Arc Flash Study

[cornbread]

NFPA 70E label question. General question when you have "main tie main", with the ties noramlly opened and lock, do you have to label the breakers with the worse case condition or the normal operating condition?

My conservative side says use the worse case condition but that adds a great deal of fault current and additional PPE that will in all likelhood never be needed. Is it permissible to use the "normal operating" configuration to calculate the Arc Flash and handle tie breakers on a case by case basis?

 

[ron]

We put worst case for automatic operation, unless breakers can be locked out then we put all operating modes with instructions on which to use when.

 

[zog]

NFPA 70E label question. General question when you have "main tie main", with the ties noramlly opened and lock, do you have to label the breakers with the worse case condition or the normal operating condition?

My conservative side says use the worse case condition but that adds a great deal of fault current and additional PPE that will in all likelhood never be needed. Is it permissible to use the "normal operating" configuration to calculate the Arc Flash and handle tie breakers on a case by case basis?

In a M-T-M configuration with proper key interlocking preventing (Without cheating) the 2 transformers to be paralelled you don't need consider that mode of operation as a worse case condition, IMHO.

 

[ron]

In a M-T-M configuration with proper key interlocking preventing (Without cheating) the 2 transformers to be paralleled you don't need consider that mode of operation as a worse case condition, IMHO.

If the mode includes paralleled sources, then the Code Committee would disagree with Zog.
There was a proposal not too many cycles ago, where the .... I think petrochemical folks, wanted to get an exception for closed transition transfers, and they rejected it, indicating that the AHJ needs to decide those types of things.
Of course, I've never ran into an AHJ that wasn't conservative on short circuit rating interpretations.

 

[zog]

If the mode includes paralleled sources, then the Code Committee would disagree with Zog.

With a M-T-M system having the proper interlocks, there is no mode that includes parallel sources.

 

[ron]

We design a bunch of MV and LV SWGR M-T-M that perform closed transition, so there is approx 100ms of paralleled sources.
That's why I wasn't sure what the OP application included.
The OP mentioned more current in one operating mode.

 

[aftelectrical]

On a main-tie-main type installation with the tie being normally open is it advisable or required to break the neutral when closing the tie and opening one of the mains. Just as bakup info the switchgear is a secondary type unit substation with a 2000 kva step-down transformer 4160V-480/277V/Y

 

[ron]

If both sources are separately derived, then transferring the neutral is needed to ensure that return current goes to the correct GF relays.
It seems weird that a 4160V SWGR would have a neutral, but I guess anything is possible.