be certain there are no excessive le
3. 00Vpc 2400V 1000 to 2500Vpc 4100V 1000 to 5000Vpc It is always advisable to contact the original equipment manufacturer to get their recommendation for the proper voltage to use when testing their equipment Gy AEMC INSTRUMENTS Technical Assistance 800 343 1391 Advantages of DC Testing Lighter size and weight of test equipment e Non destructive Historical data can be compiled Transformer Testing Transformers are tested at or above the rated voltage to be certain there are no excessive leakage paths to ground or between windings These are conducted with the transformer completely disconnected from the line and load However the case ground should not be removed Single Phase Transformer The following 5 tests and corresponding wiring diagrams will completely test a single phase transformer Allow at least 1 minute for each test or until the reading stabilizes a High voltage winding to low voltage winding and ground b Low voltage winding to high voltage winding and ground c High voltage winding to low voltage winding d High voltage winding to ground e Low voltage winding to ground Three Phase Transformer The following 5 tests and corresponding wiring diagrams will completely test a three phase transformer a High voltage winding to low voltage winding and ground b High voltage winding to ground with low voltage winding to guard c High voltage winding to low voltage winding
4. Understanding Insulation Resistance Testing Why have an insulation testing program A regular program of testing insulation resistance is strongly recommended to prevent electrical shocks assure safety of personnel and to reduce or eliminate down time It helps to detect deterioration of insulation in order to schedule repair work such as vacuum cleaning steam cleaning drying and rewinding It is also helpful when evaluating the quality of the repairs before the equipment is put back into operation What causes insulation failure Some of the more common causes of insulation failure include excessive heat or cold moisture dirt corrosive vapors oil vibration aging and nicked wiring What tests are used to detect insulation deterioration There are numerous maintenance tests for assessing insulation quality The three tests discussed here are used primarily to test motor generator and transformer insulation What equipment is necessary for conducting insulation resistance tests Megohmmeter with a timed test function Temperature indicator e Humidity meter not necessary if equipment temperature is above the dew point Test Currents in Insulation Total current in the body of the insulation is the sum of three components Capacitance Charging Current e Absorption Current e Leakage or Conduction Current Test Currents in Insulation One Line Diagram Represents charging current Represents absorption
5. akage paths to ground starter terminal and frame and ground the motor shaft or between windings These are conducted with the Discharge the field winding by grounding Then remove transformer completely disconnected from the line and the field winding from ground and connect to the Line load However the case ground should not be removed connection on the megohmmeter Connect the Earth terminal to ground The diagram shows the connection for Single Conductor testing the field insulation resistance The stator winding Connect as shown in the diagram may also be measured in a similar manner a Conductor to Line terminal and sheath to Earth Multi Conductor I a Single conductor b One conductor to all c One conductor to earth d One conductor to others minus ground a me Faa Ground Sheath single conductor Stator a Single conductor Three eponducter PANE 7 we spans b One conductor to all Earth Terminal Line Terminal G Guard Terminal s 2002 Chauvin Arnoux Inc d b a AEMC Instruments c One conductor to earth Three conductor MAME o o7 o Mope 1060 d One conductor to others minus ground Q AE MC Understanding Insulation Resistance T
6. d is fairly independent of temperature and often can give you conclusive information without records of past tests It is based on the absorption effect of good insulation compared to that of moist or contaminated insulation Simply take successive readings at specific times and note the differences in readings see curves Figure 2 Tests by this method are sometimes referred to as absorption tests Good insulation shows a continual increase in resistance see curve D over a period of time in the order of 5 to 10 minutes This is caused by the absorption good insulation shows this charge effect over a time period much longer that the time required to charge the capacitance of the insulation If the insulation contains moisture or contaminants the absorption effect is masked by a high leakage current which stays at a fairly constant value keeping the resist ance reading low R E I see curve E The time resistance testing is of value because it is independent of equipment size The increase in resistance for clean and dry insulation occurs in the same manner whether a motor is large or small You can compare several motors and establish standards for new ones regardless of their horsepower ratings Figure 2 shows how a 60 second test would appear for good and bad insulation When the insulation is in good shape the 60 second reading is higher that the 30 second reading A further advantage of this two reading test is that
