Fused Current Limitng Power Circuit Breaekrs 1

[rockman7892]

For power circuit breakers that have internal current limiters, are these current limiters intended to extend the interrupting rating of the breaker to higher interrupting values or are these current limiters intended to limit let-through current to downstream devices short circuit rating or Arc Flash purposes?

In terms of interrupting rating I've seen newer power circuit breakers that have interrupting ratings up to 100kA without any limiters. With limiters these ratings appear to be extended as high as 200kA. So are these limiters only applicable in cases where the available fault current at the breaker exceeds the maximum interrupting rating of the breaker without limiters?

If intended for current limiting or reducing peak let-through at downstream devices then I know this is a complicated subject matter with these ratings often being mis-applied.

 

[unclebob]

Both. Motor starters, soft-starts and drives, etc, need fuses for SC protection. Small caliber breakers may require current limiter if fault current is too high. For Arc Flash purposes I'm not sure.

 

[jraef]

Do you want the long story with historical context or the short one?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[rockman7892]

Jraef

I'd definitively be interested in the long story with historical context.

 

[davidbeach]

At one point the only way to get to very high interrupting ratings and any sort of current limiting with molded case breakers was to include fuses. Eliminates resetability, the biggest single advantage of the MCCB over fuses in the first place. Then the breaker manufacturers discovered that they could use blow apart contacts in a hair-pin configuration to achieve current limiting without fuses. Multiple CL breakers in series and several may "burp" with out unlatching for a downstream fault.

 

[rockman7892]

So I understand the reason for needing limiters to achieve high interrupting ratings for a given application however what would be applications where the limiters were used for "current limiting" purposes at downstream locations when used in power breakers?

I'm guessing perhaps they are used to limit available fault current downstream? From what I understand you simply cannot "blindly" apply current limiting breakers to achieve reduced fault levels downstream for device evaluation unless they are "tested" combinations. The NEC allows engineering judgment however due to dynamic impedances and other factors it becomes very difficult to apply these for fault limiting applications without tested and listed combinations?

 

[Zogzog]

They are typically used when 2 transformers are operating in parallel, like a welding bus.

 

[rockman7892]

Zog

I would think that for the parallel transformer application you mentioned it would be strictly to increase the interrupting rating of the breakers as a result of the increased fault current from both breakers operating in parallel.

 

[jraef]

Then the breaker manufacturers discovered that they could use blow apart contacts in a hair-pin configuration to achieve current limiting without fuses.

This design concept was invented and patented by Hein Moeller, of Klockner-Moeller fame, in the 1950s and he licensed it to Westinghouse. The final extension of the patent expired only recently (I think in the 90s?) so other mfrs could not yet use it and as available fault currents kept increasing as infrastructure grew, mfrs who could not use this concept had to accomplish the higher interrupts in another way, hence the in line fuses as current limiters. The fuses also took MCCBs into the 200kAIC range, higher than the Moeller/Westinghouse design, but that was not as necessary in the vast majority of cases. Now that those patented concepts have expired, the fuses MCCBs are falling by the wayside.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

If your are talking about UL then yes, the breaker with limiters could be limiting the current reaching downstream components. But as you posted, that is usually only with a tested combination.

Due to the way UL tests, a 35kA rated contactor is not capable of surviving 35kA of fault current going through it. So, you can't just say that the breaker and limiters only allows 35kA of fault current to pass so it co-ordinates with this contactor.

 

[rockman7892]

So did the "blow apart" contact configuration allow breakers to achieve higher interrupting ratings or simply just to current limit the downstream fault current? Or perhaps both? Not sure if the interrupting rating and current limiting terminology are related in this case?

I know Westinghouse/Cutter Hammer has power circuit breakers with interrupting ratings up to 100kA without limiters but haven't looked much at what other manufacturers offer.

 

[jraef]

By reacting and opening faster, the current limiting allows the breaker itself to pass Interrupting Capacity testing at higher fault current levels, which basically just means the breaker does not become a bomb when opening during a 100kA fault. The limitation of down stream current is a bonus that a manufacturer can use in specific tested and listed series combinations, but cannot be used for putting together field components that are not tested and listed in series. So for example Westinghouse, Cutler Hammer, Furnas (pre-Siemens) and A-B all used the Westinghouse fuse less CL breakers to attain a 100kAIC rating on their MCC FVNR starter buckets. But later when Furnas was bought by Siemens and they switched them back over to the old ITE breakers, the max interrupt rating they could get with the ITE breakers was 65kAIC, unless you went to the fused breaker type. Interestingly though, their Size 1 and 2 starters were still listed at 100kAIC, but that's because Siemens chose to brand label the 100A frame Westinghouse / Eaton breakers for use in starter assemblies in order to keep it.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[controlsdude]

interrupting rating
I always thought that the KA rating was a "this component will not become a bomb with that much current running thru it". I agree with jref on that statement.

current limit rating
The fault current limit rating was a rating to where you could use this info, so that you could judge what calorie rated arc flash equipment you needed.

Beyond that i am still a bit unclear on other factual statements about both ratings.

How else would you use these two ratings?

But I thought that the use of combination of components fell under the UL umbrella rating combination of components?

I am still confused based on these two questions.

 

[LionelHutz]

interrupting rating
I always thought that the KA rating was a "this component will not become a bomb with that much current running thru it". I agree with jref on that statement.

No, not always.

If a breaker is rated for 50kA then you can apply 50kA to it's incoming terminals.

A contactor rated for 50kA was not tested with 50kA flowing through it. It was tested in a circuit with a thermal magnetic breaker, some connecting wire and some wire shorting the output. The rated fault current (50kA) is the current that flowed when the cables connected to the incoming side of the breaker were shorted. By the time you add the breaker and interconnecting wire and the contactor and the wire used to short the 3 phases together (maybe 18" or 24"??), the fault current that actually flows through the contactor will will be less than 50kA.

If you want to do higher current co-ordination, then you need the actual current that flowed during the above tests. Say it was 35kA. You can then co-ordinate with a 100kA breaker without testing if you can prove the breaker current limits the 100kA input to <35kA on the output.