1 SECOND & 3 SECOND SHORT CIRCUIT RATING 1

[abrahamJP]

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

 

[Desrod2]

Think energy which is a function of current (squared) and time. A 3 second fault will required the equipment to maintain integrity while seeing 300% more energy than a 1 second fault. Fault duration will depend on your protection devices and scheme. You also have to plan for malfunctions considering operating time of your backup protection. For example, a faulty relay or breaker on your feeder might be defective (stuck breaker for example) and require a protection device closer to your power source to clear the fault. Since you usually design your protection to coordinate properly, the backup operating time will be longer than your normal protecting device time.
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?

 

[7anoter4]

See :IEC Standard 60865-1/1993
Short-circuit currents –Calculation of effects –Part 1efinitions 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.