Hello all,

I am doing an arc flash study using SKM on a facility with large incoming amps from a spot network utility system (236,902 amps). It comes into the main switchboard which has a Bussmann FRS-R 600A current limiting fuse serving a transfer switch. The transfer switch then serves a 600A switchboard. The bolted fault numbers are very high, so high that the main switchboard isn't even rated for it; 159.13kA @ transfer switch, 118.54kA @ 600A switchboard. From looking at SKM's website, I understand their reasoning using the IEEE Buff Book for the high current values. My problem arises when looking at the incident energy levels at these pieces of gear. The main switchboard is very high at over 500 cal/cm2, which is to be expected, but then the transfer switch and 600A switchboard drops to below 1 cal/cm2 each. All this equipment is right next to each other so I would expect higher incident energy values than this. Can anyone shed some light on this dilemma? Maybe I have input some wrong info or the current limiting fuse really does affect things this drastically? I've included a screenshot of this for some clarity.

Any help is appreciated,

Scott

Yes. The fuse does affect it that drastically.

No OCP ahead of the switchboard line side of main in your model so the energy is calculated using a fault that lasts the SKM default (and somewhat standard) 2 seconds whereas downstream of the fuses the fault is cleared in a fraction of a cycle, never reaching "peak" so the incident energy is low.

2 seconds is an eternity and even fairly low fault currents (at least in comparison to your example) will result in high incident energies.

I recommend you get Jim's materials (books and/or vids) on short circuit, coordination and arc flash studies. Both his free stuff and the ones for sale. They are pretty good starting materials.

The arc flash industry's best kept secret is that even a small incident energy (less than one second) can do quite a damage when delivered within short time interval. Its impact is way different comparing to same energy delivered in one second or longer time interval. Check or search this forum for more information.

Thank you both for your answers. I will definitely be checking out some of Jim's training materials.

200 la doesn't even sound very believable. Sounds like the utility is doing too many assumptions.

Agree with Paul - something sounds off. In downtown low voltage spot networks the current can approach 200 kA but I've never seen it over that. Worth digging a little deeper. Which city/utility if you can say - just as FYI?