The purpose of the IR test is to check for damaged insulation, this can be mechanical damage or damage by heat, (overloaded cables), readings less than 2 Mohm indicate damaged insulation, readings of 2-50 Mohm are indicative of long circuit lengths, moisture and contamination and do not indicate the insulation quality,
The rule may be stated: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms.
An insulation resistance (IR) test measures the total resistance between any two points separated by electrical insulation. The test, therefore, determines how effective the dielectric (insulation) is in resisting the flow of electrical current.
The Megger test is a method of testing making use of an insulation tester resistance meter that will help to verify the condition of electrical insulation. Insulation resistance quality of an electrical system degrades with time, environment condition i.e. temperature, humidity, moisture and dust particles.
In an IR or megohm test the voltage applied and the total leakage current are measured between the windings and the motor frame/ground. Ohms law is applied to calculate the resistance in megohms.
The Insulation Resistance test sequence:
- Connect one of the test leads of the Insulation Tester to the Line conductor and the other one to the Neutral conductor.
- Repeat the test process with one of the test leads on the Line conductor and the other one on the Earth conductor.
If you are testing insulation resistance to ground, place the positive probe on the ground wire or the grounded metal junction box and the negative probe on the conductor or terminal. Energize the Megger for 1 minute. Read the value of the resistance at the end of the minute test and note it in your table.
Usual megger voltage is 500 V ,Our equivalent resistance is very high about 1 M . 500 V , 1000 V or 2500 V , None of the voltages would injure you, but you will hurt yourself just trying to get away from it, fall off ladder, jump backward and bang your head, etc.
If the
motor is kept in a place with high humidity, a periodical inspection is necessary.
A general rule-of-thumb is 10 Megohm or more.
| Insulation resistance value | Insulation level |
|---|
| 2 Megohm or less | Bad |
| 2-5 Megohm | Critical |
| 5-10 Megohm | Abnormal |
| 10-50 Megohm | Good |
Read the resistance value.Resistance is a measure of how much a device or material reduces the electric current flowing through it. Higher numbers indicate a higher resistance rating, which means more energy will be required to integrate the component in a circuit.
An open circuit implies that the two terminals are points are externally disconnected, which is equivalent to a resistance R=∞ . This means that zero current can flow between the two terminals, regardless of any voltage difference.
Clamp the alligator clip on one of the megger probes to the wire or cable, touch the other probe to the exposed conductor and press the test button. The megger will generate a current between the probes, and the meter will record the resistance of the jacket to the flow of the current.
Procedure of Megger test/Insulation Resistance test of transformer:
- First disconnect all the line and neutral terminals of the transformer.
- Megger connects to LV and HV bushing studs to calculate the importance of IR insulation resistance between LV and HV windings.
Therefore, if we apply AC voltage during IR test then there will always be a flow of current through the capacitor and thus a leakage current to the ground. This is the reason, an Insulation Tester or Megger have DC voltage generator.
You use your insulation multimeter to check the insulation resistance of each conductor to ground and of each conductor to the other two.
The earth megger is essentially a direct reading ohmmeter and it has a hand driven generator which supplies the testing current. The ohmmeter essentially consists of two coils (current coil and pressure coil) mounted at a fixed angle to each other on a common axis.
For good insulation, the resistance usually reads in the megohm range. The Megger insulation tester is essentially a high-range resistance meter (ohmmeter) with a built-in direct-current generator. This method is nondestructive; that is, it does not cause deterioration of the insulation.
