Long Distance Motor Control Center Arc Flash

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nuckythompson

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Nova Scotia
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Electrical
Hi All,

At one of our plants, there is a 4160V switchgear which is equipped with arc flash protection relays. One of the breakers on this switchgear supplies a nearby 4160-600V transformer which then feeds a 600V MCC ~1200ft away. Looking at the previous arc flash studies, cable impedance was not considered for this long run. With cable impedance considered in my calculations, if there were an arc fault at the MCC, the fault current would not be high enough to activate the arc flash protection relay on the 4160V switchgear.

I am looking for help with a solution. At this time, we are requiring 40cal gear when doing any intrusive work on long distance MCCs. One option is programming a maintenance mode into the arc flash relay which would have a lower fault level, but I am not fond of this for a few reasons (extra steps, inrush, etc.).

My questions are, have you encountered this issue in the field? Are there protection devices out there to install on existing MCCs? We are planning to get a new arc flash study done, but I am looking for answers in the meantime. Appreciate any help! :)
 
With low fault current, the arc flash analysis is often artificially stopped at 2 seconds, with the assumption the worker will get away. I am not a fan of that assumption, as in an industrial environment, the worker may be blasted back into other equipment or a wall and not be able to get away.
Can the arc flash relay at the 4160V SWGR be adjusted to lower its threshold so it can activate based on the low short circuit/arc fault current?
Also, older 5kV breakers are "slow" compared to modern, so even with the arc flash relay set to pickup, the time delay may still allow a lot of current to pass before it is cleared by the breaker.
Can you replace the 480V main (if there is one, or add one) so it can detect arc flash and open quickly?
 
nuckythompson said:
I am looking for help with a solution. At this time, we are requiring 40cal gear when doing any intrusive work on long distance MCCs. One option is programming a maintenance mode into the arc flash relay which would have a lower fault level, but I am not fond of this for a few reasons (extra steps, inrush, etc.).
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Is there a dedicated arc flash relay feeding the transformer? If so, could CT's for the relay be placed on the transformer secondary instead of primary so that inrush would not be limiting how low the fault threshold can be set? This is not really my main wheelhouse, so take that suggestion "with a grain of salt".
 
Without more information on your system and a one-line, it's hard to give specifics, but maintenance-mode settings are a popular solutions for reducing incident energy by clearing the fault more quickly. You sacrifice coordination while in maintenance mode and it increases the risk for human error, but it's usually the cheapest solution.
 
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