switchgear rated for 1s is going to be much lighter construction than that rated for 3s.
It might be that you need to start a very large electric motor in which the start up current is massive and lasts for more than a second.
Your protection must trip the supply in either less than 1s or less than 3s. If your protection is set to trip at 0.5s and your motor takes 2s to start a system based on a 1s rated busbar your will not work.
Personally I would stick with 3s rated bars - much safer.
Short circuit withstand time has nothing to do with the motor starting time.
If withstand time is less, then cross section of the busbar or cable will be less (Ish = kA/ √t) and cost of the equipment/ cable will also be less. Then why should we go for higher time i.e. 3 sec ? The reason is the allowable time required for relay coordination.
In case of occurance of short circuit, the nearest circuit breaker in the distribution net work should trip and clear the fault say within 0.3 sec which is the sum total of relay sensing and operating time and breaker operating time. If this circuit breaker fails to trip then the fault will be cleared by the upstream circuit breaker after a time delay which is deliberately provided for discrimination purpose. In absence of this time delay there will be a possibility of upstream circuit breaker operating simultaneously or even before the down stream circuit breaker in case of short circuit. This is not expected from a well coordinated relay protection scheme. Hence, considering the worst situation, system designer often envisages higher short circuit withstand time.
Isn't that roughly what I said. I agree you need to achieve relay coordination but you also need to consider the purpose of the installation. Your installation will be likely be switching HV transformers with large inrush, large motors etc. The protection settings of the incomers also need to coordinate with your outgoing circuits. This is what can easily push the time to over a second.
How future proof are your 1s rated bars?
We can cut corners on the rating of a busbars etc but PSCC will remain the same and in the event of a failure no one will thank you becasue you managed to destroy an MCC with wrecked bars and take a plant out for quite a period of time.
Dear Silverfox I am sorry if I have not conveyed my idea properly to you. You must have observed that in my reply I have not advocated for 1 sec rated switchboard. I have only discussed the pros and cons of the 1 sec and 3 sec rated switchboards. The system designer is the best judge to select the switchboard based on his requirement.
Since the discussion was on short circuit protection I mentioned that starting current of motor will not play a role in selection of the switchboard (1 sec or 3 sec). Also the short circuit current setting should be such that it does not allow tripping due to inrush current when a transformer is switched on.
I would say for an LV switchboard above about 1600A there is no difference to the size of the busbars to go between 1S and 3S, since around this rating the current carrying capacity of the bars sets the size rather than the fault level/duration.
What can change is the support system as this would need to cope with the increased fault duration.
Electrical Engineering
Re: Electrical Engineering
Good Answers: