Fuse Impedance

[TexasPwrEng]

How uniform is fuse impedance? I am performing an arc flash mitigation study for a facility, and have come across a scenario in which two identical fuses (208V 400A... I don't recall the type off the top but can get it if it makes a difference) are in parallel.

Can I assume that the total available fault current at the high side divides evenly between each fuse? If not, how have any of you dealt with this situation?

 

[davidbeach]

If the fuses can actually be used in parallel, the manufacturer will have a published TCC for that parallel combination. If you can't get the TCC, the combination is probably an NEC violation. (I'm assuming NEC based on "Texas" and 208V, but that's only an assumption.)

 

[cuky2000]

Fuse impedance varies with ambient temperature, current (Joule effect), type of SC wave, X/R ratio, etc. The fusing curves published by fuse manufacturers are average RMS values usually provided with a range for clearing time performance, good enough for many practical protection applications.

The current in any parallel protective device obey the physical law and are not perfectly distributed even if you find a manufacturer that could provide this parallel device. At best, the average performance may be provided in range of time fusing fashion.

Beware that arc flash calculation is also an approximation since there is many random variables including the protective devices clearing the fault in a wide range of accuracy usually conservatively if compare with actual test values.

 

[jghrist]

There are manufacturers that provide parallel fuses. Cooper Power does for MV applications. The resistance difference is not as defining as you might first suppose. If the lower resistance fuse takes more current, it increases temperature and resistance.