Megger Test Voltages

[RobbieDicks]

We have been testing/inspecting 250 to 750 HP motors (mainly DC with separately excited fields) for several years at a customer of ours. Between my colleague and myself (both of us engineers) have developed a process to visually inspect the communtator, brush lengths, and motor insulation. We typically use a megger to test the insulation and have used the "twice the voltage" idea to safely perform the test. For example, if the output to the armature of the drive is 500VDC we output 1000V from the megger and record the results after a couple of minutes.
Likewise with the field, if the field voltage is 300VDC we output 500V from the megger.
Are these megger output values correct for testing DC motors?
I have yet to find a specific documented table of test voltages vs. motor voltages.

 

[rbulsara]

Check NETA- MTS specs..

 

[aolalde]

See IEEE Std-43 for Insulation Resistance Test.

 

[ziggyfrst]

I've been doing electrical for 29+ years. In my experience, I ahve always considered the over potential test to do no harm. Recently people in my organizations I've met believe that it does cause harm. Which is true?? I've read that recent technology shows that the megger test of cable may produce falose readings because it polorazizes tiny fissures. True??

 

[Zogzog]

I think they are thinking about a DC hipot test, DC hipot is considered destructive on field aged cables and should not be done, do a VLF or Tan delta instead. A megger test will not damage a cable when done properly and will give good evaluation data if doen right, most megger testing is not done properly.

 

[oldfieldguy]

I've had 5 kV cable in 4160 volt service fail under a 5000 volt megger test. Popped. Megger went down to zero. Changed preventive maintenance testing into a cable pull. But that is the only instance I remember in a career that tested hundreds of cables in 5 kV and higher ranges over a couple of decades.

Still, most well thought out test regimens specify a megger test before applying the much higher voltage of a hipot test for cables.

As far as motors are concerned, 1000 volts for a 480 volt motor is the standard approach, but again, if you suspect your motor to be wet, dirty, or damaged, a quick test at a lower voltage won't hurt, either.

They still pay us to think about what we're doing, after all...



old field guy

 

[Zogzog]

If your megger test fails your cable is bad, the test does not cause cable damage.

 

[Skogsgurra]

Agree. A 4160 cable should stand a 5 kV megger test. That cable was already dead. Applying 4160 to it would probably have caused breakers to pop. Why it didn't before shut-down? Because it self-heated and kept insulation up.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oldfieldguy]

Zog & Skog (Weren't they a comedy team?)

That's what I told them: "Better it should fail now than at two AM on a Saturday morning in the midst of a critical production effort..." Somehow that statement did little to soothe the guy who had to set up for the replacement job.

NETA Maintenance Test Specification 2005 gove the DC voltage for insulation tests for 600-volt class equipment as 1000 VDC.

Again, though, a thoughtful approach to investigating a piece of equipment that was functional but that also might be badly deteriorated is to use a lower voltage for evaluation purposes.

More than once I've told a client "It's getting pretty bad. You REALLY want to start lining up resources to replace this thing because it hasn't failed YET, but it's looking like it could go at any time..."

old field guy