Voltage and sustaining an arc 2

[elen09]

I am doing a arc flash hazard analysis and I have found something very interesting. There is a 500kVA transformer feeding into a 1600A fuse, then into a 208V lighting panel. The arcing current is 7140A, if this is then looked on up on the ANSI Time Current Curves, the burn through time is about 9 seconds, or 540 cycles. This causes the Incident Energy to be extremely high...about 172 cal/cm^2. What I would like to know is if it is even possible for 208V to sustain an arc for 9 seconds?

 

[mls1]

Most likely not, but arcing faults at this level are very unpredictable so no one can say for sure. However, IEEE 1584 recommends that unless the worker is unable to move out of the area (for example in a bucket truck) you can manually reduce the arc time down to 2 seconds. This assumes the worker is going to move away.

 

[sslobodan]

Don't count on it to much. If the arc go over 3 seconds it will seriously damage the cable and dielectric will lose its permeability and over voltage will burn him next time if not the first time than the next one.... If you have stress situation on equipment, the circuit should turn off in minimum reaction time and still it should not react at small voltage jumps and drops. Setting the protection is very sensitive matter. And you should not be worried about cable but transformer isolation. It will decrease and it will burn up very soon. Generally thermic stress is what burns out the gear. In theory if you have pure wire (so we can exclude dielectric malfunctions)and proper cooling you can push as much Amper rate as it can physically carry (skin effect) so if the wire will burn on lets say 130 degrees inside core value (Usual much higher) and you can dissipate heating below that you can push the currents even few hundreds times more than nominal. But that is lab experiment, in fact Temperature factor of the max cable current is in difference 20 % from nominal value up till 55% down and if you pass the value cable will burn up.

 

[dpc]

I would use a maximum arc time of 2 seconds. This is suggested in the IEEE 1584 Annex. Arcs are difficult to sustain at 208 V, but IEEE 1584 requires the arc-flash calc to be done if transformer size is 125 kVA or larger.