# Vaporized Service Entry Breaker Fuse



## Overwelmed (Mar 14, 2013)

I have a customer with a 250HP deep well pump on a VFD. They had been running the motor at less than 60hz for several hours then sped it up to 60hz. Shortly after running at full speed the motor/drive tripped off line and would not restart. This was observed via SCADA. An inspection of the circiutry revealed one leg of the service entry breaker terminals at an auto transfer switch had melted the lugs and vaporized its fuse. It had also melted the ground strap for the S.E.B. I disconnected the motor from the VFD and megged it at 1000v and got >6 Gohms on each leg. Also tested phase to phase resistance and got .1 ohms between each. The electrcian and VFD rep are saying the the drive has short circiut faults and the motor is to fault for the above damage. I am have a hard time believing that a short in the motor could cause damage to the entrance side of the VFD. I would think that it would damage the VFD or trips it's breaker before allowing that kind of damage. Any feedback would be appreciated.


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## HARRY304E (Sep 15, 2010)

Overwelmed said:


> I have a customer with a 250HP deep well pump on a VFD. They had been running the motor at less than 60hz for several hours then sped it up to 60hz. Shortly after running at full speed the motor/drive tripped off line and would not restart. This was observed via SCADA. An inspection of the circiutry revealed one leg of the service entry breaker terminals at an auto transfer switch had melted the lugs and vaporized its fuse. It had also melted the ground strap for the S.E.B. I disconnected the motor from the VFD and megged it at 1000v and got >6 Gohms on each leg. Also tested phase to phase resistance and got .1 ohms between each. The electrcian and VFD rep are saying the the drive has short circiut faults and the motor is to fault for the above damage. I am have a hard time believing that a short in the motor could cause damage to the entrance side of the VFD. I would think that it would damage the VFD or trips it's breaker before allowing that kind of damage. Any feedback would be appreciated.


Not all breakers do what they should in time to prevent this sort of damage.


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## oliquir (Jan 13, 2011)

humm pretty sure the line entrance fault (loss of phase) kill the vfd and not the motor :blink:
anyway a motor is not hard to test for motor specialist.


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## Steve01 (Mar 8, 2013)

Overwelmed said:


> It had also melted the ground strap for the S.E.B. I disconnected the motor from the VFD and megged it at 1000v and got >6 Gohms on each leg.


If the ground strap burnt because of a ground fault from the motor then the ground wire may be open somewhere else. What I am getting at is the integrity of the ground wire to the motor could mess up your megger result. Of course that depends on where you connected with the megger. It may be something you already checked but I thought it was worth putting out there.


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## Overwelmed (Mar 14, 2013)

Thanks for the replies. I megged the motor at the motor j-box and connected the ground to the motor. We are hiring a motor repair shop to come out to test the motor on site.


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## Overwelmed (Mar 14, 2013)

Would the vfd register the short circuit fault if it occurred on the line side?


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## Jabberwoky (Sep 2, 2012)

What is the make/model of the vfd? Generally short circuit faults are drive output. Faults on the line inputs are usually low line, line loss or phase loss. On an input I don't ever recall there being any ground fault detection. As far as the drive knows the line inputs are either there, low, or not there. The manual for the drive should have a list of all the possible fault codes and a brief description of what may cause them.

The only time I have ever seen an input fuse vaporized was when a line regen IGBT on the convertor failed spectacularly. Blew a hole through the cover and caught the drive and cabinet on fire. Even more amazing was that they paid us to rebuild it.


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## JRaef (Mar 23, 2009)

Overwelmed said:


> I have a customer with a 250HP deep well pump on a VFD. They had been running the motor at less than 60hz for several hours then sped it up to 60hz. Shortly after running at full speed the motor/drive tripped off line and would not restart. This was observed via SCADA. An inspection of the circiutry revealed one leg of the service entry breaker terminals at an auto transfer switch had melted the lugs and vaporized its fuse. It had also melted the ground strap for the S.E.B. I disconnected the motor from the VFD and megged it at 1000v and got >6 Gohms on each leg. Also tested phase to phase resistance and got .1 ohms between each. *The electrcian and VFD rep are saying the the drive has short circiut faults and the motor is to fault for the above damage.* I am have a hard time believing that a short in the motor could cause damage to the entrance side of the VFD. I would think that it would damage the VFD or trips it's breaker before allowing that kind of damage. Any feedback would be appreciated.


I'm having a hard time decyphering the highlighted sentence.

Are you saying that the VFD has _records of having_ short circuit faults and the motor is _AT_ fault for the above damage?

If the VFD is UL listed, it must provide Short Circuit protection for the motor because you are right, something that happens on the motor side does NOT show up on the line side (unless there is a catastrophioc meltdown of the VFD, i.e. it becomes a pool of molten metal in the bottom of the box and the wires are still in contact). So years ago, UL addressed this by requiring the VFDs to be UL listed AS the SCPD of the motor. What is put on the line side of the VFD is only there to protect the VFD circuit, (hence the new rules on conductor sizing to VFDs starting in about 2005). But that is for the CONDUCTORS, no OCPD device up stream of a VFD is going to act fast enough to protect the VFD components. They sell these high speed "semiconductor" fuses that supposedly do, but I have never seen where it prevents the damage. Usually the VFD is damaged, THEN the ultra expensive fuse blows, so the entire event costs even more. 

If the VFD HIM shows that there were short circuits on the motor side, the drive SHOULD have tripped off. If it did not, then someone has turned off that protection feature and they should not have.

But that still would not explain the melting of the lug on the line side. More likely, the lug was not sufficiently tight and over heated, then melted, which went to ground, which fried the ground strap, which means the ground reference for the VFD was lost, which the VFD may have misinterpreted.

Another possibility that I see a lot is that they used extra flexible cable (aka DLO Cable) from the ATS to the VFD because it is easier to run for large power devices like drives and ATS, but the lugs were not rated for using that. Some electricians unaccustomed to large power installations are unaware that you CANNOT use that kind of cable in standard mechanical lugs, you are supposed to use crimp lugs and they (and their dies) must be SPECIFICALLY rated for the DLO cable size used. What happens then is that because they are not properly compressed, the harmonics in the VFD circuit heat up those extra fine wires in the cable due to skin effect issues, the heat combines to over heat the lug and it's all down hill from there. I've seen that time and time again. If the proper crimps and dies are used, the fine strands become a solid mass and this doesn't happen.

Also, if they did use DLO, people often mistakenly read the Amperage ratings of it and think it is higher than it is technically rated for in an enclosed controller. The cable was intended for running on the ground or in air between locomotives, so the ratings are for 90C in free air. But if you read the NEC, when used in aceway, the ratings are the same as THHN (RHW actually) and are based on 75C.


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## Ultrafault (Dec 16, 2012)

Dear jraef I want to just publicly announce you are my new role model. I want steal your brain man youd better look out.


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## Big John (May 23, 2010)

Regardless for the reason of the lug overheating, I agree this is what happened:


JRaef said:


> ...More likely, the lug was not sufficiently tight and over heated, then melted, which went to ground, which fried the ground strap....


 Line side ground fault. You saw the evidence of the origin, you saw the evidence of the return path, and you saw it take out the associated protection.

I don't know what this grounding point was made out of, but I would not expect a motor ground fault that was severe enough to start melting conductors to not show up on a megger.


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