Series Rated Breakers

[topgone]

From what I gather from all the responses there are several factors that effect the series rating all of which must be proven by testing. The general consensus seems to be that with series ratings the upstream breaker will always trip first so that the downstream breaker is never actually interrupting the fault but is rather "withstanding it."

It sounds like the upstream breaker doesn't really "limit" the fault current to the downstream breaker but rather will either open quicker so that the downstream breaker does not interrupt, or take advantage of both upstream and downstream breaker contacts starting to part this being able to better extinguish the arc however the upstream breaker will still trip first? Or am I incorrect here and the downstream breaker may actually trip first in some cases?

Yep. That's how I see it works. The main concern is when the breaker contacts part and the existing breaker isn't rated to interrupt the fault level. That is the reason why addition of a series breaker was allowed in the NEC; to provide a leeway for existing installations with fault levels rising beyond the original levels of fault due to additions/ upgrading of supply transformers. The combination need to be listed as "series-rated" and you're good to go.

 

[kwired]

Fuse-breaker series ratings are not manufacturer dependent.
The fuse in the UL series-rating testing is a special 'umbrella fuse' designed so that its performance is always worse than the fuses sold to the public.

I think you will probably find the trip curve standards for a particular fuse type are probably a tighter window then the trip curve standards for a breaker, so if we have say an RK5 fuse we can expect similar time and current response from it regardless of who manufactured it. Switch it to an RK1 and you will get different response, but still similar regardless of who made it.

 

[jim dungar]

I think you will probably find the trip curve standards for a particular fuse type are probably a tighter window then the trip curve standards for a breaker, so if we have say an RK5 fuse we can expect similar time and current response from it regardless of who manufactured it. Switch it to an RK1 and you will get different response, but still similar regardless of who made it.

So what??

My comment was that UL uses a special fuse design for testing series ratings. The use of the umbrella fuse during testing means that fuse-breaker series ratings are not dependent on the manufacturer of the fuse.
Trip curves and their tolerances have nothing to do with and are not applicable to 'tested series ratings'. Trip curves cannot be used to show device behavior faster than 0.01s, which is longer than the clearing time of current limiting devices.

 

[electrofelon]

The general consensus is wrong.
There is nothing in the UL series rating test that requires the upstream device to open much less actually clear the fault.


Yes, it is possible that the down stream device trips first.
Most breaker manufacturers publish a table of series rated combinations that also selectively coordinate. this would not be possible if the upstream device trips first.

So I get it in the cases of the upstream breaker trips first, or they both are tripping at about the same time. But if the upstream breaker doesnt trip, how is the downstream breaker seeing less current?

 

[don_resqcapt19]

So I get it in the cases of the upstream breaker trips first, or they both are tripping at about the same time. But if the upstream breaker doesnt trip, how is the downstream breaker seeing less current?

Where the fault current is high enough to exceed the interrupting rating of the downstream breaker, both breakers will unlatch in most cases.
If the upstream breaker does not trip, it is likely that the actual fault current was below the interrupting rating of the downstream breaker.

Just because the available fault current exceeds the interrupting rating of the downstream breaker does not mean that every fault will cause the full available fault current to flow though the breaker. The impedance of the fault will limit the amount of current that flows and in many cases to a value below the rating of the downstream breaker.

 

[kwired]

The impedance of the fault will limit the amount of current that flows and in many cases to a value below the rating of the downstream breaker.

Correct, most of the time when you get full fault current effect is when the fault is at or near the branch circuit breaker. Quite often a fault occurs at an outlet or even within utilization equipment, at that point the branch circuit conductor length is going to limit available fault current and the branch breaker may not even see near the current that would flow if the fault were right at/near the breaker.

 

[jim dungar]

But if the upstream breaker doesnt trip, how is the downstream breaker seeing less current?

This is problem with deciding series rating except by actual test. There is not a single simple explanation.
For example: the current may not actually be reduced at all, the downstream device may be fully capable of interrupting the 'series-rating' it just can't pass the next standard UL test level.

 

[jim dungar]

Just because the available fault current exceeds the interrupting rating of the downstream breaker does not mean that every fault will cause the full available fault current to flow though the breaker.

The NEC 110.9 requirement is that the device be able to hand the available bolted fault current at its line side terminals. Testing is done with not more than 4' of conductor on the load side, as well as with solid bussing. The through fault currents condition, you mention, is not applicable to the AIC rating assigned by UL.

 

[don_resqcapt19]

The NEC 110.9 requirement is that the device be able to hand the available bolted fault current at its line side terminals. Testing is done with not more than 4' of conductor on the load side, as well as with solid bussing. The through fault currents condition, you mention, is not applicable to the AIC rating assigned by UL.

My point is that the actual real world fault current may be well be low the available fault current that the system was designed for. Where the actual fault current is low enough the downstream breaker of a series system may act like a fully rated breaker installed in the same circuit.

 

[jim dungar]

My point is that the actual real world fault current may be well be low the available fault current that the system was designed for. Where the actual fault current is low enough the downstream breaker of a series system may act like a fully rated breaker installed in the same circuit.

I fully agree.
I often suggest series-combinations, especially for branch circuits.