Series Rated Breakers

[philly]

I understand how series rated breakers are applied (tested and listed combinations) but I never quite had a full understanding of what the rating was based on.

My understanding has always been the upstream breaker in the series rating somehow "current limited" the fault current so that the downstream breaker would see a reduced let-through current which was below its interrupting rating.

For example if the downstream breaker (Breaker B) has a 35kA interrupting rating but a 65kA series rating with upstream breaker (Breaker A) then does Breaker A somehow current limit fault current on the load side of Breaker to below 35kA? So any fault current passing through Breaker A (up to its 65kA rating) will be current limited to Breaker B and thus Breaker B can take on the 65kA rating of the upstream breaker?

So the upstream breaker acts somewhat like a current limiting breaker/fuse?

 

[Jraef]

As I understand it, it's more based on the fact that since for the series rating to be relevant, the fault must be down stream of the lower rated breaker. So if it is high enough to make the up stream breaker trip and begin to open, it would by definition be high enough to have already started the lower breaker opening, and the two sets of contacts in series opening together effectively increases the interrupting capacity of the circuit as a whole.

 

[Carultch]

I understand how series rated breakers are applied (tested and listed combinations) but I never quite had a full understanding of what the rating was based on.

My understanding has always been the upstream breaker in the series rating somehow "current limited" the fault current so that the downstream breaker would see a reduced let-through current which was below its interrupting rating.

For example if the downstream breaker (Breaker B) has a 35kA interrupting rating but a 65kA series rating with upstream breaker (Breaker A) then does Breaker A somehow current limit fault current on the load side of Breaker to below 35kA? So any fault current passing through Breaker A (up to its 65kA rating) will be current limited to Breaker B and thus Breaker B can take on the 65kA rating of the upstream breaker?

So the upstream breaker acts somewhat like a current limiting breaker/fuse?

Essentially, the upstream OCPD will safely trip, before the downstream OCPD will catastrophically fail. Provided that the incident fault current at the upstream OCPD is within its own KA rating.

Observe that even "current limiting fuses" are not universally series rated with all breakers. Each breaker family will typically say "series rated with class X fuses".

 

[philly]

Essentially, the upstream OCPD will safely trip, before the downstream OCPD will catastrophically fail. Provided that the incident fault current at the upstream OCPD is within its own KA rating.

Ok so essentially the upstream breaker is guaranteed to trip first (faster clearing time) thus preventing the downstream breaker from opening. The downstream breaker may begin to start opening as Jraef mentioned but the upstream breaker will still trip faster thus preventing the downstream breaker from clearing the fault.

Sounds like it has nothing to do with the upstream breaker current-limiting as I had thought.

 

[topgone]

Ok so essentially the upstream breaker is guaranteed to trip first (faster clearing time) thus preventing the downstream breaker from opening. The downstream breaker may begin to start opening as Jraef mentioned but the upstream breaker will still trip faster thus preventing the downstream breaker from clearing the fault.

Sounds like it has nothing to do with the upstream breaker current-limiting as I had thought.

AFAIK, the conditions for "series-rating" requires that the downstream breaker remains "passive" and it will be the upstream breaker that "interrupts" the fault.

240.86 Series Ratings. Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in (A) or (B), and (C).

Snippets:
A) The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations.

B) Tested Combinations.. . . -->For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.

C) Series ratings shall not be used where:

1. Motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device, and
2. The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.

 

[mbrooke]

I always wanted ask, but never have. How does it work? I've seen load centers with 22KAIC mains that magically allow for a 10KAIC branch, but if not main is present, then 22KAIC branches :blink:

 

[Carultch]

Ok so essentially the upstream breaker is guaranteed to trip first (faster clearing time) thus preventing the downstream breaker from opening. The downstream breaker may begin to start opening as Jraef mentioned but the upstream breaker will still trip faster thus preventing the downstream breaker from clearing the fault.

Sounds like it has nothing to do with the upstream breaker current-limiting as I had thought.

Not necessarily.

There are four events to consider:
1. the downstream breaker trips
2. the upstream breaker trips
3. the downstream breaker catastrophically fails
4. the upstream breaker catastrophically fails

As long as event 2 happens before event 3, the breakers can be series rated. It still is of interest to use breakers that have event 1 happening before event 2, such that the branch circuit trips on a fault initiating in the branch circuit, before taking the whole building with it. This is what is known as selective coordination. It is not usually an NEC requirement for the run-of-the-mill job, but it is a design issue for serious installations.

With a 10 kaic branch breaker, and a 22 kaic main, you'd want the 10 kaic branch breaker to trip up to 10 kA, and then remain passive if the fault current is over 10 kA. The time-current curves would cross over, so that the upstream breaker trips first upon > 10 kA currents.

