Normal MCCBs vs. Current-Limiting MCCBs

Current Limiting circuit breakers (& current limiting MCCB's) operate much quicker than the normal type CB's/MCCB's under short circuit conditions, thus limiting the amount of "let through energy" (I²t). They are therefore more expensive than "normal" circuit breakers.

Is this a homework question?

Amazing what a Google search will find.

http://library.abb.com/global/scot/scot209.nsf/veritydisplay/d342027bcb18bc9585257584006b2744/$File/1SXU210127G0201.pdf

Interesting brochure from ABB, but it is essentially just a bunch of advertising drivel. They make no real explanation of WHAT is different about a CL breaker, only a vague and short reference to what actually happens inside. That's because the truth is, it is NOT "rocket science", it is just magnetics.

The current carrying components are arranged so that the current flowing through the stationary contacts must make a u-turn (horse shoe) before making contact with the moveable element. This creates a magnetic force surrounding the horse show shape, which is the same polarity of the magnetic force coming from the movable contact arm. As we know, magnetic forces of the same olarity repel each other. In general, the forces are relatively low when current is normal, but when a short circuit happens, the forces increase exponentially. So the higher the amount of current passing through the breaker, the HARDER the contacts will try to separate. Once the trip mechanism releases them then, the FASTER they open and interrupt the current flow. The faster the current flow can be interrupted, the lower the "let-through" current will be.

This technology was discovered and patented by the old Klockner Moeller company in the '60s and their patents ran out in the late '70s, at which time Westinghouse released their version, followed shortly thereafter by everyone else. Ironically, Westinghouse was bought by Eaton / Cutler Hammer, whpo now has purchased what was left of Klockner Moeller.

Hi JRaef,

Can you elaborate on the explanation provided, or do you have a site where a good explanation with diagrams exist?

thanks for your always consise answers

regards

Chas

Unfortunately, everything I learned about this predated the digital age...

Moeller or Cutler Hammer may still have some white papers on it, but the concept is now so old that it may be too passe for someone to have made a digital version or scanned an old paper to make it available.

Standard CB operated by the force from springs normally cannot separate fast enough to extinguish the arc before the fault current passed thru its 1st instantaneous peak value at half a cycle, this kind of CB is therefore unable to provide current limitation when clearing a fault & thus unable to limit the 'let thru' energy.

1 method of CL CB is Manufacturers' use the slot motor principle. On extreme large fault current, the large current thru the 2 contact arms creates a tremendous repulsion force that cause the contacts to open apart extremely fast (normally < 0.01s). Thus limit the peak current into the protected circuit.