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Insulation Resistance (Megger)

This was also outlined in Non-destructive Testing, but because it was such a large topic, we've expanded it here. Any tester that uses resistance to analyze a circuit, conductor, winding, or ground wall is non-destructive testing and is invaluable to our troubleshooting/maintenance routines. This is also a top rated commissioning tool for us as well. The following items are not in any particular order, and each topic is merely an executive summary.

Please keep in mind tthat insulation readings can be affected by temperature and humidity. They are used to troubleshoot issues, maintenance checks on equipment and conductors, commissioning of new equipment to prove integrity and accuracy, and quality control in manufacturing.

Insulation can break down over time, get damaged, moisture damage, vibration, ambient temperatures, exposure to the elements, etc which makes an easy low resistance path for leakage current. This leakage current makes the cable inefficient, and when it's really deteriorated; a hazard.

A megger has a built in dc generator which has both current and voltage coils. This all comes together to produce a high range, resistance meter. (Ohmmeter). A rule of thumb is that an insulation resistance should be roughly 1MΩ for each 1,000 volts of operating voltage, with a minimum value of 1MΩ.

Insulation Resistance (Tips)

- Make sure humidity is lower than 70 percent.
- Make sure all switches are locked out
- Make sure all cable ends are marked for "test warnings" and high voltage
- A good reading will increase in value, then it will stay constant
- A potential bad reading will increase in value, then it will decrease
- A megger should never, ever be used in an explosive or highly flammable area
- Always put up high voltage signage
- Get a radio, or form of communication between employees so you can communicate procedures, and readings
- Disconnect all leads from earth or chassis before tests

Two or Three Lead Insulation Tests

Most insulation tests involve two leads. There are three lead tests, that have a guard terminal which acts as a shunt to remove a connected part or module from the reading. This is done to either protect the part, or to get an accurate reading without added resistors, circuit boards, etc affecting the test. The leads are labelled G, L, and E. Guard, Line, and Earth.

Three Lead Testing


Two Lead Testing

When doing a two lead test, the leads are normally placed between ground (black) and line (red) to get the reading. This is the most common test done for simple maintenance and troubleshooting tasks. Please read below to get in-depth directions on how to perform specific insulation tests.
This graphic may show a test through the line, but please remember that all tests should be performed with the breaker TURNED OFF so that you are not reading potential resistance items through the breaker, or other transfer switches and transformers. There is another portion of this site that outlines breaker injection testing.

Electric Motor Insulation Testing
The motor must be checked for winding imperfections, ground wall insulation, and circuit integrity.

Winding (Spot Test - which is a short reading) Disconnected Winding Leads

To start with you have to isolate the motor by disconnecting all sources of input voltage. Lock out the circuit. You cannot megger a live circuit.

Prove your ground by putting one lead on your chassis ground screw, the other on another chassis point that looks like its clear of paint or coatings, and will accept a clamp, then press test. It should read 0Ω. This ensures that the placement of your black lead (clamp) is an acceptable ground reading. Normally the clamps can't clip on to the chassis screw location, and the exterior usually has the presence of equipment paint, finishes, that can affect your readings. Getting your valid zero reference point is important to getting a correct reading. Especially if your conditions reflect a floating ground system.

Motor winding testing is done by putting the black lead of the megger (ground) to the chassis ground of the motor and the other lead (red) to one of the disconnected winding leads. This will only work in high voltage megger settings. Do not dial in your megger at low Ω. It must be at least with: 250V-100 MΩ, 500V-200MΩ,
or 1000V-400MΩ (this is a normal 3 range insulation tester)

It is also a NOTABLE mention here, that testing an electric motor exclusively with an insulation tester will only show a faulted path from the winding to ground. Most winding faults start off as internal winding short, then deteriorate to insulation faults, but that is not always the case. A non conductive substance between the winding and ground can give a high resistance reading that can skew results. An electric motor should have several different tests to ensure the overall health, and safety of its operation.

This is high voltage testing with high resistance. It is at this point that we will mention that the manufacture's specification will tell you what setting the insulation tests should be set to for specific equipment. ALWAYS go by manufacturer specifications for equipment AND conductor recommendations. If you do not have manufacture specifications, do not go higher than your system voltage during normal usage. This is a non-destructive test. Higher voltages than system voltages can cause damage to the insulation.

If your meter reads 0MΩ, when the red lead is connected to a motor winding and the black clamp connected chassis ground, then it is grounded (fault). If not, then you have a normal winding with a resistance reading that will look like one of the following.

 100 MΩ or more Excellent
 50-100 MΩ Very good
 10-50 MΩ Good
 5-10 MΩ Abnormal
 2-5 MΩ Critical
 2 MΩ or less Bad (Take out of Service)

Your paperwork must reflect a "benchmark" or "target" or "standard" to compare against. This is normally what the equipment or material's reading was when it was new or first installed. This will give you an idea of natural age (wear or depreciation). The test should only be conducted above dew point to prevent moisture from forming on the surface of the insulation. Temperature should be 20 degrees Celsius.

This is normally done when the motor is in maintenance condition. A low setting (Ω) will not pick up the faults you need to see, nor will they give you the proper reading you need to fill out your paperwork for the spot tests. This is normally done for 60 seconds.

Don't forget to short your leads to ground after the test to dissipate the charge left in the circuit. This is a safety precaution for yourself when reconnecting the leads, and for the equipment upstream or downstream (static charges can take out electronic boards, etc).