# 480v 6ohz to 400/230v 50 hz rotary converter grounding



## Shorty Circuit (Jun 26, 2010)

I've got a client who bought equipment (including some with motors) that requires 400/230V 50 hz power. I've decided that a rotary m-g set would be best. Question about bonding the generator neutral to the ground. If I use only building steel as though it was a 60 hz wye connected transformer do I risk possible problems with circulating ground currents in the steel structure from ground loops where the local point of ground connection might not be at the same lowest voltage to true ground potential as the existing service entrance? I'm thinking if I can't tie back the grounding conductor for the generator to the existing service entrance ground I might run into problems if I don't establish a new earth ground as though this was a separate service entrance. I'm concerned about both the 50 hz generator neutral and the electronic controls. This would mean additional ground rods, building steel, welding to rebar and underground copper pipe if it's available. If I establish a new ground reference should I install an equalization ground conductor between the two ground reference points, say by connecting them with a 2/0 copper ground wire? Anyone have any experience one way or the other with this type of separately derived system?


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## Tom45acp (Sep 6, 2011)

Grounding of separately derived systems is covered in 250.30 and to my knowledge, the frequency of the system has nothing to do with meeting the requirements. As long as your building steel meets the requirements of a grounding electrode (see 250.52(A)(2), you should be good to go.


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## Shorty Circuit (Jun 26, 2010)

Tom, thank you very much for your response. This is not a code issue. I fully understand the code requirement to ground the neutral of the separately derived system. The question relates to how quiet (clean) the ground must be for the equipment to perform satisfactorily. I usually don't concern myself much with isolated grounds, ground loops, relatively small circulating ground currents that some IT people are obsessive about. However in this case I was wondering. Some of the manufacturers I've contacted have expressed some concern as well, most don't know and haven't gotten a response from their own technical people yet. My hunch is that my concern is probably not justified but I thought I'd just ask.


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## brian john (Mar 11, 2007)

You are asking if the converter will cause grounding issues? I wold say no. Even if the convertor had some weird factory internal ground the currents would only circulate between the converter and the point of the ground of the neutral. Though I doubt this would happen or be an issue.

AS you know you have to ground the neutral and no matter where you ground it to the facilities grounding point, all grounds will be common.


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## Shorty Circuit (Jun 26, 2010)

brian john said:


> You are asking if the converter will cause grounding issues? I wold say no. Even if the convertor had some weird factory internal ground the currents would only circulate between the converter and the point of the ground of the neutral. Though I doubt this would happen or be an issue.
> 
> AS you know you have to ground the neutral and no matter where you ground it to the facilities grounding point, all grounds will be common.


Again perhaps I did not express my concerns clearly enough. I'm not worried that the frequency converter will cause grounding issues but that will will be adversely affected by already exisiting circulating ground currents in the building steel. If you have ever dealt with IT or A/V technicians they always worry about this kind of thing. That's why they sometimes insist on isolated grounding schemes. For ground currents to circulate in building steel, different parts of a building must be at different potentials. This is rarely if ever a safety concern, the one area NEC purportedly addresses itself to because at any point in the structure the steel should never be more than a few volts above the service entrance ground except during an upset transient from an electrical equipment failure before the fault is cleared. However, for proper operation of equipment it can be a legitimate concern in some cases. I'm just wondering if this can be one of those instances. The answer is probably no but I've been asking if anyone has had experience with this type of equipment one way or another.


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## brian john (Mar 11, 2007)

Shorty Circuit said:


> Again perhaps I did not express my concerns clearly enough. I'm not worried that the frequency converter will cause grounding issues but that will will be adversely affected by already exisiting circulating ground currents in the building steel. If you have ever dealt with IT or A/V technicians they always worry about this kind of thing. That's why they sometimes insist on isolated grounding schemes. For ground currents to circulate in building steel, different parts of a building must be at different potentials. This is rarely if ever a safety concern, the one area NEC purportedly addresses itself to because at any point in the structure the steel should never be more than a few volts above the service entrance ground except during an upset transient from an electrical equipment failure before the fault is cleared. However, for proper operation of equipment it can be a legitimate concern in some cases. I'm just wondering if this can be one of those instances. The answer is probably no but I've been asking if anyone has had experience with this type of equipment one way or another.


And I answered that question, in my experience no. A/V techs do not know ANYTHING about grounding, they may think they do. But all their knowledge, in my experience is based upon a belief in magic.

As for the ground being a certain voltage above neutral voltage, or less than phase voltage, this is a fact YOU CANNOT AVOID in a distribution system, it has to do with voltage drop in the system. Of course you could put a isolation transformer at every point of use.


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## Shorty Circuit (Jun 26, 2010)

brian john said:


> And I answered that question, in my experience no. A/V techs do not know ANYTHING about grounding, they may think they do. But all their knowledge, in my experience is based upon a belief in magic.
> 
> As for the ground being a certain voltage above neutral voltage, or less than phase voltage, this is a fact YOU CANNOT AVOID in a distribution system, it has to do with voltage drop in the system. Of course you could put a isolation transformer at every point of use.


