The measurement of insulation resistance is very important. We usually read the measured value at some time after the test voltage is applied. The standard duration of applied voltage is 1 minute or 10 minutes. Therefore, the insulation resistance can also be called 1-minute insulation resistance or 10-minute insulation resistance, depending on the duration of the test.

Note: the voltage we use to measure the insulation resistance is dc voltage.

When we apply a dc voltage to the insulation, current begins to flow through it. The current has two main components.

1. The current flows through the leakage path on the surface of the solid insulator. The leakage path is mainly caused by water, dust, etc., which naturally accumulates on the surface of the solid insulator.

2. The volume of current flowing through the insulator.

The second part of the current is further divided into three parts, as described below.

• because the insulation is essentially a dielectric, a capacitive charging current occurs immediately after the test voltage is applied. The current is instantaneous in nature. It will effectively disappear within a few minutes. Therefore, if this current is acquired after 1 minute or more, it has no effect on the measured reading.

• the other part of the current is called the absorption current. It decays from high to zero. The value of insulation resistance obtained in the first few minutes of the test is mainly determined by the absorption current.

• the last but most important component of current is conduction current. It remained stable throughout the insulation resistance test. Therefore, after the charging current and the absorbing current become negligible, the test results are mainly determined by the conducting current.

Thus, eventually, both the current and the conduction current will leak out when the insulation resistance is read.

This is why the insulation resistance is usually read after 15 seconds or 1 minute or sometimes 10 minutes during testing.

Method for the determination of insulation resistance

There are several instruments for measuring the insulation resistance of electrical equipment.

1. Direct indicating ohm meter with manual dc generator. As the megohm meter is one of the most famous manufacturers of this instrument, it is known locally as the manual megohm meter.

2. Direct indicating ohmmeter with motor dc generator. This is known locally as an electric megohm meter.

3. Direct indicator ohmmeter with independent battery.

4. Direct indicating ohmmeter with independent rectifier. The instrument is powered from an external ac source.

5. Resistance bridge circuit with independent galvanometer and battery.

We can measure the insulation resistance with an external dc power supply. In this case, voltage and current readings are obtained by means of a dc voltmeter and a micro-range dc ammeter, respectively.

In this case, we can calculate the insulation resistance with the help of ohm's law,

Where V is the voltmeter reading and I is the ammeter reading.

Ammeters are micrometers because during the test, a small current flows through the insulation, and the current is only in that range. However, when the voltage is applied, the micrometer must use the initial capacitor charging current as well as the absorbing current. Therefore, the ammeter should be able to withstand both currents at least for the initial time. Voltmeters, ammeters and power supplies should also be able to withstand short-circuit current if insulation failure occurs during the measurement.

When we use the direct indicator ohmmeter of a simple megohm meter, the lead of the instrument is bonded to the insulator to be tested. After the instrument is driven, the value of the insulation resistance is displayed directly on the analog or digital dials of the instrument.

In both of the above methods of measuring insulation resistance, readings are taken after the standard time delay to obtain more accurate and error-free readings.