Stepdown Transformer Secondary Grounding

Technically NEC does not require this, for your case maybe a non-conductive case and thus come under the heading of double insulation. But if the case is conductive then it certainly should be grounded for safety, provided that lethal voltages referenced to earth reside anywhere inside the chassis. But there is nothing that states that your secondary voltage must be referenced to earth.

The transfomers are 480V to 240V with a center tap for 120V.

Should the 240V be grounded using the center tap(x2-x3) as a netrual point or

create the netural using X1 or X4?

Use X1 or X4 will create 230V to ground is this a code problem?

"Should" is a very interesting word. It implies a partial understanding of the situation by the questioner, combined with a reluctance to decide. The funny thing also with "should" is that regardless of how anyone answers the immediate question, the questioner will always doubt any given answer and will continue to doubt their own knowledge.

To this I say, get your own copy of the NEC code book. Do your own research. I recommend taking the 40 hour training session to accurately interpret the code. It will give you a useful certificate and line on your resume.

With all due respect.

Me hide. You really don't know me very well. As a matter of fact, you don't know me at all. You come here asking for an absolute answer from a group of strangers bringing insufficient information to answer your question. Despite the lack of information, I grant you an answer from the microscopic scraps of information given that will not endanger people.

You ignore it. You then add the briefest portion of added information that really adds nothing useful to resolving your initial stated problem. (Others here have pointed this out.) Do I point out that you clearly do not know what you are doing, no I do not. Instead I point out that a proper formal education of the wiring standard is what you need. In the NEC 2008 standard you will find the times that one must ground a neutral wire from a transformer secondary, the times one has the option to ground the neutral wire and the times one cannot ground the neutral. Let me remind you, you did initially ask for the NEC standard on this question.

I will admit that I did respond in a somewhat surly manner, for I despise people who work with potentially lethal devices that do not know what they are doing. I also did not want you to damage your self, your family, coworkers or anyone else because you could not accurately explain the circumstance that you were wiring a step down transformer and was relying on the information of a group of strangers.

Please do not kill anyone.

I am not aware of the NEC, but looking practically yes.

If the secondary one point is not grounded then the secondary is floating.

The transformer will only create the voltage differential ie secondary L-L voltage will be 240/120V (for you)

One of the voltage can as well be the pri voltage ie 0V or 480V (as it wishes to be) and the others will be additive/ subtractive, as it wishes at the moment and thus anything between 240V to 720V, and that definitely is not safe.

There is generally a practice not to keep the secondary floating.

As is so often the case... not enough data has been provided for a full and complete answer.

The following would be my answer, which is based on the Canadian code (which almost mirrors the NEC in this regard).

If your utilisation equipment is 240 volt and if no 120 volts loads are to be serviced by this transformer, there is no need to bring a neutral out from the transformer nor is there a need to ground the neutral point between the two 120 volt windings nor would you bond the neutral point of the windings to the case of the transformer.

Remember that even if you do not ground and bond the neutral, the case of the transformer must be bonded to the "bond" system in place.

Under Canadian code, if there is no neutral point brought out from the transformer, in this instance, as the system voltage would be over 150 volts relative to the other side, you may not ground the transformer. See CEC rule 10-106(1)(a).

If any of the utilisation equipment is 120 volt equipment and the neutral is to be brought out from the transformer, then...

The neutral of the transformer, the midpoint between the two windings, must be grounded and the neutral must be bonded to the case of the transformer. This "starts" the bonding system for this separately derived system. As well, the grounding conductor that grounded the neutral of this system must be tied back to the main consumer service ground through a conductor that ties the "grounds" to each other. This is to ensure no potential difference can exist across the "bond" systems.

Read the codes and standards in effect where you are located to properly understand what you must do where you are located in the world.

hello from a newbie!

as told to me by engineering support here at my plant-

" you know as well as i do, you can plan all day and electricity is still gonna do some freaky shi...(er,stuff)"

i was adding quick disconnects to some level probes, and encountered one using both sets of N.O. dry contacts in the probe. most of the others use only one set.

my QD's can accomodate an extra conductor, if the common (which is also L1 for the probe) is shared among them. in this case the extra contact is wired from a separate power source entirely, namely a flex IO serving a separate process. there are only two conductors from there, a hot and a return.

i measured between L2 and the hot wire from the separate source and read 60 vac. i new what the issue was right away, but not why. all of our L2's should all be bonded to ground at the control transformer, of course. the tech advised me to check to see if it was the same from L1b to L2a as it was from L1a to L2b. yep, 60vac. go figure.

thats when he made the "freaky shi..." statement.

i traced the circuit back to the culpret transformer, and sure enough what should have been the neutral was not bonded to the transformer frame.

the proper way to handle the situation is to shut down the process and make the bonding connection, and then remove the second power source from the probe and use the PLC to send the signal via ethernet to the other PLC that owns the flex IO.

he wants me to install a another QD to the second boss in the probe housing,which would take care of the number of conductors problem, but would will not remove the second power source from the probe. you would be able to remove power from both sources via the 2 QD's

i gotta do what he wants regardless of what i think. if the decision were yours to make, how would you make the fix? no, you can't wait for him in the parking lot after work...