600V 3phase 3 Wire (Delta) Main Service Woes
600V 3phase 3 Wire (Delta) Main Service Woes
(OP)
I have a 3 phase 3 wire 600V service to a commercial building. The tenant experiences flickering or dimming of the lights, and her UPS's are constantly switching between battery and normal. This is consistent with a sag. Measuring on the line side of the main switch, we get:
A-B, A-C, B-C all approx 600V, which is expected.
However, referencing ground we get A-G at 540V, B-G at 587V and C-G at 59V. I would have expected 347V from phase to ground. Furthermore, why is C-phase inconsistent with the others? Our initial thoughts were that perhaps a fuse was blown and that phase was backfeeding. We have not confirmed that a fuse has not blown (with the exception of the fact that 3 phase motors are all still working, but that could be attributed to single-phasing), but is there something else I should be looking for? I have a funny feeling that something is wrong with the ground at the main board. Other than that, I think that I may have to start looking to the street and the local utility for problems with their transformers.
A-B, A-C, B-C all approx 600V, which is expected.
However, referencing ground we get A-G at 540V, B-G at 587V and C-G at 59V. I would have expected 347V from phase to ground. Furthermore, why is C-phase inconsistent with the others? Our initial thoughts were that perhaps a fuse was blown and that phase was backfeeding. We have not confirmed that a fuse has not blown (with the exception of the fact that 3 phase motors are all still working, but that could be attributed to single-phasing), but is there something else I should be looking for? I have a funny feeling that something is wrong with the ground at the main board. Other than that, I think that I may have to start looking to the street and the local utility for problems with their transformers.






RE: 600V 3phase 3 Wire (Delta) Main Service Woes
Find the ground fault - and think of adding ground detector lights to your main service.
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
Check Thread238-9005 Thread238-3370 Thread237-7403 Thread238-6870 for discussions of this. There, folks talked about 480V but the same rules generally apply. Ungrounded delta [or ungrounded wye] systems can have the problems you describe.
Does the serving 600V transformer secondary (possibly utility-owned) have a solid neutral (wye-point)-to-ground connection? If not, your phase-to-ground voltage readings are at the mercy of the entire 600-volt-system insulation.
Are any lighting ballasts (or other loads) served at 347V? If yes, then you have an immediate fire hazard from a “loose neutral.”
Are the UPSs served at 600V or 208V? For 208V, then in almost all circumstances the 208Y/120V wye-point (usually an X0 bushing) must be solidly grounded; i.e., building steel/electrode.
More information about the system at hand will help yield higher-quality advice.
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
Suggestion to the previous posting marked by ///\\\:
.... aren?t the phase-to-ground voltage readings (assuming they?re not ø-n) indicative of insulation degradation of cø insulation?
///Yes, it may be the case. If the delta connection has a grounded corner, which is not so clear in the original posting, then c to ground voltage may be very small, almost zero. I mentioned "etc." in my above posting, since there are more possibilities left.\\\
The poster stated that ø-ø readings were all around 600V. Isn?t that the same situation as is described in Beeman chapter 6 for an ungrounded-delta system?
///Yes, except the grounded delta corner, which normally also has all phase-to-phase voltages 600V.\\\
That doesn?t seem like a fuse or connection problem unless it is served as solidly-grounded 4-wire wye (600Y/346V).
///I interpret a partially open circuit as one with the higher impedance in line (in series), across which the missing voltage drops.\\\
If it were, it?s likely that the original poster would have found other problems like lighting ballasts in flames. A discontinuity in the neutral conductor upstream of where the voltages were measured would manifest the described problem, but the poster specifically describes the system as 3-wire delta. [In the US, I believe lighting circuits in commercial buildings are limited to 300V to ground,
///Yes.\\\
but the poster doesn?t give a location.]
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
If the erratic UPSs are operated at 208 or 120V, then the 600−208Y/120V transformer secondary X0-connection/ground conductor may have a problem like poor continuity or possibly more than one connection to ‘local’ building steel or similar ground electrode. Both can cause the circuit neutral voltage-to-ground to be significantly above ‘zero volts,’ and make the UPS think it’s seeing a low (or high) line-voltage condition.
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
USA....
USA....
Wirenut
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
It may not have been apparent, but the hydro troubleshooter probably appreciated all your efforts. This will make it easier the next time you have to deal with such a situation. And, you have experience with a special electrical problem that a lot of electricians do not understand.
Have you been able to tell if the UPS problems have also ceased?
RE: 600V 3phase 3 Wire (Delta) Main Service Woes
Try fault locating equipment that is permanently installed and notifies immediately of a fault and its location!