How significant is insulation resistance testing? Since 80% of electrical maintenance and testing involves evaluating insulation integrity, the answer is "very important." Electrical insulation starts to age as soon as it's made. And, aging deteriorates its performance. Harsh installation environments, especially those with temperature extremes and/or chemical contamination, cause further deterioration. As a result, personnel safety and power reliability can suffer. Obviously, it's important to identify this deterioration as quickly as possible so you can take the necessary corrective measures.
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.
You can use it as:
Generally, you connect two leads (positive and negative) across an insulation barrier. A third lead, which connects to a guard terminal, may or may not be available with your tester. If it is, you may or may not have to use it. This guard terminal 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.
Obviously, 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, you apply the test voltage for 1 min. (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 1 min, 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.
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 DegrC 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 DegrC. A corrected measurement at 20 DegrC 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.
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