 

[kwired]

I always wanted ask, but never have. How does it work? I've seen load centers with 22KAIC mains that magically allow for a 10KAIC branch, but if not main is present, then 22KAIC branches :blink:

Jraef's explanation in post 2 is the most logical explanation I have ever heard on how this series rating works.

If you have more then 10kA flowing I sure hope you are within range to initiate trip sequence for the "main" device, and the combination of both devices trying to open nearly at the same time does clear arcing faster then just one set of contacts opening. If 10kA + was flowing in the secondary device don't expect it to look like brand new afterwards either though.

I got to see Square D testing lab back when I was still in college, and didn't understand what was going on as well as I would now, but I do remember them showing us a demonstration of one test where they did take a three pole breaker and tied all three output leads together, plugged it into a panel mounted inside a vault and closed the heavy vault door. They then applied a source that was supposed to have 10kA available fault current - did make a fairly large boom. They then opened the vault door and smoke was still rolling off equipment in there. The breaker handle was in trip position, they attempeted to reset it, and it apparently did reset - but looked like something that either needed replaced or at least tested before using again.

 

[mbrooke]

Jraef's explanation in post 2 is the most logical explanation I have ever heard on how this series rating works.

If you have more then 10kA flowing I sure hope you are within range to initiate trip sequence for the "main" device, and the combination of both devices trying to open nearly at the same time does clear arcing faster then just one set of contacts opening. If 10kA + was flowing in the secondary device don't expect it to look like brand new afterwards either though.

I got to see Square D testing lab back when I was still in college, and didn't understand what was going on as well as I would now, but I do remember them showing us a demonstration of one test where they did take a three pole breaker and tied all three output leads together, plugged it into a panel mounted inside a vault and closed the heavy vault door. They then applied a source that was supposed to have 10kA available fault current - did make a fairly large boom. They then opened the vault door and smoke was still rolling off equipment in there. The breaker handle was in trip position, they attempeted to reset it, and it apparently did reset - but looked like something that either needed replaced or at least tested before using again.

I can understand at 12,000amps the magnetic pickup curve exceeds both the main and the branch, however, what if one was slighly faster at unlatching than the other?

 

[kwired]

I can understand at 12,000amps the magnetic pickup curve exceeds both the main and the branch, however, what if one was slighly faster at unlatching than the other?

Not a question I can answer.

Extinguishing the arc is not as much of a problem for AC current as DC current, but regardless you either open the circuit and extinguish the arc before the other trips or you have the second opening of a contact as an assistance in extinguishing an arc. Has to be better then then the 10kA rated device alone when the fault is over 10kA.

One thing you also have to remember is if you have more then 10kA available but still use series rating of a main to allow 10k devices, that 10 kA available fault current does drop as you travel down the branch circuits. The series rating is kind of only for when the fault occurs right at or near the panel. Just 20 or 30 feet of 12 AWG branch conductor can reduce the available fault current quite a bit on the load end of that conductor.

 

[jim dungar]

It is unbelievable how much misinformation is contain in this one thread.

A tested series rating means the the two protective devices have a single combined AIC rating. Nothing more.

There is absolutely no requirement that the upstream device opens before the downstream device.
UL pretty much just cares that the fault is cleared, both devices remain functional, and that the conductors are not damaged.

There are many ways that fault current can be reduced by having multiple devices in series, which is why actual testing is pretty much mandatory.
for example
a) the lower rated device is actually fully rated at the 'non-standard' combination rating.
b) both breakers begin to open at the same time but the 'smaller' downstream device clears first, the current is limited by the two arcs in series
c) the lower rated device is very slow but is also strong enough to withstand the fault current until the upstream device clears.

 

[dkidd]

A tested series rating means the the two protective devices have a single combined AIC rating. Nothing more.

The downstream device has a higher AIC rating when used with a specific upstream device than it does with non-tested devices upstream. The upstream device has its own rating, which will generally be higher.

 

[templdl]

I can understand at 12,000amps the magnetic pickup curve exceeds both the main and the branch, however, what if one was slighly faster at unlatching than the other?

It is not all that unusual if there is a bolted fault on one of the branch circuits that the main breaker will trip and not the breaker which one expects to be protection that branch circuit.

 

[templdl]

I understand how series rated breakers are applied (tested and listed combinations) but I never quite had a full understanding of what the rating was based on.

My understanding has always been the upstream breaker in the series rating somehow "current limited" the fault current so that the downstream breaker would see a reduced let-through current which was below its interrupting rating.

For example if the downstream breaker (Breaker B) has a 35kA interrupting rating but a 65kA series rating with upstream breaker (Breaker A) then does Breaker A somehow current limit fault current on the load side of Breaker to below 35kA? So any fault current passing through Breaker A (up to its 65kA rating) will be current limited to Breaker B and thus Breaker B can take on the 65kA rating of the upstream breaker?

