SKM Power Study - Modeling a Transformer

[bpetti]

Transformer = 4160-480/277v with a Secondary Circuit Breaker

Inserting the secondary circuit breaker between the transformer symbol secondary bus will give you a lower incident engery level on the secondary bus than when the circuit breaker is placed after the secondary bus. I understand the reason behind all of that. The part that requires clarification is when to insert the cb before the bus or after the bus. The engineer here in our office just says put it on the after the bus which will give you worst case scenario. Hmm?

Looking through SKM documentation the secondary cb is shown inserted before the bus. But if the breaker is in the same compartment as the loadside lugs is this accurate? What if the cb is not in the same compartment as the loadside lugs? But then does it really matter cause even if you open the loadside breaker isn't the transformer still live until the primary protection is disconnected? What gives? Where do i put this breaker?

 

[ron]

SKM calculates things at busses.
Put the breaker after the imaginary secondary bus so you can show the incident energy on the line side of the breaker (at the bus).

 

[jim dungar]

It depends on what version of SKM you are running.

Since version 6.0xxx, SKM allows you to select if an arc flash value will be calculated on the line side of the device feeding a bus+the bus itself. SKM reports these line side values with the suffix "(lineside)"

Our policy is to put the protective device in front of the bus and use the option to calculate values at both the bus and the line side.

 

[ron]

I agree with Jim's point except that if the first breaker is any distance away from the secondary of the XFMR, you may want the bus at the secondary of the XFMR too, so you will have a sticker to place on the secondary of the XFMR and one for the line side of the breaker downstream.
The impedance between the two may impact the calculation result.