Arc Clearing Time Upstream of Transformer

[GMIEE]

Hello,

I am performing and arc flash analysis and have run into a problem figuring out what clearing time to use. I am given the bolted fault current at the transformer secondary and the transformer specifications. The 150Kva transformer has secondary of 208V and a fault current of 32000 A. I have been told there are 100 K fuses upstream of the transformer along with some time delay breakers.

How do I figure out what clearing time to use for an arc at the transformer secondary? The upstream breaker has a delay of 2.5 and the fuse clearing time varies wildly based on the current.

 

[dpc]

First you have to determine the arcing fault current. This is a function of the bolted fault current, voltage, and arc gap.

Once you have a value for the arcing current at 208V, you need to convert that to an equivalent primary current using the ratio of the secondary voltage to the primary voltage.

You then plot this current on the fuse TCC, probably to discover that it is going to take a really, really long time to clear an 208 V arcing fault.

You should probably review the basic assumptions discussed in IEEE 1584 and NFPA 70E related to arc-fault hazard analysis to get a better feel for the process used to develop the calculated energy levels.

 

[Zogzog]

Then use the 2 second rule

 

[dpc]

Probably.

 

[GMIEE]

Yeah, I thought I might have to do that. One other quick question. Should the bolted fault current be based on an infinite bus calculation, or the actual fault current?

 

[dpc]

Actual three-phase fault current.