Megger test is the test that evaluate the insulation resistance quality.

Insulation starts to age as soon as it’s made. As it ages, its insulating performance deteriorates. Any harsh installation environments, especially those with temperature extremes and/or chemical contamination, accelerates this process. This deterioration can result in dangerous conditions in power reliability and personnel safety.

As such, it’s important to identify this deterioration quickly so that corrective steps can be taken. One of the simplest tests and its required test instrument are not universally understood. To help eliminate this lack of understanding, let’s discuss in detail Insulation Resistance (IR) testing and the Megohmmeter or Megger test.

Insulation testing components

Let’s approach the subject by component.

The Megohmmeter

A basic megohmmeter schematic is shown above. The megohmmeter is similar to a multimeter, when the latter is in its ohmmeter function. There are differences, however.

First, the megohmmeter’s output is much higher than that of a multimeter. Voltages of 100, 250, 500, 1,000, 2500, 5,000, and even 10,000V are used .

Equipment AC Rating	DC Test Voltage
Up to 100V	100V and 250V
440V to 550V	500V and 1,000V
2,400V	1,000V to 2,000V and higher
4,160V and above	1,000V to 5,000V or higher

The most common voltages are 500V and 1,000V. Higher voltages are used to stress an insulation to a greater degree and thus obtain more accurate results.

Second, the range of a megohmmeter is in megohms, as its name implies, instead of ohms as in a multimeter.

Third, a megohmmeter has a relatively high internal resistance, making the instrument less hazardous to use in spite of the higher voltages.

Testing connections

A megohmmeter usually is equipped with three terminals. The “LINE” (or “L“) terminal is the so-called “hot” terminal and is connected to the conductor whose insulation resistance you are measuring.

Remember: insulation testers should never be connected to energized lines or equipment when operated in any of the Insulation Test or Resistance Test modes.

The “EARTH” (or “E“) terminal is connected to the other side of the insulation, the ground conductor.

The “GUARD” (or “G“) terminal provides a return circuit that bypasses the meter. For example, if you are measuring a circuit having a current that you do not want to include, you interact that part of the circuit to the “GUARD” terminal.

Note that Resistance Insulation is temperature sensitive. When the temperature goes up, Resistance Insulation (IR) goes down, and vice versa. A common rule of thumb is that IR changes by a factor of two for each 10°C change.

For example, suppose we obtained an IR reading of 100 megohms with an insulation temperature of 30°C. The corrected IR (at 20°C) would be 100 megohms times 2, or 200 megohms. 

For ship piping system, as below captures extracted from BV, ABS rule: