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240 volt - 125 kVA transformer exception
https://brainfiller.com/arcflashforum/viewtopic.php?f=33&t=3791		 Page 1 of 1
Author:  wilhendrix [ Thu Oct 01, 2015 8:50 am ]
Post subject:  240 volt - 125 kVA transformer exception
I'd like to hear how you treat 480 to 208 volt transformers that are rated below 125 kVA? Do you think the 480 volt portion of the transformer eliminates the exception or not? I've asked NFPA this same question and instead of answering the question, they wanted to know why people were opening transformers left energized? So I asked them how does one do an infrared inspection without removing the cover? We open switchboards, panels, distribution sections and so forth and all are left energized.

Do you think the fact there's 480 volts inside the transformer eliminate the exception or not?
Author:  stevenal [ Thu Oct 01, 2015 8:59 am ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
If both voltages are present in the same compartment, I would say yes the exception does not apply. It would apply to the downstream 208 system.
Author:  JBD [ Thu Oct 01, 2015 12:13 pm ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
wilhendrix wrote:
Do you think the fact there's 480 volts inside the transformer eliminate the exception or not?


Dry-type transformers below 175kVA in size do not have physical separation between the 480V and 208V terminals, in fact they are typically intermixed. You cannot have a 3-phase 208V arc flash event that does not also involve the 480V primary, therefore the AF incident energy on the primary side cannot be ignored.
Author:  PaulEngr [ Sat Oct 03, 2015 5:16 am ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
You run the analysis %or both and take worst case regardless of the outcome. In the case of shock, 480 drives everything. In the case of a 480/208 transformer 112.5 kva or smaller, if the 480 side is under 1.2 cal/cm2 (or 2 cal/cm2 for utilities), that drives everything. But it depends on transformer construction, too. Small pole mount utility transformers typically have open insulators on top and a small 208 compartment on the side or bottom and underground ones have elbow connectors or some other gland so there can be separation compared to wall/floor mounted dry transformers.
Author:  engrick [ Mon Oct 05, 2015 7:04 am ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
Why are you doing live work in the transformer? I guess if you need to get in there then OK put a label on it. Padmount xfmr with easy access sure but a dry type?
Author:  JKlessig [ Mon Oct 05, 2015 8:47 am ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
Some one wrote "Why are you doing live work in the transformer? I guess if you need to get in there then OK put a label on it. Padmount xfmr with easy access sure but a dry type?"
Well for example, we have one client, that thinks that ALL dry types need to be IR scanned on a regular basis. They believe this seriously enough that they have modified the covers on all of their transformers to allow for this in a (some what )safer fashion.
And seriously enough that they have been willing to take the risks associated with the covers that have not been modified. {Fairly crazy IMO}
Author:  stevenal [ Mon Oct 05, 2015 9:42 am ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
engrick wrote:
Why are you doing live work in the transformer? I guess if you need to get in there then OK put a label on it. Padmount xfmr with easy access sure but a dry type?


The transformer must be considered live until tested to ensure it is deenergized. The testing itself is live work.
Author:  wilhendrix [ Mon Oct 12, 2015 6:53 am ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
RESPONSE FROM QUESTIONER:
Everyone, thank you for your replies. Here's a summation of what I got from your collective responses to my question on the 125 kVA and 240 volt transformer exception. I tried phrase this like a rule. Hopefully, it makes sense to you.

"That if a worker or workers are performing tests and/or inspections of an energized transformer that is rated 125 kVA or less and may be exposed to 240 volts or more, the transformer must be properly labeled with arc flash/shock hazard warnings and be included in the arc flash study."

Please note that this does not say "maintenance", just test/inspect.

Does this seem like a reasonable summation of all the replies?

Again, thanks to all of you for your valuable observations and input on this question.
Author:  stevenal [ Mon Oct 12, 2015 8:19 am ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
wilhendrix wrote:

"That if a worker or workers are performing tests and/or inspections of an energized transformer that is rated 125 kVA or less and may be exposed to 240 volts or more, the transformer must be properly labeled with arc flash/shock hazard warnings and be included in the arc flash study."


Needs work I'm afraid. The false implication is that transformers exceeding 125 kVA at those voltages can be left unlabeled and unstudied.
Author:  PaulEngr [ Mon Oct 12, 2015 9:46 pm ]
Post subject:  Re: 240 volt - 125 kVA transformer exception
I have personally experienced issues with dry transformers that were "de-energized" and then failed the live-dead-live test (result was live-live-live). All equipment with the possible exception of cord-and-plug equipment is always worked live even if the only live work is the process of making it dead. The only other way to get there is to go back to the pre-1900's and stop using electricity. By the way, steam is invisible too and a "live" test is required in many cases too. And high pressure (>3000 PSI) hydraulics poses unique challenges too, and pneumatic equipment also poses hazards of pushing particles into the skin as does hydraulics, so eliminating electricity does not eliminate "live" work.
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