Arc Flash Active Mitigation
Arc Flash Active Mitigation
(OP)
NEC-2014 240.87(B)(4) requires arc flash rated breakers on feeders of 1200 A and higher. We have an MCC which is powered by 1200 A feeder, the origin of the feeder is in a different room. If we place an arc flash rated breaker at the origin of the feeder (in different room), then do we need a main breaker for the downstream MCC? Or can we skip the main breaker of the MCC and simply provide the written procedures that the upstream breaker (in different room) must be padlocked before performing any maintenance work at the downstream MCC ? Thanks






RE: Arc Flash Active Mitigation
So for your system as described, what is the protective device on the 1200A feeder supplying the MCC? Has an arc flash study been performed? The reason I ask these questions is that you may be able to reduce your arc flash incident energy with installed upstream from the MCC protective device by either making a permanent instantaneous settings change or possibly installing a new trip unit that has a maintenance switch.
RE: Arc Flash Active Mitigation
Do we need a main disconnector/breaker in the MCC (because the upstream breaker is in a different room), or can we skip the main disconnector/breaker of the MCC and provide written procedures to padlock the upstream breaker (in switchgear) before performing any maintenance on the MCC? Thanks
RE: Arc Flash Active Mitigation
An arc flash study should be performed on any equipment that will need servicing, maintenance, etc. Testing for de-energized would require properly rated AR PPE for that task.
If you are working on the entire MCC, I would de-energize and LOTO at the upstream breaker so that there is nothing energized in the MCC. Testing to verify that it is de-energized would require AR PPE that the MCC requires.
Have you had a study done so that you know what the proper AR PPE is?
RE: Arc Flash Active Mitigation
Also what is the name of the equipment which provides active arc flash mitigation system of 240.87(B)(4)? I thought it was something installed on the breaker, but as you mentioned above it has nothing to do with the breaker.
RE: Arc Flash Active Mitigation
An instantaneous trip will satisfy the requirement as long as it is set properly to trip in the arcing fault current range. This is because sometimes there is no instantaneous trip or it is turned off on larger breakers to coordinate with downstream devices.
Usually it is a relay that has both a current input and a fiber optic input. Typically the fiber optic cable is routed thru each MCC/Switchgear cubicle and will detect the flash of an arc fault. So if it sees current greater than the setpoint AND a flash, it will trip the protective device.
What software are you using for your arc flash study?
RE: Arc Flash Active Mitigation
RE: Arc Flash Active Mitigation
Of course the question is how far to go on mitigation? What value of incident energy is reasonable? And also, unless there is some reason to work it live, you should be de-energizing. Then the only "live" work is testing dead which can be done at a distance greater than 18" which should reduce the incident energy..
RE: Arc Flash Active Mitigation
RE: Arc Flash Active Mitigation
RE: Arc Flash Active Mitigation
RE: Arc Flash Active Mitigation
RE: Arc Flash Active Mitigation
Why yes. How else would you do it? Even if the MCC has a main breaker, you cannot take credit for it interrupting an arc flash event.
RE: Arc Flash Active Mitigation
I think he was talking about arc flash tested breakers. The tested clearing time often is faster than the generic curves the manufacture uses.
RE: Arc Flash Active Mitigation
To the best of my knowledge and I have been doing arc flash studies for a number of years, there is no such thing as an "arc flash tested breakers". There is arc resistant switchgear on the market which is designed and built to minimize the effects of the are flash blast and probability of an internal arcing fault. This is done thru design of the buswork, compartmentalizing and providing a means of directing the blast away from the front of the switchgear, usually thru the top.
RE: Arc Flash Active Mitigation
On page 4 of this document, they talk about the difference between tested and generic trip times.
http://www.eaton.com/ecm/idcplg?IdcService=GET_FIL...
If you are a member of IEEE-IAS, the 2011 July/Aug magazine issue briefly talks about this on page 30.
RE: Arc Flash Active Mitigation
The Summary section states that direct testing provides the most accurate information possible but I would think that it would have to be for a wide range of breakers to be useful in industry applications. Yes, the IEEE 1584 equations result in higher incident energy values over the tested values but I don't think conservative is a bad thing right now. Also the breakers tested were most likely very new from the factory assembly line. How would the values change on a breaker that has been sitting on a shelf for awhile before being used? What about one in service that may have seen some faults? What about one that is in service but has not been operated for 364 days prior to it's one year maintenance?
While the paper is interesting, I would not want to risk using test values from a manufacturer without some sort of standard behind that. Think litigation.
RE: Arc Flash Active Mitigation
RE: Arc Flash Active Mitigation