How to Size ATS

Take into account chances of actuation of ATS in the event of shortcircuit whereby one CB will trip and the second automatically starts to close due to inherent characterstics of the ATS. I would propose a CB in the outgoing circuit of ATS for the protection while leaving the ATS to be an auto changeover switch.

Impidence of generator is high so not much of a problem in short circuit rating selection of CB, you have not mentioned the other source what is it? use the KVA rating for sizing however commercially you will get some standard range CB'S so check with the equipment manufacturers.

Thank you very much for your help, the ATS has already a CB on the load side. emergency side and normal operation.

So please suggest me, should i install a separate CB for generator, which is just 3 - 5 mtr away from ATS. or you you suggest just a MCCB or whole switch gear with over load and over current relay ?

Up front you say the voltage is 13.8 KV and now you are asking for an MCCB which is a low voltage device, for the HV systems vaccum/oil/SF6 etc. would be applicable.

I do not readily agree with you for interposing a CB for generator it will icrease the cost as well as complexity without much benifit.

I prefer interlinking of existing protection relays of the generator to initiate master shut down, lockout etc. in order to protect dammage in the event of a fault in the zone; generator- ATS outgoing CB.

sorry .... I made a mistake, thankyou for correcting me, what i meant was a CB.

Sir I have much information about how to protect the GEN but in this case i'm confused because it a 2000 KVA Diesel GEN and a backup to the normal supply. Like i said GEN is just 3-5 meters away from ATS, So therefore i need help and suggestion form your side what needs to be done.

sizing factor:-100%.

It means the size of ATS should be same as the size of Generator minimum.

If you are not bothered about the protection of Generator and cables , no need additional breaker on the generator side.

Most important is you have to take care of your job security.

How will an additional CB, protect the generator from faults such as "rotor earth fault or inter-turn winding insulation failure"?

Sencibly you need to shut down the machine.

If you provide a circuit breaker in ATS you have to provide a CB in generator with proper protection to secure the generator and load. Since there is no details of generator we cannot exactly decide if MCCB can do the switching function easily in ATS.