I was recently on a PQ investigation where another PQ consultant was in the middle of a lengthy investigation into possible circulating ground currents (I was there as a kind of second opinion). In the course of my conversation with the consultant he informed me that when he had brought in a contractor to bond the building steel he specifically instructed them not to bond the motor frames to the building steel because it would violate article 250 of the NEC. I am not an expert in the application of the NEC but I don't think what he said is correct. The motors are provided with a grounding conductor which provides for fault clearing. He went on to say that bonding the motor frames would create ground loops. Could any one clear this up for me? I have always been under the impression that if you had a facility with a well bonded steel structure that it was a good practice to bond the exposed metal parts (such as motor frames)to the steel.
Bonding of Motor frames to building steel
- Thread starter jlmott
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- Location
- Bremerton, Washington
Re: Bonding of Motor frames to building steel
What would the purpose be for bonding the motor frames to the building steel?
What would the purpose be for bonding the motor frames to the building steel?
kentirwin
Senior Member
- Location
- Norfolk, VA
Re: Bonding of Motor frames to building steel
It's done where I work for static elimination. It's seldom the motor itself, but the motor frame gets bonded to building steel anyway via the frame of the machine.
It's done where I work for static elimination. It's seldom the motor itself, but the motor frame gets bonded to building steel anyway via the frame of the machine.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: Bonding of Motor frames to building steel
There is nothing inherently wrong with bonding metal parts together redundantly. It probably will have little or no effect on anything though.
The only way you would get circulating currents through these bonds would be if there was an extra bond to a neutral somewhere. That would be a bad idea, and not code compliant, and would lead to circulating currents.
I see a lot of control panels coming through our shop with ground lugs for really huge wires. I have to wonder what these people are thinking, putting a 4/0 ground wire on a control panel fed by a 15A circuit.
I have asked a few times in the past what the purpose is of such a large EGC. I inevitably get either s shrug or the answer "noise". I have to guess there are people who think you can reduce electrical noise by making the ground wire bigger. I'm not sure where they got that idea, but it seems to be widespread.
A very good question. Although the idea that you should not do it because it would cause circulating currents is a dud as well.
There is nothing inherently wrong with bonding metal parts together redundantly. It probably will have little or no effect on anything though.
The only way you would get circulating currents through these bonds would be if there was an extra bond to a neutral somewhere. That would be a bad idea, and not code compliant, and would lead to circulating currents.
I see a lot of control panels coming through our shop with ground lugs for really huge wires. I have to wonder what these people are thinking, putting a 4/0 ground wire on a control panel fed by a 15A circuit.
I have asked a few times in the past what the purpose is of such a large EGC. I inevitably get either s shrug or the answer "noise". I have to guess there are people who think you can reduce electrical noise by making the ground wire bigger. I'm not sure where they got that idea, but it seems to be widespread.
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