Arc Flash Active Mitigation

[timm33333]

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

 

[wbd]

First off the code does not require an arc flash rated breaker as there is no such thing. Two new items were added to the method to reduce clearing time: 4) Energy-reducing active arc flash mitigation system, 5) An approved equivalent means along with an informational note on an energy-reducing active arc flash mitigation system.

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.

 

[timm33333]

The protective device on the 1200 A feeder supplying the MCC is a 1200 A breaker from an upstream switchgear, this switchgear has other outgoing feeders as well. The arc flash study of this switchgear would be performed.

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

 

[wbd]

The protective device on the 1200 A feeder supplying the MCC is a 1200 A breaker from an upstream switchgear, this switchgear has other outgoing feeders as well. The arc flash study of this switchgear would be performed.

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.

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

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?

 

[timm33333]

Not yet, but I will do arc flash study. It looks that arc flash has changed a lot since I did it last time a couple of years ago. Can a regular 480 V power circuit breaker with instantaneous trip function fulfill the requirement of 240.87(B)(5)?

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.

 

[wbd]

ot yet, but I will do arc flash study. It looks that arc flash has changed a lot since I did it last time a couple of years ago. Can a regular 480 V power circuit breaker with instantaneous trip function fulfill the requirement of 240.87(B)(5)?

Not a whole lot has changed in how to do an arc flash study but more so on the labeling requirements (NFPA 70E-2015) and a guide to specification of scope and deliverables (IEEE 1584.1-2013). You should read both documents.
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.

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.

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?

 

[timm33333]

I will use ETAP. For NEC 240.87 we have to satisfy one of the 5 options. Is option 5 (instantaneous trip in the arcing fault range) the easiest option to implement?

 

[wbd]

When you model it you should be able to see the affects of lowering the instantaneous trip on a permanent basis but that may affect coordination. In that case you may have to replace it with a trip unit that may be able to go lower for a temporary basis.

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..

 

[timm33333]

It looks that 8 Cal/cm2 would be reasonable. So is arc flash analysis required for all panels, switchgear, etc; even if it is not anticipated that the workers would be working on the energized equipment ?

 

[wbd]

Please read the requirements for labeling in NFPA 70E-2015 Art. 130.5(D). That will provide your answer.

 

[timm33333]

Sure I will, thanks for help!

 

[timm33333]

If the MCC does not have main breaker on the incoming 1200 A feeder (the incoming 1200A feeder is protected by breaker in the upstream switchgear); then will the arc flash reduction switch be required in the upstream switchgear and not in the MCC?

 

[wbd]

If the MCC does not have main breaker on the incoming 1200 A feeder (the incoming 1200A feeder is protected by breaker in the upstream switchgear); then will the arc flash reduction switch be required in the upstream switchgear and not in the MCC?

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.

 

[HamburgerHelper]

WBD,

I think he was talking about arc flash tested breakers. The tested clearing time often is faster than the generic curves the manufacture uses.

 

[wbd]

I think he was talking about arc flash tested breakers. The tested clearing time often is faster than the generic curves the manufacture uses.

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.

 

[HamburgerHelper]

Wbd,

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_FILE&dID=202729

If you are a member of IEEE-IAS, the 2011 July/Aug magazine issue briefly talks about this on page 30.

 

[wbd]

Thank you for the link to the paper. I notice that it is authored by a specific circuit breaker manufacturer and probably testing on fuses would result in the same conclusion. The issue becomes is that this is not a consensus recognized standard for determining incident energy values such as IEEE 1584. The recommendations state that the instantaneous trip function should be used, should be set to the lowest level possible and to be aware that the fault current could be less than the instantaneous range of the breaker. Nothing really new there.

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.

 

[HamburgerHelper]

I suppose with anything it is how conservative do you want to be. I don't know anything about them aside from these two sources. I have hard time imagining though it is any less precise than trying to determine how well energy is redirected with arc flash switchgear during an event.

 

[wbd]

I don't think any further discussion is worthwhile unless someone else can provide more insight. You can do what you want but I will stick to recognized standards for my analysis.