Breaker Coordination in instantaneous region

[mull982]

Can someone please explain to me what happens in the circled area between the two instantaneous regions of these two breakers? It appears they overlap so I was wondering what happens for current in this magnitude?

Is it possible for both breakers to trip at the same time? Or is it possible for the Cuttler Hammer Breaker to trip before the ABB breaker?

 

[steve66]

Can someone please explain to me what happens in the circled area between the two instantaneous regions of these two breakers? It appears they overlap so I was wondering what happens for current in this magnitude?

Is it possible for both breakers to trip at the same time? Or is it possible for the Cuttler Hammer Breaker to trip before the ABB breaker?

Were you going to attach something?

 

[charlie b]

A picture would help, but I think I know what you are trying to say. The answer is that either one could trip first, and the second one may or may not also trip.

For any given level of current, what the Time Current Characteristic (TCC) curve tells us is (1) The soonest the breaker might trip, and (2) The longest amount of time the breaker might wait before tripping. So if two breakers are carrying the same fault current, and if their TCC curves overlap (in the instantaneous region or elsewhere on the curves), what it means is that both are going to get the command to open. Either one could obey that command before the other does. If one opens far enough ahead of the other, then the fault current will have disappeared before the second one gets around to tripping. In that case, only one of the breakers will trip, and there is no predicting which one it will be. Is is also possible that the second breaker will commit itself to tripping before the fault current has vanished, so that it will trip too.

 

[zog]

Charlie nailed it, you can add ZSI to prevent such an issue.

 

[jim dungar]

Can someone please explain to me what happens in the circled area between the two instantaneous regions of these two breakers? It appears they overlap so I was wondering what happens for current in this magnitude?

It is unpredictable.

Is it possible for both breakers to trip at the same time?

Yes

Or is it possible for the Cuttler Hammer Breaker to trip before the ABB breaker?

Yes


While selective coordination of breakers is possible, it can only be determined by actual testing. It is unlikely that any manufacturer will conduct testing with someone else's devices.

 

[mull982]

One of these days I'll remember an attachment :grin:

 

[templdl]

Short circuit Coordination with standard themal Mag breakers is ot luck at best.
An adjustible short time delay option is often available on breakers that are available with electronic trip units. Delays can be adjusted up to 300ms on on some up to 500ms. This limited time delay is due to the breakers ability to keep its contacts closed when subject to a fault as trhe breaker nust open its contacts to clear the fault befor it is damaged. The is what you call withstand.
Another option ti available called zone interlocking wher breakers talk to one another. If 2 or more breakers see the fault they will communicate in such a may to assure that the down stream breaker clears the fault with no time delay.

 

[steve66]

One of these days I'll remember an attachment :grin:

Since one breaker has a time delay, and the other doesn't, which one trips first will depend on the fault current.

If the fault current is high enough (above about 2200 amps), the MCC breaker should trip first since it doesn't have any delay.

However, breakers take a certain amount of time to unlatch their trip mechanism, and another time delay to actually open the circuit. So it is also possible that both might trip for currents above 2200 amps.

Steve

 

[zog]

Since one breaker has a time delay, and the other doesn't, which one trips first will depend on the fault current.

If the fault current is high enough (above about 2200 amps), the MCC breaker should trip first since it doesn't have any delay.

However, breakers take a certain amount of time to unlatch their trip mechanism, and another time delay to actually open the circuit. So it is also possible that both might trip for currents above 2200 amps.

Steve

You lost me on this post. where are you getting 2200A? it is a logrymthic scale.

Where are you getting a time delay? He is nto asking about the RL-2000.

 

[jim dungar]

However, breakers take a certain amount of time to unlatch their trip mechanism, and another time delay to actually open the circuit.

UL 489 listed thermal magnetic and electronic trip circuit breaker trip curves are supposed to include all of the normal tolerances and inherent delays, so what you see on the curve is what the breaker should be doing (this is not true of relays and fuses).

 

[steve66]

You lost me on this post. where are you getting 2200A? it is a logrymthic scale.

I'm getting that from the MCC breaker (the CH breaker). I was trying to approximate its trip point from the graph, but I see now its trip point is at 2500 amps, not 2200.

Where are you getting a time delay? He is nto asking about the RL-2000.

I assumed he is talking about the ESP Cabinet Breaker and the MCC breaker. By time delay, I was refering to the space below the ESP Cabinet Breakers curve.

UL 489 listed thermal magnetic and electronic trip circuit breaker trip curves are supposed to include all of the normal tolerances and inherent delays, so what you see on the curve is what the breaker should be doing (this is not true of relays and fuses).

OK, that makes sense. So with that, and now that we have a graph, it looks like below 2500 amps, the ESP Cabinet breaker should trip, between about 2500 and 5000 amps, it could be either or both breakers that trip, and above about 5000 amps, the MCC breaker has to trip first. Does that sound right.

Steve