What is the Difference Between Rated Short Circuit Duration 3 Seconds and 1 Seco

From the small amount of information you've provided in your question the only answer I can provide is that the difference is two seconds. I'm also certain that you do not want this obvious, obnoxious answer. Forgive me for that. Allow me to offer a few similar questions that might be what you meant to ask.

• Why does a circuit breaker rated for current X take three seconds to trip with Y current and one second with Z current? X<Y<Z
• What is the design difference between two circuit breakers rated at current X that one breaker trips in only one second with current Y while the other circuit breaker trips in three seconds with current Y? Also why would anyone desire this difference?
• You cannot find a circuit breaker rated to trip in the customer specified time of one second at current Y. You did locate a circuit breaker rated to trip in three seconds. Can you use your replacement?
• You have a circuit breaker that previously tripped in one second at current Y. The same circuit breaker now trips in three seconds with the exact same current Y. What possible change has happened in this circuit breaker to make this change?

Any of these questions may have been what you meant to ask. It may even be something else you meant to ask. Please clarify.

Difference between is exactly 1 second

i believe this is because of time-based discrimination, the cheapest and least logical fault-discrmination methods. The upstream breaker has to wait till any possible downstream breakers trip, so that there is no nuisance oss of power to healthy feeders. Nowadays, most systems use energy-based discrimination, which does not need such a long wait time.

The 3 sec breaker should withstand flow of fault current for 3 sec without any damage to the breaker. Similarly the 1sec breaker should with stand for 1 sec.

I Know what you say.

but my question is " when do we consider 1 sec. or 3Sec.?"

Select of "1 sec. or 3Sec." Depend on What?

Discrimination Requirements

If it trips in 1sec the equipment and cable connected to it will get less heat damage instead of tripping in 3 sec. Cable manufacturers give short circuit withstand current for each size of cable.

Can We Consider "short circuit withstand current =1 Sec." in 63/20KV Substation?

Why Not or Why Yes?

As has been previously said, the duration is based on the requirements of time discrimination. Traditionally with moving core protection relays and times to send trip signals etc the bands had to be quite wide, leading to 3 second delays on the incoming devices. However with modern high speed protection relays and circuit breakers clearing faults in less time this need has reduced.

The only way your question can be answered is by carrying out a discrimination study, once you have that done the answer will be clear. However I would point out that some authorities require 3 second ratings as standard, irrespective of the system, so that could override any results