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Basically, you're applying a voltage (specifically a highly regulated, stabilized DC voltage) across a dielectric, measuring the amount of current flowing through that dielectric and then calculating (using Ohm's Law) a resistance measurement. Let's clarify our use of the term "current." We're talking about leakage current. The resistance measurement is in megohms. You use this resistance measurement to evaluate insulation integrity.
Current flow through a dielectric may seem somewhat contradictory, but remember, no electrical insulation is perfect. So, some current will flow.
it's a good idea to have a basic familiarity with the item you're testing. Basically, you should know what is supposed to be insulated from what. The equipment you're testing will determine how you hook up your meghommeter.
After you make your connections, apply the test voltage for one minute. (This is a standard industry parameter that allows you to make relatively accurate comparisons of readings from past tests done by other technicians.)
During this interval, the resistance reading should drop or remain relatively steady. Larger insulation systems will show a steady decrease; smaller systems will remain steady because the capacitive and absorption currents drop to zero faster than on larger systems. After one minute, you should read and record the resistance value.
When performing insulation resistance testing, you must maintain consistency. Why? Because electrical insulation will exhibit dynamic behavior during the course of your test - whether the dielectric is "good" or "bad." To evaluate a number of test results on the same piece of equipment, you have to conduct the test the same way and under the relatively same environmental parameters - each and every time.
Your resistance measurement readings will also change with time. This is because electrical insulation materials exhibit capacitance and will charge during the course of the test. This can be somewhat frustrating to a novice. However, it becomes a useful tool to a seasoned technician.
As you gain more skills, you'll become familiar with this behavior and be able to make maximum use of it in evaluating your test results. This is one factor that generates the continued popularity of analog testers.
What affects insulation resistance readings? Insulation resistance is temperature-sensitive. When temperature increases, insulation resistance decreases, and vice versa. A common rule of thumb is insulation resistance changes by a factor of two for each 10???C change. So, to compare new readings with previous ones, you'll have to correct your readings to some base temperature. For example, suppose you measured 100 megohms with an insulation temperature of 30???C. A corrected measurement at 20???C would be 200 megohms (100 megohms times two).
Also, "acceptable" values of insulation resistance depend upon the equipment you're testing. Historically, many field electricians use the somewhat arbitrary standard of 1 megohm per kV. The interNational Electrical Testing Association (NETA) specification Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems provides much more realistic and useful values.
Remember, compare your test readings with others taken on similar equipment. Then, investigate any values below the NETS standard minimums or sudden departures from previous values.
Remember you will need you will need a meter that reads megaohms.