Current limiting breaker with si

[Evgueni]

Good morning Gentlemen

Several days ago I found in Internet(Google) one poor description about Current Limiting Circuit Breakers with combination of the dual-pivot contacts and single-pivot contacts. The work principle based on the high arc impedance during tripping action.(It could be SIEMENS product) Do you have some opinions about it?
Thanks in advance Evgueni.

 

[jraef]

The concept was invented by Klockner Moeller several decades ago (1950's?) and they held exclusive patents on the design until maybe the mid 1980's when Westinghouse challenged their hold on it and released their C-Line breakers. Everyone else quickly followed suit. Since then it has become a common technology and improved upon I'm sure. The basic concept is that by looping the current path back on itself, magnetic force is created in the conductors within the breaker mechanism that opposes the closure of the contacts. Under normal operation that force is too weak to overcome the spring pressure holding them together. Under fault conditions though, once the release mechanism has let go, the higher the fault current, the higher the magnetic forces and the faster the contacts are opened up. So the design is inherently current limiting in the respect that the level of the fault decreases the clearing time of the fault. Dual and single pivot points are mainly economic concerns, smaller breakers don't need the added forces from dual contact sets.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

Just a point of clarification - The standard Series C circuit breakers are not listed or labeled as "current-limiting" devices.

There are current-limiting circuit breakers, but these are not standard breakers.

To be considered current limiting, the fault must be cleared or limited within the first 1/2 cycle, I believe.

All circuit breakers provide some impedance to fault current, but UL has specific criteria that must be met before a breaker can be called "current-limiting", at least in the US.

 

[davidbeach]

dpc, the current has to be interrupted in less than the 1st 1/2 cycle. If the full 1st 1/2 cycle occurs prior to the interruption, the full peak fault current has passed through the device. The interruption must be significantly less than the 1st 1/2 cycle to get any meaningful limitation.

 

[dpc]

Yes, I agree. It's probably more like a 1/4 cycle.

 

[waross]

Hi dpc;
1/4 cycle theworetically gets you to the peak of the first 1/2 cycle and you may still have maximum current.
At unity power factor we need less than 1/4 cycle.
respectfully

 

[davidbeach]

waross, if the interruption were a current chop then you would be correct about reaching the peak current. But the arcing in the device (CL Breaker or CL Fuse) prior to interruption brings the current down from the peak that is achieved to zero more gradually than a chop. If CL devices chopped the current, they would not be used in sizes of more than a few amps.

 

[dewone]

Most breakers operate around 5 to 7 cycles. Fast acting breakers can operate around 2 to 3 cycles. Timeing tests for example on a Toshiba VK Series breaker is 20 to 30 milli seconds. From the time the trip coil is energized to the main contacts break. So interuption would take slightly longer, way past the 1/2 to a 1/4 cycle time.

 

[dewone]

You know maybe he's looking at like a GE AKU, or Westinghouse DBL these breakers are fused in which case we can current limit. Because they are fused there is a blown fuse trip mechanism as well to not single phase the loads.
A lot of times this is applied to make a 600 Amp Frame into a 800 Amp. The fuses used where Buss KAF-800 Limiter.
Don't know if that helps your discussion but we try.

 

[dpc]

dewone,

I think you need to look at some molded case circuit breaker time current curves. At high currents, they trip much faster than that. And the contacts part even faster. In modern MCCBs, the contacts can open before the breaker trip mechanism even operates if there is enough fault current.

 

[LionelHutz]

Dewone, The Toshiba VK you're talking about is a medium voltage vacuum circuit breakers and the origional poster seems to be asking about low voltage molded case circuit breakers.

The Siemens current limiting circuit breakers do seem to work well. We've done some fault testing using them and they have shown to be capable of really limiting the fault current. We got some fault ratings that were as good with the breaker as with a class J fuse. An example breaker would be the CED6 for up to 125A sizes.

 

[dewone]

LionelHuzt
Yes VK is Medium Voltage. The breaker times I stated earlier are for metal clad. Most instantaious curves even in molded case, insulted case are still around 0.01 to 0.05 sec's Have to admit that I haven't seen the curves for the Siemens CED6 yet.

 

[Evgueni]

I found time - current curves for CED 6. For S.C.current 31 kA rms 0.48 kV I got operation time as 0.0033 sec

 

[DanDel]

Evgueni, don't trust the low end of those curves. No LV C/B will open in less than 0.0033 sec, which is less than 1/4 of a cycle. Typically, it will take at least 3-4 cycles (0.06 sec). Most TC curves start out at 0.01 sec, anyway.

 

[dpc]

DanDel,

Why do you say no LV C/B will open in less than 0.0033 sec?

Because of arc-flash, manufacturers are re-testing their MCCBs and publishing new TCCs that are more accurate at the very high end.

 

[DanDel]

The only protective device which will open in less than one cycle is a fuse.

 

[DanDel]

Sorry, my original comment should have been: "No LV C/B will open in 0.0033 sec, which is less than 1/4 of a cycle. "

 

[davidbeach]

DanDel, there are listed LV current limiting circuit breakers. To be current limiting they have to open in significantly less than 1/2 of one cycle. There is device testing that proves your statements false.

 

[Evgueni]

Gentlemen.
Could you tell why those curves were published . If its not practical information why Engineers have used it?

 

[dpc]

We should probably distinguish between "opening time" and "clearing time" - two related, but not quite identical terms. Opening time is the time required for the contacts to part. Clearing time is the maximum time required for current to be completely interrupted.

When looking at a TCC for a molded-case circuit breaker instantaneous trip, the top line represents the maximum clearing time. The breaker contacts will have to open at some point before the maximum clearing time. But modern MCCBs with "blow-apart" contacts can actually open **before** the breaker trip mechanism even releases the stored energy spring in the breaker. This is due to the magnetic forces on the contact arms at high current levels.

These breakers can clear high current faults pretty quickly. They really have to, or else the breaker will tremble and disassemble. This fast clearing time is the only way that the very high interrupting ratings can be achieved in such a small, inexpensive device.