Understanding Insulation Resistance Testing

1 Understanding Insulation Resistance Testing (pdf) 02/06 Rev. 02 Technical Assistance (800) 343-13911of 6 Understanding InsulationResistance TestingWhy have an Insulation Testing program?A regular program of Testing Insulation Resistance is stronglyrecommended to prevent electrical shocks, assure safetyof personnel and to reduce or eliminate down time. It helpsto detect deterioration of Insulation in order to schedule repairwork such as: vacuum cleaning, steam cleaning, drying andrewinding. It is also helpful when evaluating the quality of therepairs before the equipment is put back into causes Insulation failure?Some of the more common causes of Insulation failureinclude: excessive heat or cold, moisture, dirt, corrosivevapors, oil, vibration, aging and nicked tests are used to detect Insulation deterioration?

2 There are numerous maintenance tests for assessinginsulation quality. The three tests discussed here are usedprimarily to test motor, generator and transformer equipment is necessary for conducting insulationresistance tests? Megohmmeter with a timed test function Temperature indicator Humidity meter (not necessary if equipment temperature isabove the dew point)Test Currents in InsulationTotal current in the body of the Insulation is the sum of threecomponents Capacitance Charging Current Absorption Current Leakage or Conduction CurrentInsulation Resistance ReadingsReadings 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 currentThese changing readings are best seen with analogbargraphs on digital instruments or needle movementon analog Reading TestMethodFor this test.

3 The megohmmeter is connected across theinsulation of the windings of the machine being test voltage is applied for a fixed period of time, usually60 seconds and a reading is taken. The spot reading testshould only be carried out when the winding temperature isabove the dew point1. The operator should make a note ofthe winding temperature, so that it will be possible to correctthe reading to a base temperature of 20 DurationTo obtain comparable results, tests must be of the sameduration. Usually the reading is taken after 60 of ResultsProper interpretation of spot reading tests requires access to records of results from previous spot reading tests.

4 Forconclusive results, only use results from tests performed atthe same test voltage for the same amount of time, and undersimilar temperature and humidity conditions. These readingsare used to plot a curve of the history of Insulation curve showing a downward trend usually indicates a loss ofinsulation Resistance due to unfavorable conditions such as:humidity, dust accumulation, etc. A very sharp drop indicatesan Insulation failure. See Figure VOLTAGESOURCESRLRACTest Currents in Insulation One Line DiagramC = Represents charging currentRA = Represents absorption currentRL = Represents volumetric leakage current (dielectric loss)ABSORPTIONCURRENTCONDUCTIONOR LEAKAGECURRENTTOTAL CURRENTCAPACITANCECHARGING.

5 1 .15 .2 .25 .3 .4 .5 .6 .7 .8 .9 2 341098765 CURRENT MICROAMPERESSECONDSFor more information, please visit our website Insulation Resistance Testing (pdf) 02/06 Rev. 02 Technical Assistance (800) 343-13912of 6(1) Dew point temperature is the temperature at which the moisture vapor in theair condenses as a Testing MethodThis method is fairly independent of temperature and oftencan give you conclusive information without records of pasttests. It is based on the absorption effect of good insulationcompared to that of moist or contaminated take successive readings at specific times and notethe differences in readings (see curves, Figure 2).

6 Tests bythis method are sometimes referred to as absorption Insulation shows a continual increase in Resistance (see curve D) over a period of time (in the order of 5 to10 minutes). This is caused by the absorption; goodinsulation shows this charge effect over a time period muchlonger that the time required to charge the capacitance ofthe the Insulation contains moisture or contaminants, theabsorption effect is masked by a high leakage currentwhich 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 isindependent of equipment size. The increase in resistancefor clean and dry Insulation occurs in the same mannerwhether a motor is large or small.

7 You can compare severalmotors and establish standards for new ones, regardless oftheir horsepower 2 shows how a 60-second test would appear for goodand bad Insulation . When the Insulation is in good shape, the60-second reading is higher that the 30-second further advantage of this two reading test is that it givesyou a clearer picture, even when a spot reading says theinsulation looks tests on large rotating electrical machinery especially with high operating voltage require highinsulation Resistance ranges and a very constant test heavy-duty megohmmeter serves this need. Similarly,such an instrument is better adapted for cables, bushings,transformers, and switchgear in the heavier-duty Methods Time-Resistant Tests Dielectric AbsorptionRatio (DAR) The ratio of 60 seconds/30 seconds less than 1 = failed to = OK to = excellentNote:This is not a commonly used testStep Voltage TestMethodIn this test, the operator applies two or more test voltages insteps.

8 The recommended ratio for the test voltage steps is1 to 5. At each step, test voltage should be applied for thesame length of time, usually 60 seconds. The application ofincreased voltage creates electrical stresses on internalinsulation cracks. This can reveal aging and physical damageeven in relatively dry and clean Insulation which would nothave been apparent at lower DurationA series of steps, each step lasting 60 of ResultsCompare the readings taken at different voltage levels, look-ing for any excessive reduction in Insulation Resistance valuesat the higher voltage levels. Insulation that is thoroughly dry,clean, and without physical damage should provide roughlythe same Resistance values despite changes in test voltagelevels.

9 If Resistance values decrease substantially when testedat higher voltage levels, this should serve as a warning thatinsulation quality may be deteriorating due to dirt, moisture,cracking, aging, Index (PI) =10-minute reading1-minute readingThe IEEE Std 43-2000 lists the following minimum values forthe polarization index for AC and DC rotating machines:Class A: B: C: in Megohms10000100010010 Date of TestABC 1 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 B, the sharp drop indicates an Insulation C, the Insulation Resistance value after the motor has been 345678910 Time in MinutesReading in MegohmsDE Figure 2 Absorption curve of test conducted on 350 HP Motor:Curve Dindicates agood Insulation with an excellent polarization index of 5.

10 Curve Eindicates apotential problem. The polarization index is only 140/95, or (2) IEEE Std. 43-2000, Recommended Practice for Testing InsulationResistance of rotating Machinery. Available from the Institute of electrical andElectronics Engineers, Inc., 345 E. 47th St., New York, NY Insulation Resistance Testing (pdf) 02/06 Rev. 02 Technical Assistance (800) 343-13913of 6 Utilizing the Guard TerminalThe guard terminal is useful when measuring very highresistance test voltage should I use?There are two schools of thought regarding the voltage to testinsulation at. The first applies to new equipment or cable andcan use AC or DC test AC voltage is used, the rule of thumb is 2 x nameplatevoltage + 1000.