Insulation resistance measurement is done using an IR tester. This is a portable tool that is more or less an ohmmeter with a built in generator that’s used to produce a high DC voltage. The voltage usually measures at least 500V, and causes a current to flow around the surface of the insulation.
How insulation resistance test is done?
To calculate the IR value, apply the voltage, measure the steady-state leakage current (after the dielectric absorption and charging currents have decayed to zero) and then divide the voltage by the current. If the insulation resistance meets or exceeds the required value, the test is successful.
What is a good insulation resistance test?
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.
How do I test my insulation with a Megger?
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.
What does a reading of 0 ohms mean?
Ohmmeter is an instrument used for measuring resistance of a component or a circuit. It indicates zero ohms when there is no resistance between the test points. This shows continuity of current flow in a closed circuit. It indicates infinity when there are no connections in the circuit that is as in an open circuit.
What are acceptable megger test readings?
Turn on and read the meter. Anything reading between 2 megohms and 1000 megohms is usually considered a good reading, unless other problems have been noted. Anything less than 2 megohms indicates an insulation problem.
Why do we do megger test?
Megger testing measures resistance between the phase windings of the motor under test and ground. If there is an insulation breakdown, the resistance is negatively impacted and there is a departure between phases or between one measurement and a previous one and a determination of motor health is made.
What is a megger reading?
The Megger insulation tester is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms. For good insulation, the resistance usually reads in the megohm range.
What is the minimum acceptable insulation resistance?
The rule states that insulation resistance should be approximately one megohm for each 1000 volts of operating voltage, with a one megohm minimum. (By following this rule, a motor rated at 2400 volts should have a minimum insulation resistance of 2.4 megohms).
Which equipment is used for insulation resistance measurement?
A multimeter is used to measure current, voltage and resistance.
Why do we test insulation resistance?
The purpose of the tests are to verify that the insulation of conductors and electrical accessories is satisfactory and that live conductors or protective conductors are not short-circuited, or do not show a low insulation resistance.
Is insulation resistance a dead test?
Insulation Resistance – to test whether there has been any breakdown in insulation to the installation. Polarity – to check that the installation is correctly wired.
Electrical Testing Sequence.
| Dead testing Electrical supply disconnected | |
|---|---|
| A | Continuity of protective conductors including main and supplementary equipotential bonding |
| G | Polarity |
How do you set up a megger?
How to Connect a Megger
- Disconnect the item being tested from other circuitry and the power supply if possible.
- Connect one test lead of the megger to the insulation and the other test lead to the conductor.
- Rotate the crank until the slip clutch begins to slip.
- Note the reading from the meter.
How many ohms should a good wire have?
How many ohms should a wire have? Typically, good wire connections have a resistance of less than 10 Ω (often only a fraction of an ohm), and isolated conductors offer a resistance of 1 MΩ or greater (typically tens of megohms, depending on humidity).
What is difference between resistance and continuity?
Think of it this way: Continuity is a binary version of resistance. If the resistance of the thing we’re testing—the wire we want to make sure isn’t broken, the connection we want to be certain actually goes to ground, the switch we want to know works—is low (like less than 1 ohm), we say that it has continuity.
What is the difference between ohms and resistance?
Resistance is a measure of the opposition to current flow in an electrical circuit. Resistance is measured in ohms, symbolized by the Greek letter omega (Ω). Ohms are named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current and resistance.
When using a Megger you must avoid touching?
While measurement of insulation resistance by megger, both leads at the same time must not be touched. When a megger is used, the generator voltage is present on the test leads. This voltage could injure you or damage the equipment you are working on.
What causes low Megger readings?
Two conductors have consistently lower values: Because the conductors adjacent to the ground conductor only have one layer of insulation, those conductors will have lower insulation resistance and the Megger values will be lower.
What is IR value?
IR Value for Transformer. Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual windings. Insulation resistance tests are commonly measured directly in megohms or may be calculated from measurements of applied voltage and leakage current.
Can a megger shock you?
The megger supplies over 500 volts, and that is enough to give a fatal shock. What I do not know is the internal impedance of the megger. The test is looking for very small amounts of leakage current. The internal impedance of the device might be high enough to prevent the current from reaching a harmful level.
How many types of megger are there?
A Megger insulation tester can be used to perform three different types of insulation resistance tests, short-time or spot-reading, time-resistance and dielectric absorption ratio. Each test varies with results and is used for particular purposes.
