10 Questions You Should to Know about insulation resistance tester

Good afternoon all, hope everyone is doing well.

When using an insulation tester on a powercable and motor that's fully disconnected from a servo drive, the readings should be OL from each phase to ground to ensure cable insulation is good and windings in motor are good.

However, when testing between phases, I have found some discrepancies and wanted clarification. Some of our maintenance guys say it's okay to use an insulation tester between phases to see if a motor is bad or not. I have read online that I should only use a regular ohm meter to test between phases. What is the correct method to test b/w phases?

I tested this on a huge 100lb Fanuc spindle motor (a06b--b230, 240VAC 3phase) we are shipping for repair for mechanical issues. Using a megger on both 250V and 500V, between any two phases shows >250 Megaohms and >550 megaohms respectively, however using an ohm meter, I see 0 ohms between any two phases. The guy from Fanuc says it's possible to damage the windings using an insulation tester b/w phases, so the repair shop will check on that too for me.

However, I distinctly remember testing a 480VAC 3phase allen bradley motor for a grinder infeed wheel with a megger between any two phases showed 0 ohms using 500V option.

So both of my experiences b/w Fanuc and AB motors are conflicting, and the feedback I have received from my colleagues and OEMs is also conflicting, so hoping someone could help clarify for me? Thanks a bunch. to check motor winding
Low Ohms motor lead T1 to motor Lead T2
high Ohms an Lead to ground or chassis
the feed wires should be disconnected from both the starter and the motor
and each lead should be checked separately (Lead to Ground)
motor windings should be only a few Ohms < 5 or so depending on the motor
ever ever use a high pot tested on a motor the can blow a good motor ( I have seen it happen )
It takes some experience to do it right
the best place for this information is AEMC
they make the best I have used many of their test interments over the years

Originally Posted by SecondGen A senior tech once told me the Megger guard terminal is the same potential as the negative terminal. I'm not sure if this holds true with modern test equipment, but I think this should be taken into consideration when guarding equipment that could be damaged by a high test voltage, such as a low voltage transformer winding.

As an example, let’s say you were testing a 13.2-0.48kV transformer. If the guard terminal was at the same potential as the negative terminal, it would not be a good idea to guard the 480V winding while doing a 5kV test with the negative lead on the primary winding. You would effectively be putting 5kV on the low voltage winding. In a special case like this, it would be advisable to use your positive lead on the primary side to protect guarded equipment from high voltages.

A 1kV test, for example, will apply +500VDC on positive terminal and -500 VDC on negative terminal, when one of the leads becomes grounded, the other will see +/- VDC. If the negative lead is not connected to ground the guard would also be at +/- VDC.

For this reason, when testing transformers, I will use the positive terminal for the windings under test.

I would recommend you take a look at "A Guide to Diagnostic Testing Above 1kV" by Megger for some really good technical info on insulation resistance testing. http://www.biddlemegger.com/biddle/5...ticTesting.pdf Originally Posted by suchagrrl In case anyone else is looking for this, I contacted Megger. This was the reply I got.

There is more flexibility in the manner of lead hookup than is commonly supposed. In the absence of any other considerations, industry standard is minus (-) to circuitry, plus (+) to ground. (Various labelling conventions apply to different models of tester; some are designated “L�, for “line�, and “E�, for “earth�.) This configuration baffles some operators, depending on familiarity with conventions employed in other types of testing. In most cases, it doesn’t matter; the same resistance reading will prevail if the leads are exchanged. However, it has been observed that certain types of exotic insulating materials (e.g., some ceramics) yield different readings depending on the test lead configuration. In such instances, it has been observed that the aforementioned configuration yields the lower of the two readings. This is the desired of the two readings because insulation testing is generally concerned with safety, maintenance, and troubleshooting, and therefore the worst case reading would be the one which yields the most relevant information. Adopting a standard procedure for lead hookup relieves the operator from having to establish a specific knowledge of every type of material to be encountered as to whether it exhibits this effect, and prevents the less-informative higher reading from being inadvertently accepted as the final test result.
Additionally, some authorities assert that the reverse hookup can cause small amounts of contaminants to be carried into the insulation with the leakage current, whereas the accepted configuration would have the opposite effect.
Stated more specifically, if testing wire or cable, the minus lead would go to conductor(s), the positive to ground, shield, armor, or conduit. In extreme cases like direct-buried single conductor, a ground rod can be driven into the soil in the vicinity of the test, and the positive lead connected to it. The additional resistance of the soil as the leakage current travels to the rod is irrelevant when compared to the resistance of the insulation. With motors, generators, and transformers, the negative lead is to windings, the positive to case. With electrical tools and other equipment, negative is to circuitry, positive to frame.
However, the operator has additional freedom to employ other hookup configurations. Just be careful to avoid inadvertent continuity tests when elements thought to be isolated are in fact connected. Be familiar with the basic wiring diagram of the test item. Remember, there is supposed to be an insulation barrier between the two elements to which the leads are connected. As an example, wire and cable can be tested hot-to-neutral or phase-to-phase, but don’t forget to disconnect at the other end of the circuit. Otherwise, it’s only a high-voltage continuity test, and the resultant “zero� reading will be misinterpreted as indicating faulty cable. Worse, if there is equipment left connected, you could end up sending a high voltage through its circuitry.
The operator is free to make a judicious choice whether to test the entire piece of equipment as a single test, or to sectionalize. As an example, hot and neutral conductors can be clipped together and tested to ground; similarly, with three phases. Or, each conductor can be tested separately, either to ground or between each other. The choice is largely the operator’s, but standard procedure is to do a complete test first, then proceed with sectionalized tests only if the first test resulted in an unsatisfactory reading. Remember, testing the entire piece of equipment at once yields a worst case result, because electrically, the insulation is only as good as its weakest point. If the entire piece tests “good�, its individual elements will read even higher.
Finally, many models have a third terminal. This is a guard, not a ground, as operators sometimes misinterpret from the “G� designation. Connecting it to ground will only serve to short-circuit the test and give an invalid reading. Its actual purpose is to act as a shunt circuit to remove parallel leakage paths from the measurement. If the test item has more than one leakage path in parallel, one can be shunted around the measurement circuit by connecting it to the guard, leaving a more specific measurement of the other path. Therefore, the guard acts as an extra diagnostic tool to permit more depth and scope to analytical testing and troubleshooting. A reasonably thorough knowledge of the test item is required, but when employed, the guard can yield invaluable detail.


Please let us know if you have any additional questions.

Regards,
Brian

Brian Hammerschmidt
Applications Specialist
Megger
Valley Forge Corporate Center
Van Buren Ave.
Norristown, Pennsylvania - USA.