TCC Curve Overlap - Instantaneous Region

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Designer69 said:
Can the TCCs of two different breakers overlap at the bottom instantaneous region or is this badly designed? see attached pic

Thanks
View attachment 22210

better pic: https://imgur.com/a/m6t7AIF
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IMO, the usual rule of thumb is to have the downstream breaker to be 1 frame lower than the upstream to coordinate well.

CBs lower than 100A usually do not have adjustable inst settings that's why it's either you will have to 1) tweak the available fault current level (longer lines, inductors/reactors, etc.) up to a point where you can see the TCCs do not overlap (1,500A on your TCC plot) or 2) you find another CB with TCC that has a characteristic curve not overlapping the other.
 
Designer69 said:
Can the TCCs of two different breakers overlap at the bottom instantaneous region or is this badly designed?
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Realistically this overlap would only come into play for a bolted fault current, which would most likely be due to a wiring error (equipment failures would typically be arcing faults). So for normal operating conditions, many engineers will accept this type of overlap for general circuits.

I know of at least one one state that does not accept this overlap for Article 700 and 701 applications.
 
CB1 is ahead of CB2. it appears there are no other breakers associated with CB1 or than CB2. If so, who cares if they overlap. Either one takes out the whole service.
 
At the risk of repeating some of the other answers, yes, its common to have the overlap for thermal magnetic breakers. Or maybe I should say that its just sometimes hard to avoid.

Notice that there isn't any point in worrying about currents higher than the bolted fault current available at the breakers.

Most faults will be downstream, and thus have some length of smaller wire in the circuit.

So real world fault currents are frequently much less than the bolted fault current, unless there is a fault right inside a switchboard or distribution panel.

So a lot depends on what you are trying to achieve. Most places this wouldn't be a problem. But for emergency systems, or critical health care facility emergency systems, you would probably try to eliminate or minimize the overlap, or go to the tables Ron mentioned.

This is why there was such a fuss when the code requirement for selective coordination was added to the code. Fuses are much easier to coordinate and much less likely to overlap at high fault currents.
 
steve66 said:
Fuses are much easier to coordinate and much less likely to overlap at high fault currents.
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Only sometimes, particularly when there is a 2:1 ratio between the devices. For example; It is very hard to get a 200A fuse to selectively coordinate with a 150A one, but it can be done with electronic trip breakers.
 
Designer69 said:
I just did a simple one for learning purposes.
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In the region where the inst overlaps, it will be a race as to which device trips first. If you don't care if the main takes out the whole service then its ok as is. If you want to try and trip the CB2 before CB1 takes out the whole thing, then you will have to get a breaker with adjustable time delay, or a breaker that is LT, ST and inst can be turned off.
 
kingpb said:
In the region where the inst overlaps, it will be a race as to which device trips first. If you don't care if the main takes out the whole service then its ok as is. If you want to try and trip the CB2 before CB1 takes out the whole thing, then you will have to get a breaker with adjustable time delay, or a breaker that is LT, ST and inst can be turned off.
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My experience with this kind of problem was really pitiful. The 3,200 A secondary main CB tripped ahead of a 600A feeder breaker which experienced a bolted fault. Upon investigation, the TCCs overlap at the instantaneous region, hence the mis-coordination! The 600A breaker was a non-adjustable one!
 
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