This problem does not relate to A/V or even IT equipment. It relates to the question of whether or not volatges at sum and difference frequencies generated by leakage current in either or both systems of two different frequencies could affect the performance of the 50 hz MG set. The correct answer is that nobody knows, there's no way to tell in advance if this could happen or not. It depends on a large number of circumstances particular to both the MG set selected including its control systems and to other circumstances of the building electrical distribution system and the loads. Especially worrying would be a 10 hz difference voltage that the control system's power supply is probably not designed to filter out. 

I've decided that the best way to handle this is to add an equalization ground conductor between the service entrance ground and the steel at a point close to where the generator neutral is to be grounded. This will act as a low impedance shunt for any circulating ground currents and will put the generator neutral at a very close potential to the cleanest ground available in the building. It is cheaper than a whole new service entrance ground and less risky than ignoring the issue for the time being wondering if and when it will crop up as an unexplainable problem.


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## RIVETER (Sep 26, 2009)

Shorty Circuit said:


> I've got a client who bought equipment (including some with motors) that requires 400/230V 50 hz power. I've decided that a rotary m-g set would be best. Question about bonding the generator neutral to the ground. If I use only building steel as though it was a 60 hz wye connected transformer do I risk possible problems with circulating ground currents in the steel structure from ground loops where the local point of ground connection might not be at the same lowest voltage to true ground potential as the existing service entrance? I'm thinking if I can't tie back the grounding conductor for the generator to the existing service entrance ground I might run into problems if I don't establish a new earth ground as though this was a separate service entrance. I'm concerned about both the 50 hz generator neutral and the electronic controls. This would mean additional ground rods, building steel, welding to rebar and underground copper pipe if it's available. If I establish a new ground reference should I install an equalization ground conductor between the two ground reference points, say by connecting them with a 2/0 copper ground wire? Anyone have any experience one way or the other with this type of separately derived system?


Ground is Ground. It is attaching Mother Earth to an electrical system. The ONLY reason that a person would do that is to give ANY aberrant voltage that may occur in the system, a path to where it's opposite polarity is satisfied, whether it be a lightning strike, or a primary to secondary short at the service transformer. Most buildings that are supported by steel columns are sufficiently grounded via rods and grids at their base. You cannot establish a NEW earth ground that would be any better than a properly installed FIRST ground...but it does not hurt to have redundancy.


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## brian john (Mar 11, 2007)

Shorty Circuit said:


> This problem does not relate to A/V or even IT equipment. It relates to the question of whether or not volatges at sum and difference frequencies generated by leakage current in either or both systems of two different frequencies could affect the performance of the 50 hz MG set. The correct answer is that nobody knows, there's no way to tell in advance if this could happen or not. It depends on a large number of circumstances particular to both the MG set selected including its control systems and to other circumstances of the building electrical distribution system and the loads. Especially worrying would be a 10 hz difference voltage that the control system's power supply is probably not designed to filter out.
> 
> I've decided that the best way to handle this is to add an equalization ground conductor between the service entrance ground and the steel at a point close to where the generator neutral is to be grounded. This will act as a low impedance shunt for any circulating ground currents and will put the generator neutral at a very close potential to the cleanest ground available in the building. It is cheaper than a whole new service entrance ground and less risky than ignoring the issue for the time being wondering if and when it will crop up as an unexplainable problem.


And did you read what I posted

*NO*

As stated above several times ground is ground.


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## Shorty Circuit (Jun 26, 2010)

brian john said:


> And did you read what I posted
> 
> *NO*
> 
> As stated above several times ground is ground.


You an make the letters as large as you like but experience with many different kinds of systems has demonstrated time and again that that is not necessarily true and can be a dangerous assumption. What appears to be a good ground in some systems or circumstances can be a very poor one in others and there are many things that can happen within a grounding system that wouldn't be possible if all points on a grounded system were equivalent. I think think this discussion is now closed, at least as far a I am concerned.


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## brian john (Mar 11, 2007)

Shorty Circuit said:


> You an make the letters as large as you like but experience with many different kinds of systems has demonstrated time and again that that is not necessarily true and can be a dangerous assumption. What appears to be a good ground in some systems or circumstances can be a very poor one in others and there are many things that can happen within a grounding system that wouldn't be possible if all points on a grounded system were equivalent. I think think this discussion is now closed, at least as far a I am concerned.


Why have you asked a question, when you know the answer you want?

I have worked on more convertors, perverters and diverters, than you can shake a stick at, WHEN PROPERLY designed, when PROPERLY installed and the system is free of problems prior to installation, then you should have NO problems.

That is that answer based on 42 years of MY experience, and logic if YOU know what you are doing. So take pick the answer you want. 

Yes I will screw the world with my electrical magic.

Or no.


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