Low voltage control grounding

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I normally ground one of the conductors of the secondary of a control transformer 24ac (277 to 24v). I know others in my area claim its not necessary according NEC 250.20, 21. Since most of the controls we install are not necessary to be 'fail' proof, I dont see a reason not to ground.

Do most of you ground the secondary?
 

iwire

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Location
Massachusetts
I normally ground one of the conductors of the secondary of a control transformer 24ac (277 to 24v). I know others in my area claim its not necessary according NEC 250.20, 21. Since most of the controls we install are not necessary to be 'fail' proof, I dont see a reason not to ground.

Do most of you ground the secondary?

No, but only because I never think of it.


Grounding it will not make anything safer but it does make troubleshooting easier as you have a ground reference to measure against.
 

tom baker

First Chief Moderator
Staff member
Take a look at 250.20(A)
(A) Alternating-Current Systems of Less Than 50 Volts.
Alternating-current systems of less than 50 volts shall be
grounded under any of the following conditions:
(1) Where supplied by transformers, if the transformer supply
system exceeds 150 volts to ground

Grounding is required, however, what you are doing is bonding the secondary.
 

Tony S

Senior Member
A grounded leg gives:

A fault return path
A reference for testing
It also removes the chance of multiple faults starting plant unexpectedly. Worse still, as happened to me, being unable to stop the plant.

It may not be required by code, you’re a damn fool if you don’t.
 
Take a look at 250.20(A)
(A) Alternating-Current Systems of Less Than 50 Volts.
Alternating-current systems of less than 50 volts shall be
grounded under any of the following conditions:
(1) Where supplied by transformers, if the transformer supply
system exceeds 150 volts to ground

Grounding is required, however, what you are doing is bonding the secondary.

Tom,

that's basically what that article is talking about, the system has to be grounded. The secondary side of the control transformer will have to have one of the conductors connected to the earth,(the metal can) :)
 

Tony S

Senior Member
Again you seem to be some nexus for bad things to happen. :lol:

Forty years in engineering, what would you expect?

Do I put my rose tinted glasses on and ignore the dangerous mistakes of the past? Like hell I will and I’ll do everything in my power to stop the same mistakes being made again.

Codes and regulations aside, what is your prime objective? Mine is to make electrically operated machinery as safe as possible.
 

Tony S

Senior Member
A memo sent to my boss many years ago



Here’s a conundrum for you,
Installing the earth links as I said could reveal years old faults. (So long as there’s only one fault per transformer there’s no great problem when the earth link is missing).
As it was we had faults on both sides of the board. Think of the consequences had they been on the same side of the board or the bus-section switch had been closed:
The earth fault on No.1 HP pump delivery filter blocked tell-tail switch wouldn’t have shown at all.
BUT No.4 LP combined with the No.1 HP fault, No.4 LP pump would have started on its own and have been impossible to stop!
I found a washer lodged behind one of the aux contacts on No.4 LP contactor. I think it had been there from the day the panel was built. No.1 HP pump delivery filter blocked tell-tail was a cable fault.
[h=1]How many more panels are like it?[/h][h=1]Tony S[/h]06-01-09

PS a label needs fitting to the hydraulic house Bus-Section switch to state that if the Bus-Section is closed the control cubical for the unused incoming feeder should be isolated and locked off.
 
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