Current Limiting Circuit Breakers and Cascading

[Elecme]

Would you please explain your understanding for the theory of operation of the current limiting circuit breakers?
Is their contacts impedance higher than other normal circuit breakers?
Or they interrupt the fault current earlier?
How they could be working with cascading?
Elecme

 

[gordonl]

Current limiting breakers interupt fault current within the first quarter cycle to limit the let through current.

As for cascading, I assume you're refering to current limiting devices in series. I would suggest that series combinations need to be tested because of the very short time periods involved.

 

[electricpete]

There may be new technologies available. Many years ago current-limiting circuit breakers referred to molded-case circuit breakers with integral fuses. The fuses had a higher short-circuit interrupt rating than the breaker due to fast interruption and extended the interrupting capability of the breaker. Fuses were not expected to trip for faults within the capability of the breaker.

 

[gordonl]

They now make breakers with operating times fast enough to perform current limiting on their own. I beleive they use magnetic repulsion due to the fault currents to break the fault.

 

[dpc]

All circuit breakers are current-limiting to some degree. UL defines a current-limiting breaker as one that limits the I2t let-through during the first half-cycle to less than would have occurred without it.

As Gordonl mentions, these current-limiting breakers use reverse current flow through the breaker contacts to create a repulsive magnetic force that causes the contacts to open before the breaker trip mechanism actually trips. The arc resistance effectively limits the maximum fault current that is passed through the breaker.

This is the same principle that has allowed molded case breaker interrupting ratings to jump from 22kA to 65kA in the past few years.

 

[jbartos]

Suggestion to the original posting marked by ///\\Would you please explain your understanding for the theory of operation of the current limiting circuit breakers?
///The current limiting circuit breaker is supposed to be an approximate equivalent to a current limiting fuse, which some circuit breaker actually have to attain the current limiting circuit breaker time current characteristics including the peak let-through. As mentioned in above posting, there are circuit breakers that are current limiting, e.g. magnetoelectric type acting very fast within the first quarter of a cycle. DC circuit breakers tend to be all fast, therefore, they tend to be current limiting.\\Is their contacts impedance higher than other normal circuit breakers?
///The contact impedance is somewhat secondary aspect since the contact impedance or resistance is very small, e.g. 18microohm (for 4000A rating) or so depending on the circuit breaker or switchgear current rating.\\Or they interrupt the fault current earlier?
///Yes, very earlier.\\How they could be working with cascading?
///The upstream circuit breaker has to have bigger let-through than the downstream circuit breaker. Contact manufacturers for details since this is not very frequent application, or attempt to analyze let-through characteristics.\\\

 

[busbar]

At one time weren't series-rated combinations understood to sacrifice selective coordination for fault levels over the interrupting rating of the smaller breaker? Isn’t this a condition of NRTL listing for the breaker pair?

 

[Elecme]

Thanks all; I refer to a statement heard from a merlin gerin sales representative "you can get a total discrimination with cascading".
This could be happened by their new technology now involved in their current limiting circuit breakers.
This new technology appearing in the breaker contact shape, which designed to create impedance increasing proportionally with the increase of fault current, and thus in turn limits the fault current value. It's not about time only.
The sales representative has explained this, what do you think?
Elecme