Impact of Short Circuit x/r Ratio on Circuit Breaker

[ish1284]

Hello,

I am well versed in evaluating low voltage molded case circuit breakers against calculated short circuit currents. I understand that if the x/r ratio of the calculated RMS symmetrical current is greater than the breaker's UL text x/r that the breaker must be de-rated based on a multiplication factor per IEEE-242.

I also understand how to calculate this multiplication factor.

What I don't understand is the mechanics. What does a higher x/r ratio do to the circuit breaker. I understand that the higher x/r ratio, the higher dc component, and also the higher peak current. Does this make the breaker harder to interrupt? Does it make the breaker more susceptible to being damaged or melting contacts?

Manufacturers only provide a symmetrical RMS rating for UL listed low voltage circuit breakers and typically do not want to bless off on any values given to them in terms of peak asymmetrical. Is it an absolute must to consider the peak asymmetrical current or is this just recommended practice?

Let's discuss!

 

[ron]

The UL listing is based on the 3 cycle symmetrical and the x/r test results like you said.

The x/r outside of the test criteria is harder for the contacts to break the arc, which is why you derate based on the multiplier associated with the x/r you determine during calculations. This is how you take asymmetrical into account without directly considering asymmetrical

https://www.powerstudies.com/sites/www.powerstudies.com/files/ImportanceofX-over-RRatios.pdf

 

[MRKN]

If a higher X/R ratio only affects the DC offset, then we know di/dt at the zero-crossing does not have an affect. I believe the reason you need to de-rate the breaker interrupting capacity depending on the X/R ratio is that the higher the peak amplitude, the more energy dissipated in the arc which is established upon mechanical separation of the contacts, which in turn heats up the air / insulating medium (you have to consider worst-case of a full half cycle of arcing) and increases the electric charge carrier density in the ionized dielectric. Due to the thermal inertia time constants being large compared to the frequency of 60 Hz zero current crossings, this in turn makes it more difficult for the air gap / insulating medium to remain insulating after current interruption at a zero-crossing.

 

[kingpb]

Check the Eaton publications on Power System Analysis, used to be Section 1.2 in old Cutler Hammer book. It talks about X/R and gives examples of how it is applied. Too much info to show here.