# That'll never work...



## MDShunk (Jan 7, 2007)

Just something I noticed the other day that you hardly ever see. I only had my cell phone along to take pics, but this is sufficiently rare that I thought I should take a couple pictures anyhow. 

Picture one is the face of a combination motor starter. On the face, you'll notice that this starter has a GE ground fault relay, and the 'on/off' switch for the ground fault relay sticks through the cover.

Picture two is the inside of the starter. The picture is dark and grainy, but you can make out what I'm trying to show. This picture is specifically of the zero sequence CT that goes with the ground fault relay. That zero sequence CT is around all of the starter's load side conductors. What do you see wrong? If you see what I see, you'll know too that this will never work as intended.


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## CJE (Oct 10, 2010)

Where is the green going?


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## macmikeman (Jan 23, 2007)

Are we interupting the ct conductors with the gfp ?


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## MDShunk (Jan 7, 2007)

CJE said:


> Where is the green going?


One end is on the equipment grounding terminal in the starter can, and the other end goes to the ground lug in the motor's peckerhead.


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## MDShunk (Jan 7, 2007)

macmikeman said:


> Are we interupting the ct conductors with the gfp ?


No. Everything that's wrong is in the second picture. The GFP relay actually kills the coil power to the starter when (and if) activated, but that's not the problem. That's proper.

Before someone mentions it that knows their starters, I'll say ahead of time that this is an Allen Bradley starter (you can tell from the style of the disconnect handle), that has been fitted at some point in time with the GE ground fault relay. That has nothing to do with the problem here, but I thought I'd throw that in anyhow.


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## Jlarson (Jun 28, 2009)

What the hell


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## HARRY304E (Sep 15, 2010)

Three wires going through one CT looks like the installer was on a donut diet:laughing:


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## MDShunk (Jan 7, 2007)

HARRY304E said:


> Three wires going through one CT looks like the installer was on a donut diet:laughing:


Well, that's how it should be for a zero sequence CT. Harry, you see the problem?


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## HARRY304E (Sep 15, 2010)

MDShunk said:


> Well, that's how it should be for a zero sequence CT. Harry, you see the problem?


 I can see the three wires and the grounding conuctor going through one CT..

I cannot make out any wires on the load side of the fused..


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## MDShunk (Jan 7, 2007)

HARRY304E said:


> I can see the three wires and the grounding conuctor going through one CT...


yup, that's all you need to see. You know what's wrong with that install?


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## CJE (Oct 10, 2010)

I'm trying to work this out in my head. I can't see a lot in the pic, but if what goes out comes back, even if on a different line, how would it trip? would you not still show a zero sum?


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## HARRY304E (Sep 15, 2010)

MDShunk said:


> yup, that's all you need to see. You know what's wrong with that install?


Yes if i am right this is what your talking about....Heres a good link..http://www.gedigitalenergy.com/multilin/family/motors/principles3.htm


There are several ways by which a ground fault can be detected. The most desirable method is to use the zero sequence CT approach, which is considered the best method of ground fault detection methods due to its sensitivity and inherent noise immunity. All phase conductors are passed through the window of a single CT referred to as a zero sequence CT. Under normal circumstances, the three phase currents will sum to zero resulting in an output of zero from the zero sequence CT’s secondary. If one of the motor’s phases were shorted to ground, the sum of the phase currents would no longer equal zero causing a current to flow in the secondary of the zero sequence CT. This current would be detected by the motor relay as a ground fault.


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## leland (Dec 28, 2007)

So if thats the case- why would one run the ground thru the CT?


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## MDShunk (Jan 7, 2007)

leland said:


> So if thats the case- why would one run the ground thru the CT?


S'zackly right.

This installer ran the ground through the CT too! That's as wrong as can be. On a ground fault, that CT will never show anything other than a zero to the ground fault relay, and the ground fault relay will never trip. You don't run the ground through a zero sequence like that.


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## CJE (Oct 10, 2010)

Hey,I got one! Even a blind squirrel finds an acorn occasionally.


