Current Limiting Capability in MV Circuit Breakers? 7

[hassankh]

I wonder if MV CBs have, just like LV CBs, current limiting capability? The current limiting capability makes us able to lower the size of downstream breakers, cables and other equipment.
Thanks in advance

 

[davidbeach]

Nope, never heard of such.

 

[ScottyUK]

Quite common for MV contactor-feeders to have current limiting fuses. Personally I'm not a fan of contactor feeders in distribution applications although we have a large installed base doing exactly that which I've inherited from previous ill-judged design and am likely to be maintaining until my retirement.

 

[hassankh]

Thank you guys.
Yes, I too have only heard of fused-contactors to limit current in MV.
As for the LV, correct me if I'm wrong but I believe that is why LV cables don't need to be sized for short circuit condition.
Regards
H

 

[sibeen]

As for the LV, correct me if I'm wrong but I believe that is why LV cables don't need to be sized for short circuit condition.

No, definitely not.

Also, when using the current limitation to lower the kA rating of downstream circuit breakers means that you lose discrimination. In the IEC world called cascading.

 

[squeeky]

All our cable sizing contains a check to the short circuit conditions. Absolutley vital. First fault and all your cables cook.

 

[dpc]

In the US, the NEC has no requirement for the short circuit rating of cables. But this has nothing to do with current-limiting devices since current-limiting devices are not required to be used. NEC rules for sizing of feeders are pretty conservative, so short circuit ratings are generally not an issue. I've done a lot of system studies and when we plot the cable damage curves, there is almost never an issue with short circuit current withstand capability.

If someone has seen actual cases of LV cables being thermally damaged from short circuit current on a through-fault, I'd be interested to hear about it. I've never seen it.

 

[FreddyNurk]

Its only certain types of LV circuit breaker that have the inherent current limiting ability, and I'd expect that the construction of such breakers doesn't scale well to higher voltages.

LV air circuit breakers do not, and I don't believe that SF6 is used for LV breakers as its not really needed.

As I understand it, its the magnetic forces during short circuit conditions that force the circuit breaker contacts open, thus limiting the through current and giving the inherent 'current limiting' condition. Of course, this means that the upstream source must be stiff enough to provide enough current to allow for this, thus the breakers can experience issues on soft systems (such as small generator powered sites).

 

[dpc]

Not sure about the rest of the world, but in the US, the term "current-limiting" is used only for devices that are capable of interrupting fault current before the first peak occurs. Current-limiting fuses only do this if the fault current is in the current limiting range of the fuse. For lower values of fault current, it is not current limiting.

 

[Kiribanda]

hassankh,
Current limiting action is achieved by cooling the arc when the arc is drawn thereby increasing the arc resistance which will limit the current.
In a LV current limiting bkr (mcbs), the arc is drawn into a seperate chamber and the arc resistance is increased and thereby limiting the current.
In side a current limiting fuse the arc is drawn inside the fuse barrel and cooled by sand thereby limiting the current.
Therefore, beyond 1000V, to draw the arc and cool, it needs considerably large arc chutes. Therefore. over 1000V current limiting breakers are
not practicable.

 

[ScottyUK]

Some ACBs do have a current limiting area, although it is fairly high before it starts to take effect. ABB SACE's E-Max series of ACBs has a current-limiting variant as one example.

 

[davidbeach]

In a breaker, current limiting is a result of magnetic repulsion of contacts in a hairpin configuration, blow apart contacts. Just not the way that higher voltage breakers are built.

 

[hassankh]

Thank you guys all for your great info. Shneider's current limiting capability works based on power electronic devices built into the CB which create high voltage opposed to the terminal voltage of the breaker thus suppressing the current.
To dpc:
interesting fact dude.

To ScottyUK and Squeeky:
Thank you very much for your insight. But do you accept Mr. / Ms. dpc's comment that current limiting has nothing to do with cable short circuit sizing? He /She says that short circuits (according to NEC) will not create any problems to cables regardless of provision of current limiting capability for CBs or cable short circuit sizing.

 

[ScottyUK]

In the UK our calculations consider the thermal effects on the cable of a short circuit. The calculations are based on the adiabatic equation, whereby the energy input is assumed to all contribute to heating the cable with no energy losses to the outside in the timeframe being considered. The fault current is clearly significant in this calculation, so the effect of a current-limiting CB would be considered.