Maximum Circuit Lenghth Help

[Mbrooke]

I have a few questions regarding this table from page 4 of this PDF:

https://steeltubeinstitute.org/wp-c...EC_Requirements_EquipmentGrounding_042820.pdf

1) Why settle on 400% current as the minimum allowable short circuit current? Time current curves of fuses and breakers indicate that current at 4 times the rating takes about 30-50 seconds to clear. Id imagine sputtering or arcing would increase this time.

Reference time current curves:

https://library.industrialsolutions...t Curves|GES-6112A|PDF&filename=GES-6112A.pdf

http://www.cooperindustries.com/con...tasheets-b/Bus_Ele_DS_1002_LPS-RK_70-600A.pdf

http://www.cooperindustries.com/con...asheets-b/Bus_Ele_DS_1009_KRP-C_2001-6000.pdf

Would this time not be a fire and shock hazard, especially at on circuits 30 amps and below?

2) Why do they list short circuit currents at 80% of what chapter 9 table 9 indicates?

3) Why do sizes 350kcm and above require calculations? And if so what type/kind of calculations?

 

[Mbrooke]

Says who? They are absolutely the primary mode of protection against electrocution. In general, the NEC is a FIRE protection code. If you are concerned about shock hazards, then normal overcurrent devices are not the primary protection.

The NFPA CMPs, along with the fact GFCIs have been documented to have a high failure rate.

 

[dpc]

David,

How bizarre. Not me.