Megohmmeter Test

A megohmmeter, or megger, is a device used to test electrical and insulation resistance. This is usually done by sending a high-voltage signal into the object being tested, typically a wire or motor. Using a megger is important for avoiding electric shock and equipment damage.

What are good megohmmeter readings?

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 megger test is done?

The Megger test is a method of testing making use of an insulation tester resistance meter that will help to verify the condition of electrical insulation. Insulation resistance quality of an electrical system degrades with time, environment condition i.e. temperature, humidity, moisture and dust particles.

How does a megger test work?

Megger works on the principle of electromagnetic attraction. When a primary coil that is carrying current is placed under the vicinity of a magnetic field it experiences a force. This kind of force generates a torque that is made to deflect the pointer of the device which gives some reading.

What do megger readings mean?

If the megger reads a resistance under 1 (1,000 ohms) on your meter after the initial 60-second interval, the cable has failed and the cable should be removed. If the megger reads a resistance between 1-1.25 on your meter, then the cable passes. Any reading above 1.25 is considered excellent.

How do you megger a circuit?

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 is the minimum megger value?

Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm.

What is the minimum acceptable value of 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).

What does a 0 reading on an ohmmeter mean?

Resistance, symbolized by the (Ω) symbol and measured in Ohms, is a measurement of how well a current can travel through a circuit or a given path. A circuit with no resistance (0) would indicate a complete circuit, or one that has no short.

What causes low megger readings?

Longer runs will result in lower insulation resistance values. 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 the difference between megger and ohmmeter?

The chief advantage of the megger over an ohmmeter is its capacity to measure resistance with a high potential or breakdown voltage. Impedence of insulation is very high. A megger is specifically designed to accurately measure high impedences. An ordinary multimeter is designed to measure lower, more finite impedences.

How do you test insulation resistance?

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.

What is a good insulation reading?

Readings should be greater than 1MΩ for an Earthed system, or 2MΩ for Class 2 or “floating” systems. Ideally this reading should be as high as possible. PV150 can read up to 19.99MΩ, and the PV200/210 up to 199.99MΩ. A good result would be “>19.99MΩ” on the PV150, and “>199.99MΩ” on the PV200/210.

Can a megger damage a motor?

Any megger with a higher voltage output than 500 volts DC should be used by an experienced technician. A high voltage for too long of a time may further weaken or fail motor windings and the winding insulation could be damaged by the testing procedure.

Why megger is used instead of multimeter for insulation testing?

Megger is used to test the insulation resistance of a conductor while multimeters are used to measure the voltage,current and resistance of a circuit. Megger is por isolation measure.

Can you test insulation resistance with a multimeter?

You use your insulation multimeter to check the insulation resistance of each conductor to ground and of each conductor to the other two.

What is the megger reading in a dead short wiring installation?

If the megger pointer shows the infinity reading, it shows the best insulation. When the reading is less than infinity, it shows uncertain value of insulation. While zero reading shows the short circuit condition of the circuit.

What is a good Megger reading for a 480V motor?

IEEE recommends that for motor starting that the operational voltage, expressed as a meg-ohm value doubled plus 1000 is the minimum meg value for starting. For a 480V motor that would mean . 480Meg x 2= . 960 Meg + 1000, so a minimum ground resistance for starting a 480V is 1.960 Meg or round up to 2 meg-ohms.

What is a pass on insulation resistance?

The insulation test pass mark will be 1MΩ. This means that the Insulation reading must be over 1MΩ to Pass the test. The PAT tester will measure the resistance between the live parts (Live and Neutral) connected together and earth.

Can a Megger damage insulation?

Using a megger meter is not destructive. When insulation has been damaged, a megger tester is used in determining the cause by testing. The test is typically conducted at rates that are low as compared to the insulation.