delta to delta transformer

[lquadros]

We have an equipment which has delta 3ph.480VAC primary to delta 3ph.240VAC secondary tranformer. There has been a dangerous voltage build up accross this equipment and surrounding metal objects. Upon checking, we found out that one phase was grounded. The details are not very clear yet. How can we ground a delta connected circuit to make it safe? What are other alternatives? Thanks.

 

[augie47]

There is not a problem with grounding one leg of the delta secondary (it might have some advantages over an "ungrounded" delta). I would make sure your corner grounded secondary is bonded 250.30.
If you are experiencing potentials between equipment and/or surrounding objects it is usually a sign of poor bonding.
If you make sure all equipment and the transformer are properly bonded, there should be no potential.

 

[lquadros]

delta to delta transformer

I do not understand how can bonding drop the voltages to safe level, when the delta circuit in question is not a part of the system. Please explain.

 

[augie47]

I may be misreading your post, but if you are getting potential between objects, bonding them together removes or reduces that potential.
If you are saying you are reading a higher voltage to ground than the transformer secondary, this is common on a "ungrounded" system. My terminology is "capacitive voltage reading" although more informed forumites can give you a better description. If thats is your concern, grounding the secondary directly or thru a high impedance ground will eliminate that "problem"

 

[brian john]

What voltages are you reading on the secondary of the transformer.

A phase to ground 240
B Phase to ground 240
C Phase to ground "0"


Phase to phase 240

Or?????

 

[wirenut1980]

lquadros, is your system supposed to be ungrounded delta, or corner grounded delta?

What is the magnitude of the dangerous voltage you measured, and did you measure it between equipment enclosures?

It sounds like you have an ungrounded system and you do not have proper equipment grounding conductors run to equipment to clear two separate phase to ground faults. The result of this would be measuring phase to phase voltage between equipment enclosures.

 

[lquadros]

The equipment is on wheels. The manufacturer has advised to have a delta/delta transformer. So a 480vdelta to 240 v delta has been installed. The readings are attached.
So far the conclusion is, a open ground which caused these voltages, and the person felt a tingle. Because this is a delta system, is it possible that all delta systems have this drawback? What about motors, welders etc.? Your feedback is appriciated.

 

[augie47]

It appears that you have a number of "ungrounded" systems and, in my opinion, poor grounding/bonding. This is especially important with equipment on wheels.
If all the equipment has a low impedance ground and proper bonding is installed there should be little if any potential.
Your readings indicate you have "leakage" to ground. The variations on L to G readings insdicate to me, poor grounding.
There are some advantage in an ungrounded system, primarily the ability to located faults without having downtime from overcurrent devices opening on ground fault, however, for that system to be useful, you need ground fault detectors as required by 250.21(B).
I still standby my earlier post that you need to review your bonding/grounding system to assure all is compliant. Special attention should be given to 250.30 (if you keep an ungrounded system 250.30(B) in particular).
Installation of ground fault detectors or going to a corner grounded system would also be paramount in my opinion.