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Mad as Hell Member
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Discussion Starter · #1 ·
Scenario:

  • Motor driving a pump tripping 60a GFCI breaker.
  • Electrician on-site uses a digital multimeter to verify cable/motor integrity. Multimeter shows no faults.
  • Electrician on-site then bypasses GFCI breaker with temporary cable laid on the ground, 60a (non GFCI) breaker trips instantly.
What is the maximum amount of resistance a typical multimeter can read?

Would a megger have shown the fault?

If there is a fault large enough to trip a 60a non GFCI breaker, how can that fault not be scene by a DMM?


This wasn't me on site, so I am unsure of exactly where my guy measured from (line to line, line to ground, etc.). It's very possible he measured line to line and had a low reading, but thought it was normal.
 

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Scenario:

  • Motor driving a pump tripping 60a GFCI breaker.
  • Electrician on-site uses a digital multimeter to verify cable/motor integrity. Multimeter shows no faults.
  • Electrician on-site then bypasses GFCI breaker with temporary cable laid on the ground, 60a (non GFCI) breaker trips instantly.
What is the maximum amount of resistance a typical multimeter can read?

Would a megger have shown the fault?

If there is a fault large enough to trip a 60a non GFCI breaker, how can that fault not be scene by a DMM?


This wasn't me on site, so I am unsure of exactly where my guy measured from (line to line, line to ground, etc.). It's very possible he measured line to line and had a low reading, but thought it was normal.
What was the temp cable for?
 

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isolate motor from cable. assuming this is a 3 phase motor? have to take phase to phase resistances on the motor, as well as phase to ground for motor. do the same for the supply cable and starter. Megger will show faults that a multimeter won't pick up, also, depending on the model of multimeter it may only read up to 1,000 ohms, then indicate infinite resistance for anything over that. (Fluke clampmeter comes to mind for that...)
 

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Scenario:

  • Motor driving a pump tripping 60a GFCI breaker.
  • Electrician on-site uses a digital multimeter to verify cable/motor integrity. Multimeter shows no faults.
  • Electrician on-site then bypasses GFCI breaker with temporary cable laid on the ground, 60a (non GFCI) breaker trips instantly.
What is the maximum amount of resistance a typical multimeter can read?

Would a megger have shown the fault?

If there is a fault large enough to trip a 60a non GFCI breaker, how can that fault not be scene by a DMM?


This wasn't me on site, so I am unsure of exactly where my guy measured from (line to line, line to ground, etc.). It's very possible he measured line to line and had a low reading, but thought it was normal.

Faults on motors that only show up under full operating conditions are not unknown !

The conditions under load would be completly different to what a battery powered meter could deliver.

If it's not in the cables to the unit ? then it could only be in the motor itself !
 

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It is possible to have a multimeter (DMM) read an acceptable reading and a megger show a fault. But generally after tripping a CB a few times as the electrician did the DMM will show the fault due to the additional tracking or insulation damage from the faults.
 

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Scenario:

  • Motor driving a pump tripping 60a GFCI breaker.
  • Electrician on-site uses a digital multimeter to verify cable/motor integrity. Multimeter shows no faults.
  • Electrician on-site then bypasses GFCI breaker with temporary cable laid on the ground, 60a (non GFCI) breaker trips instantly.
What is the maximum amount of resistance a typical multimeter can read?

Would a megger have shown the fault?

If there is a fault large enough to trip a 60a non GFCI breaker, how can that fault not be scene by a DMM?

Open motor start capacitor? Your guy would not see this with a DVM checking for resistance values.
 

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A Dmm ohm meter output is less than 9 volts so any resistive fault may not show up until some higher voltage level is achieved. A meggar can have a 500 volt output so it will break down resistive faults.
A GFCI breaker trips at 6ma and many pumps in normal operation can leak more than 6ma.
Why there a GFCI? what is the pump motor running? Could a gfi do the job? (it has a higher trip setting than 6ma, but does not provide life safety, so not in a hot tub.
 

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Donuts > Fried Eggs
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...What is the maximum amount of resistance a typical multimeter can read?
Often many megaohms, but they don't have enough voltage to push through anything but a bolted fault, so it will likely miss it.
Would a megger have shown the fault?
Depends on the fault. If you can't separate the windings you'll never see an fault between them, and even if you can, no megger can pick up an intrawinding short.

Also, if you have a mechanical failure preventing the rotor from turning, no electrical test will show it but you'll still pull LRA and dump the breaker.
If there is a fault large enough to trip a 60a non GFCI breaker, how can that fault not be scene by a DMM?
See above.
 

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Mad as Hell Member
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Discussion Starter · #11 ·
So aside from closing the breaker into a potential fault, what's the best way to write off a motor in the field?
 

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Often many megaohms, but they don't have enough voltage to push through anything but a bolted fault, so it will likely miss it.

I have seen meggers that claim they put 500 VDC on a cable or motor. Will these still pick it up or not? I never understood the voltage part of meggers fully.



Depends on the fault. If you can't separate the windings you'll never see an fault between them, and even if you can, no megger can pick up an intrawinding short.

Would an ohm meter help?


Also, if you have a mechanical failure preventing the rotor from turning, no electrical test will show it but you'll still pull LRA and dump the breaker. See above.
All excellent points btw.:)
 
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