It is permitted to tie one side of a secondary control circuit to the primary's EGC?

[theColonel1026]

Hi I'm an Electrical Tech student and this is my first post so if it's in the wrong place please move it.

So I want to know if this is legal per the NEC 2011. This is how my motor control instructor told us to wire control circuits. However I was talking to my Low Voltage instructor and this subject came up and he flipped his lid when I told him about it. He said not only is it against code but someone is liable to get electrocuted.

here is the diagram I whipped up in Microsoft Visio. the point of tying one side to earth ground is for ease of trouble shooting I am told. Also some electronics like it. as far as I can tell our trasnformers aren't autotranfomers since I read )v on my meter when I put one lead on earth ground when the secondary is ungrounded.

On a side note who ever wrote this I love you. I argued with my Residential instructor for an hour after class one day about this. He just couldn't get it through his head that the earth is a conductor. He just kept saying that electricity seeks ground and and then it just "dissipates".

Electricity does not seek the path of least resistance to the earth. It seeks all available paths back to it’s source, in proportion to their resistance. The reason that a person gets shocked when touching an ungrounded conductor and the earth is because the neutral of the system is repeatedly connected to earth in a grounded electrical system. The earth becomes part of a return path to the transformer – it’s part of one route back to the source; the earth is not the destination for the electricity.

 

[augie47]

IMO, in terms of grounding, what you have drawn meets the requirements of 250.30. Note, if the transformer is 1000 VA or less, a grounding electrode conductor might not be needed per exception 3.

 

[don_resqcapt19]

Most MCC motor starter buckets with 120 volt control power transformers are wired that way. Please ask you low voltage instructor what section it violates and why he thinks it is dangerous.

 

[tom baker]

Its from Mike Holt.

 

[theColonel1026]

Sorry guys I redid my drawing to be more accurate.


After looking at 250.30 in the code book and in the mike holt's book it seems that you need a separate grounding conductor, IF you want to ground the secondary (it's optional per section 250.30(5)ex 3). So would I be correct in assuming this is not permitted? or is it still OK?

As for the reason he said it was dangerous: he said that "the control panel would become energized and anything that is bonded to the grounding conductor, any other panels or conduit etc would be energized as well". I replied "no because the transformer is its source. It is electrically isolated from the primary side. Why would it try to go back to the service panel (then to the service transformer)? he said that "the transformer isn't an "isolation" transformer and there for not electrically isolated". I said "then why is it that when it is still floating and I use earth ground as a reference with my meter and test one of the secondary leads I get 0v?" he never actually answered that last question. he said just to trust him. :slaphead:


anyways here is the new drawing that should add more context

 

[augie47]

IMO, the diagram as drawn would be permissible for a transformer 1000 VA or less if all the stipulations in 250.30 exception 3 are met.,
Larger than 1000VA a grounding electrode would need to be added for the transformer.

 

[hillbilly1]

Your teacher probably believes that corner grounding a delta transformer would make everything "hot" too. You appear to have a better understanding of transformers than your teacher does.

 

[theColonel1026]

IMO, the diagram as drawn would be permissible for a transformer 1000 VA or less if all the stipulations in 250.30 exception 3 are met.,
Larger than 1000VA a grounding electrode would need to be added for the transformer.

thanks, I reread 250.30(a) and I now understand what you are talking about. :thumbsup: It's a lot easier to interpret the code and the terminology when you have some context to go with it.

Your teacher probably believes that corner grounding a delta transformer would make everything "hot" too. You appear to have a better understanding of transformers than your teacher does.

You know I actually brought that up too. I was like "well if what you say is true, Why is it that I don't get shocked every time I touch something bonded to ground?" He said "that's different". he never actually explained why.

I actually had another hour or so arguement with my motor control insturctor about whether a trasfomer has polarity or not. He wouldn't believe me even after I showed him the voltage of two transformers opposing each other with my meter. I had to find an online E-book that talked about it, and then he was like "oh!, yeah that". wouldn't even admit he was mistaken.

thanks again guys

 

[BPoindexter]

"saying that electricity seeks ground "

That is an incorrect statement as well. Electricity seeks it's source not ground. Something that Mike Holt has tried to get through to people for years.

 

[dicklaxt]

What happens to your grade if the instructor reads this......oooops

dick

 

[John120/240]

What happens to your grade if the instructor reads this......oooops

dick

He would be a more knowlegable instructor. A little salt & pepper with some Wild Turkey to make

that crow go down smoother.