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insulation resistance = (2 * operating voltage +1000V)
at operating temperature
if you define what you mean by minimum insulation resistance maybe i could help
132,000V on the towers is usually happy with 6' of air
some insulation is far better than air
dry air 500v/mill
thermoplastics 6000v/mill
your power drill has 1/8" - 1/4" of plastic on the cord
and perhaps .001" on the wire in the coils
the cord gets rougher handling
anything practical insulation resistance (2 * operating voltage +1000V)
witch means a 1500Vdc megger is ok for 120V appliances
should be safe to handle if its more than 10megohms
the 1500V will bridge any pinholes or airgaps
NEVER use a vom the 9v battery wont find a pinhole or an air gap
ceramic on heaters and toasters has a way of letting metals diffuse in it lowering the resistance over time also it usually lowers with temperature
so the resistance reading will be a lot lower while its hot
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Is there a standard and safe range for insulation resistance of motors or it's dependent on type of motorsif 30 Mohm is safe for a motor with 1000 Mohm of insulation resistance in normal conditions?
"INSULATION resistance is not about standard nd safe range because resistance value depend upon the application for which we are providing it in what range the current flows." The answer above is incorrect. Application and current rating & range have nothing to do with insulation resistance. There are standards available for insulation resistance testing both in terms of the test procedure and the values. IEEE Standard 43 provides a basis for insulation resistance testing. Both NETA ATS (Acceptance Testing Specification) and NFPA 70B gives criteria for insulation resistance testing. Insulation Resistance (IR) testing, also known by the slang terms meggering or megging, is a procedure where the quality of the electrical insulation is evaluated to determine if it is acceptable for service. It is also used to compare against previous measured values to determine if there has been any degradation to the equipment being tested. In this case we are talking about motor insulation. The question deals with two comparative readings and wants to know if the value of 30 megohms is acceptable. First we should clarify the test methods and results. NETA ATS 2007. Section 7.15.1 covers AC induction motors and generators. The testing involved is broken down into those motors 200 hp (150 kw) and less, and those > 200 hp (150 kw). The test voltage value is based upon the voltage rating of the motor's winding and is found in table 100.1: Rating = 250V; Test Voltage = 500 VDC; Minimum Resistance = 25 megohms Rating = 600V; Test Voltage = 1000 VDC; Minimum Resistance = 100 megohms Rating = 1000V; Test Voltage = 1000 VDC; Minimum Resistance = 100 megohms Rating = 2500V; Test Voltage = 1000 VDC; Minimum Resistance = 500 megohms Rating = 5000V; Test Voltage = 2500 VDC; Minimum Resistance = 1000 megohms Rating = 8000V; Test Voltage = 2500 VDC; Minimum Resistance = 2000 megohms Rating = 15000V; Test Voltage = 2500 VDC; Minimum Resistance = 5000 megohms Rating = 25000V; Test Voltage = 5000 VDC; Minimum Resistance = 20,000 megohms Rating = 34500V; Test Voltage = 15000 VDC; Minimum Resistance = 100,000 megohms It is important to note that the values given are based on a standard temperature of 40C (or sometimes 20C depending on the engineer's specification). You must correct your readings to a standard temperature as the value on the insulation's resistance is going to vary inversely with temperature. That is as temperature increases the resistance will decrease. The rule of thumb is that the measured value will halve for every 15C above standard and will double for every 15C below standard. As an example let us say that we have a 25 hp induction motor rated 480 VAC. The ambient temperature is 15C. Using our table we would set the tester to 1000 VDC and take a reading for one minute. At the end of the minute we get a reading of 450 megohms. Per NETA ATS Table 100.14 the correction factor is 0.31 so IR = 450 megohms x .031 = 139.5 megohms. The minimum acceptable value is 100 so this motor is acceptable. On the flip side if the motor is in a very warm process area, say 50C, then temperature correction factor is 1.59 thus IR = 450 megohms x 1.59 = 716 megohms. As you can see the temperature makes a very large difference in the results! This discussion up to this point has been about a spot-reading check. However for a true check we want to know the Dielectric Absorption value. There are two different standard tests for Dielectric Absorption. The first is the Dielectric Absorption Ratio (DAR) and the second is the Polarization Index. DAR = Reading @ 60 sec / Reading @ 30 sec. Let us say that the 30 sec reading = 325 megohms and the 60 sec = 450 megohms. Thus: DAR = 450 megohms / 325 megohms = 1.38 The minimum DAR per NETA is 1.4 so this particular motor is borderline at best and should be investigated further. PI = Reading @ 10 min / Reading @ 1 min. Let us say that the 1 min reading = 450 megohms and the 10 min = 1100 megohms. Thus: PI = 1100 megohms / 450 megohms = 2.44 The minimum PI per NETA is 2.0 so in this case the motors is acceptable. One final factor that should be taken into consideration is Relative Humidity (RH). The amount of moisture present in the air also affects the measured test values. The more moisture then the lower the reading. There is no published standard correction factor for RH however when NETA Techs perform these tests then they always record the RH for baseline comparison. The bottom line is that the readings will vary based upon temperature and humidity. A reading on warm humid day may be acceptable whereas the same reading on a cold dry day may not. So the question asked here is unanswerable as there is not enough information given. What were the temperatures at the time of the readings? Was it dry during one and raining during the other? What does the person asking mean by "normal conditions"? I highly recommend that anyone interested in this subject get the free book "A Stitch in Time" by Biddle at: http://www.biddlemegger.com/biddle/Stitch-new.pdf Please note that I also changed the category from Health/pregnancy to Electrical Engineering.
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