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## MDShunk (Jan 7, 2007)

CJE said:


> Hey,I got one! Even a blind squirrel finds an acorn occasionally.


Yup. You did good.


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## macmikeman (Jan 23, 2007)

Here is a very similar picture of some electrical equipment that is wired wrong. Can any of you make out the problem?


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## CJE (Oct 10, 2010)

MD, can you tell me what is the advantage of the type that uses individual CT's over this type?


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## HARRY304E (Sep 15, 2010)

macmikeman said:


> Here is a very similar picture of some electrical equipment that is wired wrong. Can any of you make out the problem?


 Yes there is too much hair in that picture:laughing:


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## CJE (Oct 10, 2010)

macmikeman said:


> Here is a very similar picture of some electrical equipment that is wired wrong. Can any of you make out the problem?


No, it burnt up and turned black before I got to see it.


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## MDShunk (Jan 7, 2007)

CJE said:


> MD, can you tell me what is the advantage of the type that uses individual CT's over this type?


The only time I know of that you'd use more than one zero sequence CT for ground fault detection would be if the conductors you were worried about wouldn't fit into one of the largest commercially available zero sequence CT's. You'd do each set, in that case. There might be some other reason, but I can't think of one.

Also, don't get zero sequence CT's confused with CT's used for metering or instrumentation. Different uses of CT's entirely.


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## Rikki (Mar 23, 2007)

I've never seen a ground fault relay. 

What is it for?


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## Jefro (Jul 28, 2009)

macmikeman said:


> Here is a very similar picture of some electrical equipment that is wired wrong. Can any of you make out the problem?


Yes. They ran the ground thru the furry carpet with the phase conductors. That furry carpet will never operate as intended.


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## CJE (Oct 10, 2010)

Rikki said:


> I've never seen a ground fault relay.
> 
> What is it for?


Rikki, Take a look at this link. When you develop a fault the relay should drop power to the coil of your starter.
http://www.globalspec.com/LearnMore...tion_Protection_Equipment/Ground_Fault_Relays


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## Big John (May 23, 2010)

MDShunk said:


> This installer ran the ground through the CT too! That's as wrong as can be.


 I agree with this part.


> On a ground fault, that CT will never show anything other than a zero to the ground fault relay, and the ground fault relay will never trip....


 If there's any metallic path between the starter and the motor then I disagree with this. 

Any conduit/other metal would be a parallel path with the EGC, so there would be some current that didn't return on the EGC and you'd never have zero sequence. Depending on the settings of the relay, it might take a much larger ground fault to trip it than anyone would like, but it may very well still trip.

-John


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## MDShunk (Jan 7, 2007)

Big John said:


> Any conduit/other metal would be a parallel path with the EGC, so there would be some current that didn't return on the EGC and you'd never have zero sequence. Depending on the settings of the relay, it might take a much larger ground fault to trip it than anyone would like, but it may very well still trip.
> 
> -John


Yes, indeed. Never gave that a thought. In this particular case, it's VFD cable in PVC conduit, but you really didn't have a way to know that.


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## Thedroid (Dec 6, 2010)

On a different note there is a way to get ground fault protection using 3 ct's instead of the one zero sequence. I believe it's referred to as 51N, and senses residual current in the ct's circuits neutral. I believe it was used more commonly before the zero sequence was invented.


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## John Valdes (May 17, 2007)

Very interesting post. Thanks Mr. Shunk.


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## MDShunk (Jan 7, 2007)

John Valdes said:


> Very interesting post. Thanks Mr. Shunk.


Gee, that's the nicest thing you've ever said to me. 

I feel a tear welling up. :thumbsup:


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## MDShunk (Jan 7, 2007)

Thedroid said:


> On a different note there is a way to get ground fault protection using 3 ct's instead of the one zero sequence. I believe it's referred to as 51N, and senses residual current in the ct's circuits neutral. I believe it was used more commonly before the zero sequence was invented.


Sounds like you know more about it than I do. I don't know anything about 51N relays, but I have seen 51G's in bigger generators around the ground. Instrumentation isn't really something I know very much about.


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