KAIC Rating

[AAEB]

Guys,

Good Day!

I just want to seek expert advice regarding the KAIC rating of a Busduct Plug-in Circuit breaker. My calculated 3phase bolted fault current at the line side of the plugin cb (PICB) is 34.3kA, now the contractor purchased a 35kAic PICB. Is it still acceptable? Are there some safety factor needed, let say KA x 1.25?

thanks,

AAEB

 

[jim dungar]

I just want to seek expert advice regarding the KAIC rating of a Busduct Plug-in Circuit breaker. My calculated 3phase bolted fault current at the line side of the plugin cb (PICB) is 34.3kA, now the contractor purchased a 35kAic PICB. Is it still acceptable? Are there some safety factor needed, let say KA x 1.25?

The AIC rating of the bus plug must be adjusted for the X/R Ratio of the installation versus that of the UL test circuit, other than that there is no safety margin required. Also, how much safety margin did you build in when you made your calculations?

Your power analysis software should mention this as part of their Device Evaluation procedures. The adjustment factor depends on the base AIC of the device (e.g. there is one factor for <22kA and a different one for >22kA).

I usually like to see a 10% margin, if I have not been able to make all of the calculations.

 

[kingpb]

As Jim stated, the X/R needs to be considered.

Keep in mind, that unless the fault is on the bus itself, the breaker will never see the full 34.3kA. Upstream of the breaker the fault current through the breaker is very low.

For a fault on the load side the breaker will see the +/- 33kA value. If the fault is on the bus, well, the rating is a mute point because the bus is destroyed and breakers smoked anyway.

The value is still a little close for me and would have trouble saying it was OK without doing some more detailed calcs, such as verifying X/R.