Low SCCR

[petersonra]

Suppose one crunches the numbers and finds that the available SCC at a particular breaker is lower than what would typically be required to trip the circuit breaker on the ST setting.

Would it be prudent to lower the ST setting so the breaker trips on ST in the event of a SC?

Or is this just ignored.

 

[ron]

Most important would be that the long time pickup will eventually activate.

The breaker is rated to handle the amount of current that its trip unit will allow, so if it is needed to suport better selectivity, you can leave the STP setting high if needed. Be sure you realize that by doing that, your arc flash inceident energy availabile will be super high since it will take a long time for the breaker to trip.

 

[templdl]

Suppose one crunches the numbers and finds that the available SCC at a particular breaker is lower than what would typically be required to trip the circuit breaker on the ST setting.

Would it be prudent to lower the ST setting so the breaker trips on ST in the event of a SC?

Or is this just ignored.

You have to be referring to an industrial/commercial breaker that would have a mag trip adjustment. If so it would be strange to have this type of breaker in an industrial commercial location supplioed by a POCO that would have such a low available short circuit.
I have never had that scenario presented to me. Is this just a hypothetical question because of curiosity?
However, it is not unusual for power being supplied by a generator to provide a lower available fault current which deffintely would be an issue. Soemtimes even overloads can be an issue where a generatom may not provide a high enough overoad current for a long enough period of time to even trip a breaker on thermal.

 

[petersonra]

You have to be referring to an industrial/commercial breaker that would have a mag trip adjustment. If so it would be strange to have this type of breaker in an industrial commercial location supplioed by a POCO that would have such a low available short circuit.
I have never had that scenario presented to me. Is this just a hypothetical question because of curiosity?
However, it is not unusual for power being supplied by a generator to provide a lower available fault current which deffintely would be an issue. Soemtimes even overloads can be an issue where a generatom may not provide a high enough overoad current for a long enough period of time to even trip a breaker on thermal.

I actually saw an RFQ from a regular customer recently that spec'ed this exact thing. A main 800A CB with 10kA panel SCCR. Most of their RFQs in this size are for 22-65kA, I figure it must be physically located way out on the end of a feeder somewhere.

But it occurs to me that with such a low SSC, the instantaneous trip may well not trip at all if on a normal setting and we were to get a SC at the panel. It made me wonder how much extra damage might happen before the the Cb finally tripped if the instantaneous trip didn't happen.

 

[kwired]

I actually saw an RFQ from a regular customer recently that spec'ed this exact thing. A main 800A CB with 10kA panel SCCR. Most of their RFQs in this size are for 22-65kA, I figure it must be physically located way out on the end of a feeder somewhere.

But it occurs to me that with such a low SSC, the instantaneous trip may well not trip at all if on a normal setting and we were to get a SC at the panel. It made me wonder how much extra damage might happen before the the Cb finally tripped if the instantaneous trip didn't happen.

Don't confuse AIC with instantaneous trip levels. The instantaneous trip point is likely well below the AIC of almost any standard inverse time breaker, even a 10kAic rated breaker.

 

[petersonra]

Don't confuse AIC with instantaneous trip levels. The instantaneous trip point is likely well below the AIC of almost any standard inverse time breaker, even a 10kAic rated breaker.

I didn't.

Maybe I was confusing.

An 800A CB will typically have an instant. trip point of 10X or about 8000A. If there is not 8000A of short circuit current available, what happens when you have a short circuit? Do you have to wait for it to trip on the short time setting? Is that the only option?

 

[kwired]

I didn't.

Maybe I was confusing.

An 800A CB will typically have an instant. trip point of 10X or about 8000A. If there is not 8000A of short circuit current available, what happens when you have a short circuit? Do you have to wait for it to trip on the short time setting? Is that the only option?

Same thing happens often on a smaller scale, especially with control circuit transformers.

I think an 800 amp conductor can handle < 8000 amps for the few seconds it may take to operate the OCPD

It is an inverse/time device. The less available fault current the slower the trip time, but also means there is less incident energy involved in the event. If there is concern of the need for faster tripping then careful selection of the overcurrent device comes into play and will likely mean a fuse device with certain characteristics is what is chosen.

 

[don_resqcapt19]

... The less available fault current the slower the trip time, but also means there is less incident energy involved in the event. ...

If you are talking about arc flash (70E) incident energy, maybe not. The longer trip time with lower current may very well result in a higher incident energy.

 

[kwired]

If you are talking about arc flash (70E) incident energy, maybe not. The longer trip time with lower current may very well result in a higher incident energy.

I am not talking about arc flash incident energy, just the energy present to damage conductors and equipment.

It may be wrong to assume less energy will flow in a particular event. Total energy needs to take VA and time into consideration. So lower SSC may mean longer trip time but still may be similar energy in the event, because it lasted for longer duration.