Mbrooke
Electrical
- Nov 12, 2012
- 2,546
I have a few questions regarding this table from page 4 of this 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:
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?
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:
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?