Arc flash calculation for an existing building

[steve66]

In most of the cases utility generally gives the available short circuit on the xfmr's secondary.Then you have the cable length and size to consider before it gets to the building or sometimes they give the short circuit where the service enters the building...

For a short circuit study, the info. provided above usually assumes infinite current available on the primary, and would not be suitable for an arc flash study.

Here is an example of the type of info. the utility will provide for an arc flash study:

The fault currents at the 34.5KV Tap are as follows:
3LG = 15475A
2LG = 7908A
LG = 10474A
LL = 13400A
34.5KV BUS THEVENIN IMPEDANCE (OHM)
Z(+) = 0.17454+j1.30862 Z(-) = 0.17506+j1.30891 Z(0) = 0.75464+j3.12885

Using realistic info. on the primary side of the utility transformer along with the actual transformer KVA rating and %Z will often result in a significant reduction in the calculated fault current on the secondary side. That reduction often translates to much longer trip times for circuit breakers and fuses, and much higher arc flash energy than one would otherwise calculate.

 

[texie]

For a short circuit study, the info. provided above usually assumes infinite current available on the primary, and would not be suitable for an arc flash study.

Here is an example of the type of info. the utility will provide for an arc flash study:

The fault currents at the 34.5KV Tap are as follows:
3LG = 15475A
2LG = 7908A
LG = 10474A
LL = 13400A
34.5KV BUS THEVENIN IMPEDANCE (OHM)
Z(+) = 0.17454+j1.30862 Z(-) = 0.17506+j1.30891 Z(0) = 0.75464+j3.12885

Using realistic info. on the primary side of the utility transformer along with the actual transformer KVA rating and %Z will often result in a significant reduction in the calculated fault current on the secondary side. That reduction often translates to much longer trip times for circuit breakers and fuses, and much higher arc flash energy than one would otherwise calculate.

I agree. Using an inflated available fault current value for equipment withstand ratings is fine and may be prudent to allow future increases in AFC. But as you say, this can be a dangerous practice when calculating clearing times and thus incident energy. We are seeing wildly inflated figures used to specify equipment in an effort to "be on the safe side" and to comply with 110.9 and 110.10. Now with the new requirement in 2017 110.16(B)(3) to post clearing times I'm afraid that people might use these inflated AFC values and post clearing times that are unrealistic and give a false sense of security.
I'm of the opinion that 2 calculations should be performed. One on the conservative side to ensure compliance with 110.9 and 110.10 and a 2nd precise calculation for clearing times to comply with 110.16(B)(3).