Arc Flash Incident Energy on Secondary of Transformer

[PE (always learning)]

Hello everyone,

Hope you are all doing well. I have a question pertaining to an arc flash study that I'm doing for a University building. To put things into perspective, I have a 800 amp, 120/208V, 3 phase, main circuit breaker panel on the secondary side of a 225 KVA transformer. The incident energy is understandably high on the secondary side, around 23 cal/cm^2 to be exact. The university engineer is trying to get me to drop the instantaneous on the primary over current protective device in order to drop the incident energy down, but this causes another issue. The transformer inrush is now tripping the breaker if I try to do this. I have tried arguing the point with the university engineer that they should not be working on the secondary panel live anyways because we are not dealing with a hospital or critical operations center here. He argued back to me saying that they don't want to have to "suit up" every time they have to flip a breaker on the secondary panel. He also talked about taking the secondary panels main and placing it outside of the panel at a different location in order to reduce arc flash incident energy. I would rather not go a route that involves making changes at this point because most of the equipment is on site. Am I crazy or is telling them to shut down the power not the right thing to say? I want to be careful because this is my client and I don't want to tell them I can't help them.

Best Regards,

Engineer in training

 

[PE (always learning)]

Also, another piece of information I left out is that they want the incident energy below 8 cal/cm^2 which again is hard to do on the secondary of a 225 KVA transformer.

 

[roger ober]

Suite up for operating a breaker in a panel board?
NFPE 70E does not require this. They require the the installation meets code, meaning installed correctly, and maintained.
I know some industries require the operator to suite up even when they are not exposed conductors.
Could the primary instantaneous protection be configured two ways? One for initial power up and another setting for operation or working on the secondary main? This is done here where I am.
I’ve been suggesting that the main disconnect be located outside the panel it feeds for this reason.





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[Jraef]

You can now get what is referred to as a Maintenance Mode (MM) breaker that has a temporarily selectable lower trip threshold specifically for this purpose. The lower trip threshold is selected by a contact closure, so you simply wire that up to a key switch that your electricians have access to. When they need to interact with the equipment, the indigent energy is lowered only AFTER the transformer was energized. Then when they are done, they switch back and remove the key.

 

[wsbeih]

You can now get what is referred to as a Maintenance Mode (MM) breaker that has a temporarily selectable lower trip threshold specifically for this purpose. The lower trip threshold is selected by a contact closure, so you simply wire that up to a key switch that your electricians have access to. When they need to interact with the equipment, the indigent energy is lowered only AFTER the transformer was energized. Then when they are done, they switch back and remove the key.

Ditto for Maintenance switch option. When the switch is enabled (on), the instantaneous setting takes over and other settings (short time pickup/long time pickup) are disabled. Typically existing breakers can be to retrofit add this switch. All the best.

 

[PE (always learning)]

Thank you for the responses and I agree with you guys that the maintenance mode switches would have been a good idea before the equipment was onsite, but everything is already there and I’d rather not have to reinstall and redesign. I guess this is a discussion between the engineer and the client as I was only hired to do the study. Also am I out of line for telling them that they should shut off the power? I mean the NFPA-70E clearly states that you shouldn’t be working on anything live unless it would endanger people or wouldn’t be feasible.

 

[JFletcher]

Just curious, how are you calculating that level of arc flash energy? Is that based on an infinite primary, actual transformer specs along with feeder size and length, etc.??

Along with what was mentioned above, you can get breaker operator gear that can operate breakers remotely... Not sure how they attach, maybe magnetically, however a person need not be in the same room as the breaker to open or close it.

I did not think one had to suit up to operate a breaker, regardless of incident energy.... where is it written that you have to put on PPE to operate a breaker?

 

[PE (always learning)]

Just curious, how are you calculating that level of arc flash energy? Is that based on an infinite primary, actual transformer specs along with feeder size and length, etc.??

Along with what was mentioned above, you can get breaker operator gear that can operate breakers remotely... Not sure how they attach, maybe magnetically, however a person need not be in the same room as the breaker to open or close it.

I did not think one had to suit up to operate a breaker, regardless of incident energy.... where is it written that you have to put on PPE to operate a breaker?

Hey,

I’m calculating incident energy with actual feeder lengths, available fault currents documented from the utility and actual transformer impedances. Everything is as accurate as possible and I’m using the Incident Energy Analysis method utilizing SKM Power tools. I agree with you on the statement about where does the NFPA-70E require you to wear PPE for breaker maintenance. Are you only required to wear PPE if live parts are exposed? For instance if the panel cover is taken off?