Arc Flash Cat. vs. Coordination

[adamscb]

One recent issue I've been running into when trying to coordinate protective devices is the balance between perfect coordination, and the arc flash category #. In our plant, most of the feeder breakers to our new MCC lineups have the instantaneous trip setting enabled. While this provides a low arc flash category rating, this introduces a possible coordination problem, where the MCC feeder breaker could trip before the motor feeder breaker when a fault occurs that produces high enough amps to be in the instantaneous region. Our engineering group at corporate likes the arc flash rating to be below a 2, but this is hard to achieve while still trying to achieve good coordination. Anyone have any thoughts on this?

 

[big john]

Assuming the MCC feeders are not molded-case many manufacturers have an optional "maintenace mode" feature that can be manually enabled on the trip unit to temporarily drop "instantaneous" down during hot-work activity.

This allows easy reduction of incident energy, without sacrificing coordination during normal operation.

The other option is to look into breakers with "zone select interlocking." This would allow the branch breakers to communicate with the feeder breakers, and will actively inhibit time delays only on the breaker closets to the fault allowing for much faster clearing time and lower IE. But again, if any of these are MCCBs, I don't know if that technology exists for those.

 

[wbdvt]

It depends on several things. One is can the system process tolerate some lack of coordination without causing major issues or does the safety aspect out weigh coordination? If the system can tolerate some lack of coordination and this would improve personnel safety, then opting for less IE would be preferred, IMO.

I have had to defer more to coordination than lowering IE in areas where coordination and selectivity are paramount, such as in health care facilities.

I think the best path is to layout the options to upper management and let them decide.

 

[petersonra]

One recent issue I've been running into when trying to coordinate protective devices is the balance between perfect coordination, and the arc flash category #. In our plant, most of the feeder breakers to our new MCC lineups have the instantaneous trip setting enabled. While this provides a low arc flash category rating, this introduces a possible coordination problem, where the MCC feeder breaker could trip before the motor feeder breaker when a fault occurs that produces high enough amps to be in the instantaneous region. Our engineering group at corporate likes the arc flash rating to be below a 2, but this is hard to achieve while still trying to achieve good coordination. Anyone have any thoughts on this?

can you disable them in some way?

or did you mean there is a lower setting being used to reduce the IE?

 

[gray.one]

Keep coordination. Add ZSI or maintenance mode to lower AFIE.


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[gray.one]

Keep coordination. Add ZSI or maintenance mode to lower AFIE.


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Maintenance mode just sets the instantaneous low. You can just add that to the procedure.


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[topgone]

Maintenance mode just sets the instantaneous low. You can just add that to the procedure.


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Better still, don't allow working hot--> nothing is too important that we can't shut things down before working on them!

 

[petersonra]

Better still, don't allow working hot--> nothing is too important that we can't shut things down before working on them!

a lot easier said than done. very hard to debug electrical problems with the power off.

 

[shoaib10]

One recent issue I've been running into when trying to coordinate protective devices is the balance between perfect coordination, and the arc flash category #. In our plant, most of the feeder breakers to our new MCC lineups have the instantaneous trip setting enabled. While this provides a low arc flash category rating, this introduces a possible coordination problem, where the MCC feeder breaker could trip before the motor feeder breaker when a fault occurs that produces high enough amps to be in the instantaneous region. Our engineering group at corporate likes the arc flash rating to be below a 2, but this is hard to achieve while still trying to achieve good coordination. Anyone have any thoughts on this?

Its sometimes a trade off between cordination and arcflash...I have done a project where there was a higher arcflash at the SWBD but the coordination was perfect.It depends on the client as well...In my case the client wanted to have a maximum coordination even though the arc flash is higher cause they can wear the suit and work on it rather than having problem with coordination down the road...It also depends the load MCC is feeding,if it feeding something like CWP,s or AHU's then you dont want to loose the cooling or heating just because you want your arc flash to be lower...I would say talk to your cleint and explain the siutation..I am sure there would be a common grounds on this issue...