Delta-Delta

[bjohnson64]

I recently found out that I have a 112 1/2 KVA, 480 to 240 Volt, three phase Delta-Delta transformer suppling the panel board in my metal working / weld shop. During preventive maintenance, I found that there was a grounded leg and I corrected it. Which leads me to my questions that I hope someone can help me with. Does the NEC require that the secondary of the Delta-Delta transformer be grounded? Or, if not, does the NEC require that a ground detection system be installed to alert personnel that an undesired ground in the system exists? Any help would be appreciated. Bill

 

[nvcape]

Re: Delta-Delta

NEC 250.20(B): Alternating systems 50V to 1000 V that supply premises wiring shall be grounded under the following.
NEC 250.20 (B)(3): "Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor"

 

[charlie]

Re: Delta-Delta

Originally posted by Bill:
Does the NEC require that the secondary of the Delta-Delta transformer be grounded? Or, if not, does the NEC require that a ground detection system be installed to alert personnel that an undesired ground in the system exists?

It was not required that you "correct" a grounded leg if it was intentionally grounded. If it was an accidental ground, it should have been corrected.

First, the secondary of a delta transformer is not required to be grounded unless the midpoint of one phase winding is used as a circuit conductor.

Secondly, ground detectors are not required by the Code but I don't know of anyone who feels like they should not be used. They let you know when you should troubleshoot the system to find an accidental ground and lift it. That will keep the circuit (perhaps two circuits) from letting go if a second ground is established at an inopportune time.

In general, I am not a fan of an ungrounded SDS or service. The problem is that the AC voltage can ride on a very large DC voltage and can be very harmful to an unwary (unqualified?) person that is working on the system.

 

[gwz2]

Re: Delta-Delta

Just - what did you do " to correct it "?

 

[bjohnson64]

Re: Delta-Delta

The problem I corrected was a reversal of L2 and the equipment grounding conductor in a three phase receptacle. When I was checking voltages (Line side) at the panel board I found it had a grounded leg. After isolating each circuit, to determine if the ground was on the line side or the load side, I found one circuit grounded. Then discovered that the ground was caused by incorrect wiring to a receptacle.
This is a Delta-Delta three wire system.
Bill