TCC Curve for Circuit Breaker

[jim dungar]

Remember the TCC is a function of the trip element and it's ambient and operating temperatures. The ability to follow the curve has absolutely nothing to do with its fault handling capability.

The AIC rating typically has to do with the strength of the fasteners holding the device together. Newer generation devices may employ special internal construction in order to have high AIC values, but in these cases they usually have a different/special trip element.

 

[Grouch]

Remember the TCC is a function of the trip element and it's ambient and operating temperatures. The ability to follow the curve has absolutely nothing to do with its fault handling capability.

I follow and I don't follow.

So if we have a 22k AIC breaker... let's say the curve terminates at 15k, and you have a calculated fault of 18k. How do you interpret the curve then after 15k? The curve won't show the breaker opening, since it cuts off at 15k.

 

[wwhitney]

The basic question here is this: the time-current curve shows how long the breaker is supposed to take to interrupt a given level of current. So suppose the published TCC stops at 15kA, but the breaker is rated for 22 kAIC. Then what will the breaker behavior be if it is presented with a current between 15 kA and 22 kA?

So far, nobody has contradicted the idea the breaker would interrupt currents between 15 kA and 22 kA in the same amount of time that it would interrupt a current of 15 kA. If that is correct, the choice of stopping the graph at 15 kA was arbitrary; the graph could be extended out to 22 kA as a horizontal line and still accurately reflect the breaker behavior.

An alternative possibility, but not one that I've seen anyone suggest, is that the TCC stopping at 15 kA is actually conveying some information. E.g. that with a 22 kAIC rating, the breaker will still open on a fault above 15 kA, but that it may start to get damaged and that damage may slow the opening time. So rather than advertise a specific opening time, the TCC is silent about behavior in that region.

So which one is it?

Cheers, Wayne

 

[lielec11]

The basic question here is this: the time-current curve shows how long the breaker is supposed to take to interrupt a given level of current. So suppose the published TCC stops at 15kA, but the breaker is rated for 22 kAIC. Then what will the breaker behavior be if it is presented with a current between 15 kA and 22 kA?

So far, nobody has contradicted the idea the breaker would interrupt currents between 15 kA and 22 kA in the same amount of time that it would interrupt a current of 15 kA. If that is correct, the choice of stopping the graph at 15 kA was arbitrary; the graph could be extended out to 22 kA as a horizontal line and still accurately reflect the breaker behavior.

An alternative possibility, but not one that I've seen anyone suggest, is that the TCC stopping at 15 kA is actually conveying some information. E.g. that with a 22 kAIC rating, the breaker will still open on a fault above 15 kA, but that it may start to get damaged and that damage may slow the opening time. So rather than advertise a specific opening time, the TCC is silent about behavior in that region.

So which one is it?

Cheers, Wayne

It is the former.

 

[Grouch]

So far, nobody has contradicted the idea the breaker would interrupt currents between 15 kA and 22 kA in the same amount of time that it would interrupt a current of 15 kA. If that is correct, the choice of stopping the graph at 15 kA was arbitrary; the graph could be extended out to 22 kA as a horizontal line and still accurately reflect the breaker behavior.

Ah. Now I follow.

 

[wwhitney]

It is the former.

Very good. So post #8 is correct, and I don't understand post #9.

Cheers, Wayne

 

[lielec11]

Very good. So post #8 is correct, and I don't understand post #9.

Cheers, Wayne

If you're referring to this:

The AIC rating does not affect the TCC at all.
The AIC is about the protective device not turning into shrapnel while it is clearing a fault.

You should always extend your instantaneous region up to the fault current 'flowing through' the protective device.

I think what he's trying to say is simply the AIC rating and the time current curve (TCC) are mutually exclusive. The shape of the curve is not affected by the AIC rating, and vice-versa. As for the last sentence, I believe he means if you're evaluating the trip curve, you should make sure the curve extends to all possible fault currents.

 

[wwhitney]

I think what he's trying to say is simply the AIC rating and the time current curve (TCC) are mutually exclusive. The shape of the curve is not affected by the AIC rating, and vice-versa. As for the last sentence, I believe he means if you're evaluating the trip curve, you should make sure the curve extends to all possible fault currents.

I agree with all the above, but the part you quoted was preceded with "No". Seems like it should be "Yes, and . . ." That "no" was very confusing.

Cheers, Wayne

 

[jim dungar]

I agree with all the above, but the part you quoted was preceded with "No". Seems like it should be "Yes, and . . ." That "no" was very confusing.

Cheers, Wayne

The OP mentioned extending a cut off TCC out to the AIC of a breaker.

My comment was No, because the TCC only needs to show the amount of current that would flow through it.

If the breaker is rated 22kAIC but the system is only capable of delivering 15kA during a bolted fault, what additional information would be provided by extending the TCC?

 

[wwhitney]

If the breaker is rated 22kAIC but the system is only capable of delivering 15kA during a bolted fault, what additional information would be provided by extending the TCC?

I guess this comes down to whether the TCC is meant to describe the behavior of the breaker in general, or if it is meant to describe the behavior only within a particular system. If the former, then extending the TCC would represent how the breaker would behave in a different system that was capable of delivering a higher fault current.

Cheers, Wayne

 

[lielec11]

The curve is the curve regardless of the system. The only thing that changes depending on the impedance and source(s) of the system, is the available fault current. I think you're looking into this too deeply.

 

[jim dungar]

I guess this comes down to whether the TCC is meant to describe the behavior of the breaker in general, or if it is meant to describe the behavior only within a particular system. If the former, then extending the TCC would represent how the breaker would behave in a different system that was capable of delivering a higher fault current.

Cheers, Wayne

My reports were for the systems as installed. The included TCCs were cut off at their available fault current. This made the curves easy to explain rather than having to say stuff like "just ignore that overlap it doesn't occur in your system".

A manufacturer's TCC should be extended to the full range of the device.

A What If analysis is a separate report.

 

[Tainted]

I've searched and searched through Google, but cannot find the TCC curve for a Siemens Q3100 circuit breaker. It's a 3 pole, 100 amp circuit breaker. Can someone please help?

All of the Siemens TCCs for the QP breakers are the same regardless of size.

Finding them on Siemens website is challenging though. I found them in the past, can’t any more.

https://tcc.nyc/ is your friend.

Thank me later

 

[Jraef]

One thing worth pointing out in this "TCC vs AIC rating" debate.

A manufacturer published TCC is expressed in terms of MULTIPLES of the breaker rating, not finite amp amounts. Manufacturer's TCCs do not show the curve going out beyond the point where the curve doesn't change, so they don't go all the way to the IC rating. No point in it.

When you look at TCCs from software like SKM or ETAP, they custom make that curve from the manufacturer's data for the specific breaker you put in the query, so it is showing actual amps, not multiples of the breaker rating. So the actual amps will go out to whatever you state is the AFC on that project.