Selective Coordination to 0.01

[mshields]

I have a colleague who contends that most 480V systems require a 1 to 1 transformer from the incomming service switchgear to 480V panels in order to knock down the fault current thereby permitting 0.01 selectivity. i.e. to meet the very stringen trequirements of Article 700 and by extension Emergency Systems in Hospitals.

Thanks,

Mike

 

[Shoe]

While that is one way to knock down available fault current, it does not strike me as something that should be applied universally. Fault current and selective coordination analysis should be performed on the system before this can be determined.

 

[jim dungar]

I have a colleague who contends that most 480V systems require a 1 to 1 transformer from the incomming service switchgear to 480V panels in order to knock down the fault current thereby permitting 0.01 selectivity. i.e. to meet the very stringen trequirements of Article 700 and by extension Emergency Systems in Hospitals.

We regularly (i.e. weekly), choose breakers that selectively coordinate without the use of transformers. It probably means using 1200A frame breakers or small electronic trip units, but it is not that hard to do for new designs.
Schneider Electric has a bulletin 0100DB0501 that is shows witness tested breaker combinations into the 35K and 65kA range. Eaton's equivalent is #IA01400001E. I am pretty sure Siemens and GE have similar publications

 

[mshields]

Tested combination tables

Tested combination tables

Thank you for the input.

All 4 venders, do, I can confirm have tested combination of breakers and I think it's ridiculous to automatically throw in a 1 to 1 transformer to resolve this issue unless as a last resort.

I do agree that this is on the down stream side of Emergency ATS's only. If my normal feeder to the ATS meets .01 great. If it's close but no cigar and meets 0.1, I'm ok with that too.

Mike

 

[jim dungar]

I do agree that this is on the down stream side of Emergency ATS's only. If my normal feeder to the ATS meets .01 great. If it's close but no cigar and meets 0.1, I'm ok with that too.

:thumbsup:

A big problem I have is coordinating +4 devices at a time (especially when the largest one is only 400A), why do some designers think every panel needs to use a protective device to feed another panel?

 

[hurk27]

Putting a 1 to 1 transformer after the ATS only defeats the GF protection that might be ahead of the Ats, any GF on the load side of a transformer will only appear as a line to line current to the GF device so it will never trip, this would be bad if the ATS is over the 1ka that requires GF protection to start with and the NEC would require another GF system to be installed after the transformer, but once you get below the 1ka requirement then I could see where it would help, but to me is an expensive way to go?, In most cases if the GF settings are set correctly it shouldn't be a problem.

 

[hurk27]

I would have to say that if your colleague is suggesting to do this on feeders that exceed the 1k amps that the NEC requires GF protections without installing GF protection after the transformer is setting up a very high liability if a grounding path was to fail and should some one be hurt or killed, as I said above the secondary of transformers will not trip a GF system on its primary and could subject the grounding paths to very high current in a bolted ground fault.

 

[mshields]

A very interesting point you make about ground fault!

A very interesting point you make about ground fault!

But keep in mind that we are only talking about Emergency Systems. As such, on the secondary side of the ATS's, GF is not permitted. That said, if we were feeding a 1000A plus ATS, there'd we'd have Ground Fault and if I understand your point correctly, this breaker with ground fault would never see a ground fault down stream of the 1:1 transformer on the secondary side of the ATS's. In this case, I would think my colleague envisions multiple ATS's, (30 to 45kVA) like one on every floor feeding the Life Safety and Critical system (we primarily do hospitals), etc. Since we don't want these branches to trip on ground fault and since we don't want to have a 1000A plus breaker upstream of the ATS tripping on ground fault and shutting down a very large section of the hospital, I wonder if the 1:1 transformer is actually providing us a relevant service?

 

[jim dungar]

Since we don't want these branches to trip on ground fault and since we don't want to have a 1000A plus breaker upstream of the ATS tripping on ground fault and shutting down a very large section of the hospital, I wonder if the 1:1 transformer is actually providing us a relevant service?

Selective coordination should also include GF on feeders versus normal branch breakers. I find it fairly easy to coordinate feeders with almost any 30A and smaller breaker. This means a fault to ground on a typical branch circuit (by far and away the most likely fault in a hospital) would be pretty much of a non-issue.

For larger branches and feeders, isn't it possible that the best thing to do would be to trip the feeder circuit and transfer to the generator, because the arcing fault might be cleared and not restrike during the several seconds of no power? If the fault is not cleared, how well will the voltage be maintained if there is an arcing fault to ground of several hundred amps?