1 SECOND & 3 SECOND SHORT CIRCUIT RATING
1 SECOND & 3 SECOND SHORT CIRCUIT RATING
(OP)
1.What is the difference in 1 second & 3 second rating.For eg: 65ka,3 second.
2.In our plant,11 kv primary feeders (main incomers)busbars is rated for 25ka,3 seocond. But in 11kv secondary feeders(to hv equipment),25kA,1second is allowed.How we can know from theortical point of view,1 second is sufficient.
thanx in advance
Abraham
2.In our plant,11 kv primary feeders (main incomers)busbars is rated for 25ka,3 seocond. But in 11kv secondary feeders(to hv equipment),25kA,1second is allowed.How we can know from theortical point of view,1 second is sufficient.
thanx in advance
Abraham






RE: 1 SECOND & 3 SECOND SHORT CIRCUIT RATING
If you exceed the current and duration limit, you are likely to damage your cables and even have energy confinement problems at breakers/reclosers resulting in an explosion.
SO in theory is your clearing time smaller than 1 sec?
RE: 1 SECOND & 3 SECOND SHORT CIRCUIT RATING
Short-circuit currents –Calculation of effects –Part 1:Definitions and calculation methods
ch.3.2.4.1 Electrical Equipment:
Ith<=Ithr for Tk<Tkr Tk=actual fault clearing time Tkr=rated fault clearing time
Ith =thermal equivalent short-time withstand current
Ithr= thermal rated short-time withstand current
That means if the rated fault clearing time is 3 sec and the actual is 1 sec no change for Ith.
Ith<=Ithr*SQRT(Tkr/Tk) for Tk>Tkr
That means if the actual clearing time is more than rated [3>1 sec] Ith=65/sqrt(3)
Briefly:
If the short-circuit protection will clear the fault within 1 sec or less then 65kA will be enough.
If the short-circuit protection will cut the supply in 3 sec or less then you need 112 kA rated for one second withstand current.