Grounding transformer secondary

[BKearney]

I often desing/layout electrical boxes for HVAC equipment we manufacture. These boxes often have 2 or 3 voltage levels required to operate various pieces of equipment and controls. So I may have up to two transformers in a box. These pieces of equipment are always grounded and the supply system to the equipment is grounded.

I may have for example a unit with a 460v/3phase supply, and then transformers that are:
- 460v primary / 120v secondary
- 120v primary / 24v secondary

In the past I have always grounded one leg of the 24v secondary, but so far I have never grounded one side of the 120v secondary.

Recently I had someone question this practice. He thought I should ground one side of the 120v secondary...

So I spent some time reviewing NEC and looking online for sources to explain if in this case I should be grouding one leg of the 120v secondary of my transformer, and I could not find a good matching example with a simple explanation...

My main concern is to make sure I am being safe. If I am doing something wrong I want to correct it and do what is correct and safe.

Should I ground one side of the 120v secondary on a 460v / 120v transformer in this case? If no, why, and if yes, why?

Thanks in advance for any replies!

 

[templdl]

NEC art250.20 would be excellent for you to review. "Alternating-Current Circuits and Systems to Be Grounded."
Then look at 25021 and 250.22 to see if there is the option that allows you either ground or not ground or that requires that it not be grounded.
There should be no questions after you review these articles.

 

[BKearney]

Reading through 250.20,21 and 22, it sounds like I need to ground the 120v secondary...

I think "250.21(3)d" would allow me not to ground the 120v secondary but only if I had some type of 120v controls that could detect a ground condition and due to this failed to operate...

It seems to me, in my case, that I have no other choice but to ground one side of the 120v secondary...

It just seems odd to me to ground the 120v secondary on my first transformer and then to also ground the 24v secondary on my second transformer...but it seems thats what I should do after reading through these...


Thanks!

 

[templdl]

That's the way I read it.
In addition to what is a code requirement I personally like to ground if at all possible. When you consider an ungrounded scheme if one line goes to ground there is no issue. However, if the other line goes to ground in another location you have not control over the path the current takes between the two grounded locations. By grounding one side if the ungrounded line goes to ground the return path should now be defined and it should take out the OCPD and clear the fault.
Even realizing the 24v is minor when compared to 120v it is a simple thing to do and makes sense. A simple jumper from the X1 or X2 terminal and bonded to a mounting screw of the transformer base should do the job.

 

[boboelectric]

GROUNDING TRANSFORMERS

GROUNDING TRANSFORMERS

It makes troublechasing easier when secondarys are grounded.

BOB O. 84,PA.

 

[don_resqcapt19]

BK,

It just seems odd to me to ground the 120v secondary on my first transformer and then to also ground the 24v secondary on my second transformer...but it seems thats what I should do after reading through these...

If the primary of the 24 volt transformer is fed from the 120 volt power, there is no code rule that would require the secondary of the 24 volt transformer to be grounded. Assuming that you have grounded the secondary of the 120 volt transformer.
Don

edited to correct the last line...It had said "... secondary of the 24 volt ..."

 

[kiloamp7]

Yes, generally one side of the 120V. secondary should be "case grounded".

It is not required to "case ground" the 24V. sec., especially if it is a Class 2 xfmr. It is okay to do so if you wish.