- Insulation Resistance Definition: Insulation resistance is defined as the ratio of applied direct voltage across an insulation to the current through it.
- Importance of Measurement: Measuring insulation resistance is crucial for electrical safety and ensuring the reliability of equipment.
- Current Components: The current during insulation resistance measurement includes surface leakage current and volume current, with the latter having three parts: capacitive charging current, absorption current, and conduction current.
- Measurement Methods: Insulation resistance can be measured using various methods, including direct-indicating ohmmeters and resistance bridges.
- Instruments: Common instruments for measuring insulation resistance include hand-driven ohmmeters, motorized ohmmeters, and devices with built-in batteries or rectifiers.
Insulation resistance is the ratio of applied direct voltage to the current through the insulation. Testers take the reading after applying the test voltage for a set time. Common timed readings are 1 minute and 10 minutes, so the result is called 1-minute insulation resistance or 10-minute insulation resistance.
The voltage applied for measurement of insulation resistance is a direct voltage.
When that direct voltage is applied across the insulation, a current starts through the insulation. That current has two main components.
- The current flowing through the leakage path over the surface of the solid insulator. Moisture and dust on that surface form most of this path.
- The current flowing through the volume of the insulator body.
The current through the insulator body then splits into three parts.
- Insulation materials are dielectric, so a capacitive charging current appears right after applying the test voltage. That current dies away quickly on most items, so a reading taken after 1 minute or more is usually free of it. Large capacitance can take longer to charge.
- Another component of the current is called absorption current. It falls from a high value toward a small value over minutes. The insulation resistance value taken in the first few minutes of the test is largely set by this absorption current.
- The remaining component is conduction current. It stays steady throughout the insulation resistance test. After the charging current and then the absorption current have fallen, this conduction current mainly sets the reading.
Surface leakage current and conduction current are therefore the currents still present when the reading is taken.
That is why an insulation resistance reading is commonly taken after 15 seconds, 1 minute or 10 minutes. IEEE Std 43 uses the 1-minute value as the usual insulation resistance reading for rotating machines and the 10-minute value in the polarisation index. Other equipment may follow a different timed method.
Method of Measuring Insulation Resistance
These instruments are used to measure the insulation resistance of electrical equipment.
- Direct-indicating ohmmeter with a hand-driven dc generator. This is often called a hand-driven megger because Megger is a well-known manufacturer of the instrument.
- Direct-indicating ohmmeter with a motor-driven dc generator. This is often called a motorized megger.
- Direct-indicating ohmmeter with self-contained battery.
- Direct-indicating ohmmeter with self-contained rectifier. This instrument takes power from an external AC supply.
- Resistance bridge circuit with self-contained galvanometer and battery.
The measurement of insulation resistance can also use an external dc supply. In that case, take the voltage and current readings with a dc voltmeter and a micro-range dc ammeter.
The insulation resistance then follows ohm’s law
where V is the voltmeter reading and I is the ammeter reading.
The ammeter is micro-ranged because only a very small current passes through the insulation during the test. At the moment of voltage application, that microammeter must also carry the initial capacitive charging current and the absorption current. The ammeter must withstand both of those currents at the start. The voltmeter, ammeter and source must also withstand the short-circuit current if the insulation fails during the measurement.
When a direct-indicating ohmmeter or megger is used, connect the instrument leads across the insulation under test. After the instrument is driven, the insulation resistance appears on the analog or digital dial.
In both methods above, take the reading after a standard time delay so charging and absorption currents have fallen and the result can be compared with other timed readings.
