corner grounded delta.

[g@friendly]

Can anyone tell me if a corner grounded delta 240v is legal? Will the grounded corner phase trip to ground if a circuit breaker is protecting it? I have to install a single phase 240 heating press at a print shop and the b phase is the grounded phase. I am concern that the grounded phase would short; energizing the the frame of the equipment and creating the operator to be shocked. Please no guesses on this one. Thank you

 

[jtester]

A corner grounded delta is legal, but you need to verify the L-G rating of the breakers, etc. If the phase that is grounded shorts to the equipment frame there will be no hazard because that phase wire is the "ground". I wouldn't recommend touching the phase wire, but theoretically you could with no danger, if you were grounded also.

Jim T

 

[winnie]

A fault between the grounded conductor and the machine frame will _not_ cause a short circuit, and will not open the OCPD. This would be just like any other grounded conductor faulting to the machine frame, eg. the neutral faulting to the chassis in an ordinary 120V circuit. The machine frame would be energized, but only to the level of the voltage drop in the conductor. Ground fault detection can be set up to detect this sort of condition.

As to legality, corner grounded systems are code compliant, however they are not common which means that you can get bit on rules regarding switching of the grounded conductor and voltage ratings of breakers. See the appropriate code sections, but in general you can't switch the grounded conductor unless you disconnect _all_ supply conductors at the same time (there are exceptions for motors), and you would need to use straight 240V breakers, not 'slash' rated breakers that are rated for lower voltage to ground.

-Jon

 

[wanderer20001us]

The phase that is corner grounded becomes a Grounded Conductor and presents no more hazard that any other Grounded Conductor in a distribution system. In your example, phase B is grounded. Assuming that the equipment frame in properly bonded, there would be little potential for operator shock.

 

[g@friendly]

thank you very much for your help I did even think about the 240 volt only breaker and I guess the standard 200 panel that was just installed would be incorrect also due to fact that it is a 120/277 main breaker rating. I will look into this further and if anyone has the code article cool. You are righ twith it grounding to teh fram and I guess in my case going from a to c phase is the way to go (b grounded) I asked the man. aboutthe rating on the boards due to phase to ground is 240 not 120 and the said all is go they are mail it to me in writing thank you

 

[rcallen]

Corner-of-the-delta grounding

Corner-of-the-delta grounding

These systems are unusual but legal. A 480 volt version of these was commonly used by Union Electric in the St. Louis area years ago.

A discussion of this system can be found in the IEEE Green Book (Std 142-1991) pages 31-33. It is necessary to use circuit breakers which are rated for this service and it is necessary to positively identify the grounded phase throughout the system. The interrupting duty of breakers is an issue because the available fault current for a ground fault exceeds the three-phase fault current. This connection is also mentioned in the NEC, 250-26(4).

One major concern is the confusion factor. I have had electricians come to me declaring that "one leg is dead" because they connect their Wiggy from phase A to conduit and get a reading, phase B to conduit and get a reading, and phase C to conduit and get no reading.

For this reason alone I would suggest avoiding "corner-of-the- delta" grounding if at all possible.

 

[Ben There]

Most utilities try to avoid a 240 volt grounded delta. Reason being if a single phase load is connected between the grounded phase, and only one of the other phases, only 1/2 of the load will be metered. IE: 240V single phase lighting etc. ,
with 480V it'd NOT a problem, since there are seldom (or almost Never) any 480V single phase loads.
YMMV

 

[brian john]

They better be legal, I had a call from an electrical contractor that had a problem today, service had 236 to ground on A and C phases and nothing on B phase, had current on all 3-phases. he was going bonkers trying to resolve the issue.

The site had a second service that was 120/240 single phase for lighting 120 power.

You get into old industrial areas or what was industrial, and you see some different set-ups. We had a customer that set up an office in an old industrial warehouse converted to offices, burnt up a bunch of equipment connected the B phase center tapped delta (high leg). Just have to be careful.