GFI question -unique scenario

J

jcrafty

New User
Location
New York
Occupation
Electrical Engineer
Hello,
I have a question regarding GFI circuit breakers. We currently have a situation that is kind of unique and want to get some thoughts. We have a 3 phase system that is an old antiquated grounded B phase system. We use 240 volt single phase power to power 4 receptacles which come from 4 standard 20a breakers on a machine and some of these receptacles reference A-B , B-C and A-C for balancing as we have over a 125 machines with this setup. We are looking at installing a 2 pole GFI breaker for each of these receptacles which i see requires a neutral to the breaker for the electronics. However, the grounded B system does not have a neutral with it. We do have a control transformer on the machine that steps down the 240 to 120v with a neutral to run the 120 v power on the machine. So my question is two-part. First. Do you think the GFI would work correctly if the neutral on the breaker was tied to this secondary neutral. Second, if this did work, if the breaker was tied across A-B phase would it function properly as well if the one phase is grounded. In this system we measure 240v to ground on phase A and C but 0 volts to ground on B phase. My preliminary thought is this grounded phase may trip the breaker as it will think it is a ground path. Possibly a secondary transformer install would isolate the grounded B phase but unsure of that either. Appreciate any thoughts on this scenario. Thx
 
GFCIs can only be used where the voltage to ground does not exceed 150 volts, and that is not the case with your installation.
I think if the breaker is connected to the conductors from the corner grounded system and you used the neutral from the CPT, the breaker electronics will still have 240 volts, assuming the secondary of the CPT is grounded. If it is not grounded, there will be no real voltage between the CPT neutral and the conductors of the corner grounded system.

If you can use 208 volts at the receptacles the easiest would be 240 delta, to a 208/120 wye transformer.
If not you could use single phase 240 to 12/240 transformers for each receptacle.
 
To provide ground fault protection for 240V single phase line to ground loads off a corner grounded delta I would recommend keeping the panelboard as is and using a IEC supplemental Residual Current Device (RCD).
Manufacturers such as a ABB and Allen Bradly offer them.
 
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