So the upstream breaker acts somewhat like a current limiting breaker/fuse?

Circuit breakers do have some degree of current limiting ability when they interupt. As such by their ability to limit current the breakers down stream may have a kaic rating less than that of the breaker which feds them which would be refered to as a series combination rating. In the process of the moving contact moving up and pulling the arc up and through the arc chute there is a dynamic impedance that takes place which increases to the the point where the arc is extinguished decreasing the let through current when this happens.
BUT!!! In order to obtain an actual series combination "UL Iisting" the devices MUST BE ACTULLY TESTED in series to confirm that they can can actually pass a series combination rating test. A set of listed series rated breskers by a manufacturer may even also include a list of fuses with their series combination ratings. What they don't have is a combination rating with other manufacturers devices other than fuses.
Knowing that breakers do have to some degree of current limiting ability the only breakers that can be listed as current limiting though must be able to limit their let through current to the I2t of less than that of the first 1/2 cycle. It must be able to open and clear, limiting the current as such.
So, even though the common breaker does limit let through current to some degree when it trips the only breakers that can be listed as current limiting devices must go one step farther by meeting the first 1/2 cycle requirement. To accomplish this there are various designs of moving which may involve stationary contact design to speed up the separation of the contacts and even modifying the arc chutes in order to deionize and extinguish the arc. Thus the magic of a breaker which can interupt and clear tens of thousands of amps in such a small space and live to do it over and over again. This also is how higher kaic ratings are obtained.
By understanding how a breaker interupts an understanding of why series combination rating can be obtained in addition to their interupting ratings.

 

[jim dungar]

The downstream device has a higher AIC rating when used with a specific upstream device than it does with non-tested devices upstream. The upstream device has its own rating, which will generally be higher.

No.

The individual device AIC ratings does not change. It is the combination of the devices that has the 'new' rating.

This is from a 1994 article "Series Ratings by the Molded Case Circuit Breaker Section of the National Electrical Manufacturers Association"

Series Rating‑ A short‑circuit interrupting rating assigned to a combination of two or more overcurrent protective devices which are connected in series and in which the rating of the downstream device(s) in the combination is less than the series rating.

 

[GoldDigger]

A current limiting fuse has a time characteristic at very high current (near its AIC) that essentially interrupts the current quickly enough in the first 1/4 cycle that the current does not have time to rise above the let through current value. It is not so much a dynamic impedance effect as a rapid complete quenching of the arc. As noted, it is far more complicated to achieve this with a resettable breaker. And it may be necessary to rely more on dynamic impedance than ultra-rapid quenching.
For those of you who wonder what happens when the fault occurs at the voltage peak of the applied voltage, part of the answer is that even then there is a time constant for the rise in current.

Sent from my XT1585 using Tapatalk

 

[philly]

BUT!!! In order to obtain an actual series combination "UL Iisting" the devices MUST BE ACTULLY TESTED in series to confirm that they can can actually pass a series combination rating test. A set of listed series rated breskers by a manufacturer may even also include a list of fuses with their series combination ratings. What they don't have is a combination rating with other manufacturers devices other than fuses.

Is the fuse typically listed as a specific manufacturer/model fuse or just a type of fuse (Class J for instance) when listing series rated combinations with fuses?

It has always been my understanding that fuses need to be "Listed" to be used for series ratings with downstream breakers.

I have seen many cases where a panelboard is overdutied and someone believes that simply placing a current limiting fuse upstream of the panel will lower the fault current at the panel to within its ratings. To me this is wrong since as mentioned the fuse must be specifically "listed" as a combination rating with the breakers in the panel? I have always understood that there is a dynamic impedance created by the opening of the downstream breaker that would make methods such as the Up-Over-down method incorrect when applying a current limiting fuse.

 

[philly]

It is unbelievable how much misinformation is contain in this one thread.

A tested series rating means the the two protective devices have a single combined AIC rating. Nothing more.

There is absolutely no requirement that the upstream device opens before the downstream device.
UL pretty much just cares that the fault is cleared, both devices remain functional, and that the conductors are not damaged.

There are many ways that fault current can be reduced by having multiple devices in series, which is why actual testing is pretty much mandatory.
for example
a) the lower rated device is actually fully rated at the 'non-standard' combination rating.
b) both breakers begin to open at the same time but the 'smaller' downstream device clears first, the current is limited by the two arcs in series
c) the lower rated device is very slow but is also strong enough to withstand the fault current until the upstream device clears.

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?

 

[jim dungar]

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."

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.

Or am I incorrect here and the downstream breaker may actually trip first in some cases?

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.

 

[jim dungar]

Is the fuse typically listed as a specific manufacturer/model fuse or just a type of fuse (Class J for instance) when listing series rated combinations with fuses?

It has always been my understanding that fuses need to be "Listed" to be used for series ratings with downstream breakers.

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.