Understanding ATS Withstand Ratings

[philly]

I was hoping someone could help me understand ATS withstand ratings when looking a the manufacturers datasheet. In looking at one particular datasheet I see there are two diferent withstand ratings given when used with "any manufacturers breakers". One rating is a 3-cycle rating at 50kA and the other rating is a 18-cycle rating at 36kA.

I'm trying to determine which of the two ratings to use for this ATS for the purposes of equipment evaluaiton. Do you have to look at the upstream breaker and see where the short time delay is set at to determine the number of cycles referecned above? For example if at 50kA the upstream breaker has its short time delay set at .5s then this 30 cycles delay is greater than the 3 cycle rating listed for 50kA. Do you then do the same comparison with 36kA and the upstream breaker to see if it complies?

I appreciate any guidance.

 

[jim dungar]

For example if at 50kA the upstream breaker has its short time delay set at .5s then this 30 cycles delay is greater than the 3 cycle rating listed for 50kA. Do you then do the same comparison with 36kA and the upstream breaker to see if it complies?

This is the correct methodology.

 

[philly]

This is the correct methodology.

Thanks. So really you need to examine the settings of the upstream breaker and copare it to fault current. You cant just take the withstand values given at face value.

So in the example given above if the upstream breaker has its Instantaneous setting set at 20kA and takes less than 3 cycles to clear than you can rate ATS at the 50kA withstand?

I would also imagine that you need to look at both normal and emergnecy fault contribuitons to ATS and the upstream breakers. In most cases fault current from a generator will be signifiganty small. So lets say in the same example above that the fault contribution from the generator is ony about 10kA. This 10kA value is much less than both the withstand ratings listed, so I guess the emergency side fault contribution would become irreveelent and we'd then only be looking at the normal source fault current?

 

[JoeStillman]

A quick rule of thumb is to divide the generator FLA by the subtransient reactance - usually in the range of 10-25%, just like you would with a transformer and it's impedance. This is the maximum fault current the emergency generator can supply and it's usually dwarfed by the utility.

 

[philly]

I have also recently seen "Service Entrance Rated" ATS's which appear to only have one withstand rating and not two different withstand ratings like non-service entrance rated switches. I guess the service rated ATS's have the one rating dependent upon the breaker located in the switch whie other ATS's have a two ratings depending on the clearig time of th upstream breaker?