Measurement of Insulation Resistance

We define insulation resistance as the ratio of applied direct voltage across an insulation to the corresponding current through it.

Mesaurement of inslation resistance is quite important. We normally take the reading of the measurement at a certain time after application of test voltage. The standard duration of voltage application are 1 minute or 10 minutes. Because of that, insulation resistance can also be referred as 1 minute insulation resistance or 10 minute insulation resistance depending upon the duration of the test.

NB: - The voltage, we apply for measurement of insulation resistance, is direct voltage. When we apply direct voltage across the insulation, a current starts passing through the insulation. This current has two main components.

1. The current flowing through the leakage path over the surface of the solid insulator. This leakage path is formed mainly due to moisture, dust etc. which are naturally accumulated on the surface of the solid insulator.
2. The current flowing through the volume of the insulator body.

The second component of the current is further divided in three components as mentioned below.

• As the insulation materials are essentially dielectric in nature, there will be a capacitive charging current, appears just after application of test voltage. This current is instantaneous in nature. It will effectively disappear within few moments. Hence, this current does not have any effect on the reading of measurement if it is taken after 1 minute or more.
• There is another component of current called absorption current. It decays from high value to zero. The insulation resistance value taken within first few minutes of test is largely dominated by absorption current.
• Last but most important component of the current is conduction current. It remains steady throughout the insulation resistance test. So after, charging current then absorption current becomes insignificant, the test result is mainly predominated by this conduction current.

Thus finally, leakage current and conduction current come into picture at the time of taking reading of insulation resistance. This is why the reading of insulation resistance is normally taken after 15 seconds or 1 minute or sometimes after 10 minutes during the test.

Method of Measuring Insulation Resistance

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

1. Direct-indicating ohmmeter with hand driven dc generator. This is locally known as hand driven megger since Megger is one of the best known manufacturer of this instrument.
2. Direct-indicating ohmmeter with motor driven dc generator. This is locally known as motorized megger.
3. Direct-indicating ohmmeter with self-contained battery.
4. Direct-indicating ohmmeter with self-contained rectifier. This instrument takes power from an external AC supply.
5. Resistance bridge circuit with self-contained galvanometer and battery.

We can conduct the measurement of insulation resistance with an external dc supply. In that case, we take voltage and current reading with the help of a dc voltmeter and a micro ranged dc ammeter, respectively.

In that case, we can calcula the insulation resistance with the help of ohm’s lawWhere V is the voltmeter reading and I is the ammeter reading.

The ammeter is micro ranged because, a very tiny current passes through the insulation during test and he current is in that range only. But at the moment of voltage application, the micrometer has to take initial capacitive charging current as well as absorption current. So the ammeter should be capable of withstanding both of these currents for at least initial duration. The voltmeter, ammeter and source should also be capable of withstanding short circuit current in the case of insulation failure if occurs during measurement.

When we use direct indicating ohmmeter of simply megger, the leads of the instrument are connected across the insulator to be tested. After driving the instrument the value of insulation resistance is indicated on the analog or digital dial of the instrument directly. In both of the above-mentioned methods of insulation resistance measurement, the reading is taken after a standard time delay to get more accurate and error-free reading.