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ekstra   ara
 Post subject: Arc Flash and Generator Decrement Curve
PostPosted: Wed Apr 28, 2021 10:38 am 

Joined: Wed Apr 28, 2021 10:18 am
Posts: 1
Hiya,

Just got a question regarding a scenario where a 4160V switchgear is fed by a Generator.
A generator decrement curve has been provided by the manufacture.

I've typically always used the 1/2 cycle method calculation for Arc Flash calculations, but I'm not sure if this is an adequate number to use in this case.
Looking at my TCC, the protection relay curve doesn't intersect with the current decrement curve until close to steady-state.
If i wanted to be on the safe side of things, would it not be acceptable to assign a fixed FCT based on where the two intersect? I believe at 4160V the bolted fault and arcing current should be similar enough for this assumption?

I only ask this as if I assume the calculation is purely 1/2 cycle method, my IE is significantly lower due to a much faster FCT.


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 Post subject: Re: Arc Flash and Generator Decrement Curve
PostPosted: Mon May 03, 2021 8:21 am 

Joined: Wed Feb 01, 2012 10:44 am
Posts: 23
Location: Atlanta, GA
Is your generator protection relay voltage controlled (VC) or voltage restrained (VR)? This will lower the effective response time than just the TCC portion. I would expect a 4160V generator to have this type of relaying.


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