7. esting pdf 02 06 Rev 02 INSTRUMENTS Technical Assistance 800 343 1391 5 of 6 EJ AEMG INSTRUMENTS CHAUVIN ARNOUX GROUP Contact Us United States amp Canada Chauvin Arnoux Inc d b a AEMC Instruments 200 Foxborough Blvd Foxborough MA 02035 USA 508 698 2115 Fax 508 698 2118 www aemc com Customer Support for placing an order obtaining price amp delivery customerservice aemc com Sales Department for general sales information sales aemc com Repair and Calibration Service for information on repair amp calibration obtaining a user manual repair aemc com Technical and Product Application Support for technical and application support techinfo aemc com Webmaster for information regarding www aemc com webmaster aemc com South America Central America Mexico Caribbean Australia amp New Zealand Chauvin Arnoux Inc d b a AEMC Instruments 15 Faraday Drive Dover NH 03820 USA 978 526 7667 Fax 978 526 7605 export aemc com www aemc com All other countries Chauvin Arnoux SCA 190 rue Championnet 75876 Paris Cedex 18 France 33 1 44 85 45 28 Fax 33 1 46 27 73 89 info chauvin arnoux com www chauvin arnoux com Understanding Insulation Resistance Testing pdf 02 06 Rev 02 Q INSTRUMENTS Technical Assistance 800 343 1391 6 of 6
8. ge levels Insulation that is thoroughly dry clean and without physical damage should provide roughly the same resistance values despite changes in test voltage levels If resistance values decrease substantially when tested at higher voltage levels this should serve as a warning that insulation quality may be deteriorating due to dirt moisture cracking aging etc 10 minute reading Polarization Index Pl g 1 minute reading The IEEE Std 43 2000 lists the following minimum values for the polarization index for AC and DC rotating machines Class A 1 5 Class B 2 0 Class C 2 0 1000 D Sy a E 3 100 E 2 3 10 1 4 4 4 d 1 2 3 4 5 6 T 8 9 10 Time in Minutes Figure 2 Absorption curve of test conducted on 350 HP Motor Curve D indicates a good insulation with an excellent polarization index of 5 Curve E indicates a potential problem The polarization index is only 140 95 or 1 47 2 IEEE Std 43 2000 Recommended Practice for Testing Insulation Resistance of Rotating Machinery Available from the Institute of Electrical and Electronics Engineers Inc 345 E 47th St New York NY 10017 Understanding Insulation Resistance Tes
9. it gives you a Clearer picture even when a spot reading says the insulation looks ok Time resistance tests on large rotating electrical machinery especially with high operating voltage require high insulation resistance ranges and a very constant test voltage AEM iNStauMENTs Technical Assistance 800 343 1391 A heavy duty megohmmeter serves this need Similarly such an instrument is better adapted for cables bushings transformers and switchgear in the heavier duty sizes Test Methods Time Resistant Tests Dielectric Absorption Ratio DAR The ratio of 60 seconds 30 seconds e less than 1 failed 1 0 to 1 25 OK e 1 4 to 1 6 excellent Note This is not a commonly used test Step Voltage Test Method In this test the operator applies two or more test voltages in steps The recommended ratio for the test voltage steps is 1 to 5 At each step test voltage should be applied for the same length of time usually 60 seconds The application of increased voltage creates electrical stresses on internal insulation cracks This can reveal aging and physical damage even in relatively dry and clean insulation which would not have been apparent at lower voltages Test Duration A series of steps each step lasting 60 seconds Interpretation of Results Compare the readings taken at different voltage levels look ing for any excessive reduction in insulation resistance values at the higher volta
10. sults Proper interpretation of spot reading tests requires access to records of results from previous spot reading tests For conclusive results only use results from tests performed at the same test voltage for the same amount of time and under similar temperature and humidity conditions These readings are used to plot a curve of the history of insulation resistance A curve showing a downward trend usually indicates a loss of insulation resistance due to unfavorable conditions such as humidity dust accumulation etc A very sharp drop indicates an insulation failure See Figure 1 Understanding Insulation Resistance Testing pdf 02 06 Rev 02 1 of 6 10000 1000 Reading in Megohms Bo 1998 1999 Date of Test 1996 1997 2000 2001 Figure 1 Example of the variation of insulation resistance over a period of years At A the effect of aging and dust accumulation is shown by decreasing values At B the sharp drop indicates an insulation failure At C the insulation resistance value after the motor has been rewound 1 Dew point temperature is the temperature at which the moisture vapor in the air condenses as a liquid Time Resistance Testing Method This metho
11. ting pdf 02 06 Rev 02 2of6 Reading in Megohms No Connection to To Guard Guard Terminal Terminal B 1 4 To EARTH To LINE To EARTH To LINE Terminal Terminal Terminal 7 g Terminal Applied Voltage in kV Before and after repair Curve F shows a downward trend of insulation resistance values as the test voltage is increased This indicates a potential problem with the insulation Curve G shows the same equipment after it has been repaired Utilizing the Guard Terminal The guard terminal is useful when measuring very high resistance values The guard terminal is useful when measuring very high resistance values Conductor Insulation Shield to to to Line Guard Earth Terminal Terminal Terminal Exposed Surface What test voltage should use There are two schools of thought regarding the voltage to test insulation at The first applies to new equipment or cable and can use AC or DC test voltages When AC voltage is used the rule of thumb is 2 x nameplate voltage 1000 When DC voltage is used most common on megohmmeters manufactured today the rule of thumb is simply 2 x nameplate voltage except when higher voltages are used See chart below for suggested values Equipment Cable Rating DC Test Voltage 24 to 50V 50 to 100Vpc 50 to 100V 100 to 250Vpc 100 to 240V 250 to 500Vpc 440 to 550V 500 to 10
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