Etap Arc Flash Support

[R_Roc]

I have conducted an arc flash analysis in Etap 14.1.
I'm getting a warning message that states "Exceeds Max. PPE Arc Rating"
I believe this is due to my bolted fault current or Ia (Arc Current) of 2.9KA.

I have a LV (480VAC) Main CB Cutler and Hammer KD Series Breaker, with a Trip Device Part # KT3400T, 4,000 amp trip).
I believe the Fault Current Calculated is too low for the Main CB to trip. My FCT is 136 sec with an Ie = 1277 Cal/CM2.

What is normally done with cases like mine?
The breaker is installed in the MCC and the facility is up and running.
I doubt they would want to change our the breaker. Also another breaker type might break the UL rating of the MCC as well.

Also I have graphed a TCC curve and it shows the fault is before the the trip curve so the main breaker will never trip.

See attached of One Line Diagram.

 

[oldfieldguy]

This is not uncommon.

Obvious remedy is to upgrade the circuit breaker. Many modern types have various 'smart' trip units that can be set to reflect your actual fault current parameters, allow you an instantaneous trip. This reduces fault clearing time. If the low FCT interferes with system operation, you can add a 'maintenance switch' which, when active, reduces instantaneous pickup values to a low and safe level while downstream maintenance is performed.

In your case, you show the issue with your system on generator. do you have similar issues when on utility power?

old field guy

 

[wbd]

In IEEE 1584 there is a time limit of 2 seconds that can be applied in certain conditions. Limiting the time of the arc flash event to 2 seconds will greatly reduce the incident energy. There should be a way in ETAP to set the time to 2 seconds

 

[R_Roc]

When the System is on Utility Power no issue at all. I get low incident energy levels, since the Main_CB has a different type trip curve.
For the mean time I will note and state my findings. If they Client wants to Invest/go another
route I will go with your suggestion.


Thanks again,
R_Roc

 

[Skythian]

You wouldn't expect someone staying and frying in front of the arc for 136 seconds waiting until the arc is cleared by a protection device, would you? It's usually assumed that the outer limit for the arcing time is no more than 2 seconds. Although this is not a hard and fast rule, it accounts for the likelihood that the arcing material in an arcing field will likely be either burned off or expelled by the force of the blast. In any case, this would extinguish the arc event. Also, IEEE 1584 states "If the time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible and two seconds is a reasonable maximum time for calculations." [ Annex B page 76] Not sure about ETAP but the Arc Flash Analytic software I'm using allows adjusting upper time limit for incident energy calculations:

 

[jghrist]

You might also want to look at the generator decrement curve to see how the fault contribution decreases with time. The motor contributions will also disappear after 5 cycles or so; this can be accounted for in SKM, but I don't know about ETAP.

Is there a 51V relay protecting the generator that would trip faster for low level faults with low voltage? There will probably be under/over frequency protection that would trip the generator after a short time delay.

 

[R_Roc]

In my Arc Flash Study Case I can limit the current to 2 sec.
I'll state in my report that I clamped the FCT to 2 sec due
to low calculated fault current or should I not mention it
since this sounds pretty typical and the client will see my TCC curves
and data anyways.

 

[jghrist]

Also I have graphed a TCC curve and it shows the fault is before the the trip curve so the main breaker will never trip.

It's not a good idea to have protection settings such that the breaker will never trip for a fault.

 

[R_Roc]

@ Skythian

I understand what you are saying about limiting the FCT to 2 sec, but what if it was a bolted fault that occurred. The breaker would still not trip.
I plan on making a recommendation anytime the Ie is high turn off genset for trouble-shooting/maintenance.