Arc flash on secondary of 480V Transformer
Arc flash on secondary of 480V Transformer
(OP)
I keep coming across this problem and I am hoping to find out what everyone else is doing. I have a pad-mounted 12.47kV/480V, Wye-Wye, 500kVA transformer with a C12 bay-o-net fuse on the primary.
My arc flash analysis shows that the hazard exceeds category 4 between the transromer secondary and the main breaker in the MCC.
How, if at all, is everyone handling this? Thanks.
My arc flash analysis shows that the hazard exceeds category 4 between the transromer secondary and the main breaker in the MCC.
How, if at all, is everyone handling this? Thanks.






RE: Arc flash on secondary of 480V Transformer
Solutions:
Work De-energized
Investigate other/better primary fuses
Install pad-mounted VFI or Vista switch, or recloser and provide secondary relaying to trip the primary device.
The only way to lower the arc-flash level in this situation is to find a way to clear the faults more quickly.
Pad-mounted transformers with 480 V secondaries are a MAJOR arc-flash hazard.
RE: Arc flash on secondary of 480V Transformer
Thanks for the quick feedback and advise.
If I understand correclty the only way you could de-energize is by opening the feeder breakers on the bus. My understanding is that you can not operate the main breaker with these high arc flash values on the main. Do I understand this correctly?
RE: Arc flash on secondary of 480V Transformer
RE: Arc flash on secondary of 480V Transformer
RE: Arc flash on secondary of 480V Transformer
If no main breaker, then NFPA-70E would say that none of the breakers on the bus can be locally operated while energized. You certainly do not want to open any doors or otherwise provide exposure to live parts.
If you need to work on the main breaker, it would be necessary to de-energize it first by opening the primary circuit (somehow).
One partial solution is to install a main secondary breaker at the transformer. If this breaker has appropriate time-current characteristics, it will lower the arc-flash level at your main switchboard to a much lower level. But it's only moving the problem back to the transformer, not eliminating it.
RE: Arc flash on secondary of 480V Transformer
I'm confused by your statement that if there is no main breaker, none of the breakers on the bus can be operated while energized. Why would operating any of the branch breakers be more hazardous than operating the main (assuming all breaker are applied within their fault interrupting rating)?
I've had clients ask me to install main breakers at transformers as suggested, but as you say it just moves the problem.
RE: Arc flash on secondary of 480V Transformer
RE: Arc flash on secondary of 480V Transformer
So the underlying assumption here is that breakers or switches exposed to that incoming line fault level cannot be operated under any condition? This was discussed in thread238-184025: Arc Flash disagreement.
This interpretation would seem to present a serious problem for many facilities.
RE: Arc flash on secondary of 480V Transformer
The tables in NFPA 70E list arc-flash protection required for operating 480 V breakers with the doors closed, even though technically there are no exposed live parts.
So my recommendation would be to treat manual breaker operation just like any other maintenance activity for 480 V arc-flash determination. And this does result in serious operational issues for unit substations with no main breaker.
RE: Arc flash on secondary of 480V Transformer
RE: Arc flash on secondary of 480V Transformer
But actually working on the main is a problem. DPC says to de-energize. This can be accomlished by pulling the high side elbows. NFPA 70E 120.1 (1-3) are now satisfied. After tagging comes step 5, testing for potential. This puts our worker back in the zone where proper PPE is required. This is risk category 4 per Table 130.7(C)(9)(a) if under 65kA and 1s. The high side fuse will likely take longer than that to clear.
RE: Arc flash on secondary of 480V Transformer
Wye-wye connection may also be a problem, allowing zero-sequence fault current to flow through the transformer.
RE: Arc flash on secondary of 480V Transformer
I'm always a little uncomfortable relying on a primary fuse for a low voltage arc-flash rating since it could be changed out with a different size/type in the future. But if you're stuck with existing equipment, it is a low-cost option that can sometimes get the energy level down below 40 cal/cm2 (still a whale of a lot of energy, but within the realm of being able to operate locally with appropriate PPE).
When you actually do the arc-flash calculations, it is generally the case that you are better off with MORE fault current at 480 V, not less, because of the impact on clearing times. Seems counter-intuitive, but it usually works out that way.
RE: Arc flash on secondary of 480V Transformer
RE: Arc flash on secondary of 480V Transformer
I know that trying to co-ordinate protection can be a problem in such a situation - we use restricted earth fault protection to protect the transformer secondary and bus bars feeding the main breaker. One issue with this is that the neutral earth link has to be at the panel and not the transformer itself. The protection CTs being before the earth link.
Also this may not protect against a phase to phase fault and such a fault would depend upon the hv protection seeing the fault and tripping.
Rugged
RE: Arc flash on secondary of 480V Transformer
But IEEE 1584 arc-flash calculations are based on three-phase faults, so even if you reduce the energy level for a ground fault, the required PPE does not change.
I have a hard time convincing some plant engineers that converting to a high-resistance grounding system at 480 V will drastically lower the risk of having an arc-flash event because the Hazard/Risk Category doesn't change. If it doesn't reduce the required PPE, they often don't see the point.
RE: Arc flash on secondary of 480V Transformer
rugged
RE: Arc flash on secondary of 480V Transformer
We use many different approaches to remediate the incident energy depending upon the clients' particular situation and/or budget. I feel that fuses are a bad idea in most situations because it then makes protective device coordination difficult at best.
If you have solid state relays, it is possible to have a 'maintenance' setting with a TCC set for faster trip time. It would be activated by a maintenance switch. This may not be advisable if you are having large motors automatically starting while the relay is in maintenance mode - or if you have forgetful electricians. Another option to explore is a differential relay that includes the downstream main in the zone of protection, since differentials have a faster trip time. Also, you could just replace your equipment. I have one client whose equipment is 30+ years old and was planning to have a major upgrade soon, so it did not make sense to spend money on equipment that was to be replaced. Instead, he has decided to just accelerate his replacement schedule – and that project now has the blessings of his plant management too! So, it can really depend on your situation which path will work best.
These were just a few ideas, I hope they are helpful.
Best regards,
-Shane