Reduced current let-through

[jbach_943]

I understand that using current limiting fuses can accomplish this. However I was under the impression that lowering the instantaneous settings within the trip unit IE: GE Entelliguard RELT mode (Reduced Energy Let Through) would accomplish this as well. I am not sure why magnetic repulsion of contacts would be the correct answer in this instance. Can someone please explain.

[Warrengarber]

Waht test question is this from?


I understand that using current limiting fuses can accomplish this. However I was under the impression that lowering the instantaneous settings within the trip unit IE: GE Entelliguard RELT mode (Reduced Energy Let Through) would accomplish this as well. I am not sure why magnetic repulsion of contacts would be the correct answer in this instance. Can someone please explain.

[cycle61alt]

Waht test question is this from?

Circuit Breakers:
Reduced current let-through can be accomplished by which two technologies (one older and one newer)?
a. magnetic repulsion of the contacts
b. Shorter opening distances for contacts
c. Lowered pickup settings on the instantaneous
d. Current-limiting fuses

Your answer: C and D
Correct answer: D and A

I was working through an L2 practice test with one of my newer guys yesterday and we came across this question. The answer seemed pretty obvious to me, we talked it through and went with C+D. The hint/reference after the test takes us to this thread. It took some work to start finding engineering, code, spec, or whitepaper describing option a) as a reduced current let-through strategy. Tons of info about reducing incident energy, but not so much for let-through current outside of fuse literature.

I did find this from an old post on Mike Holt forum:

One on the original CL breaker was designed be Westinghouse which used a slot motor concept where the moving contact is propelled within a slot motor for which the magnetic field used for the slot motor is as a result of the fault current.
Another common design is where the station a ry contact on the bottom actually is in the shape of a loop which caused the mag fields of the stationary and moving contacts to opposed one another when they open when speeds up opening.

Good review here as well: https://www.sciencedirect.com/science/article/pii/S2213020916301197

492

[GrayTard]

Circuit Breakers:
Reduced current let-through can be accomplished by which two technologies (one older and one newer)?
a. magnetic repulsion of the contacts
b. Shorter opening distances for contacts
c. Lowered pickup settings on the instantaneous
d. Current-limiting fuses

Your answer: C and D
Correct answer: D and A

I was working through an L2 practice test with one of my newer guys yesterday and we came across this question. The answer seemed pretty obvious to me, we talked it through and went with C+D. The hint/reference after the test takes us to this thread. It took some work to start finding engineering, code, spec, or whitepaper describing option a) as a reduced current let-through strategy. Tons of info about reducing incident energy, but not so much for let-through current outside of fuse literature.

I did find this from an old post on Mike Holt forum:


Good review here as well: https://www.sciencedirect.com/science/article/pii/S2213020916301197

492


Let through current seems to refer to limiting the available fault current. Current limiting fuses have an up-over-down method that i was gained is discredited in industry. The circuit impedance during a fault is dynamic.

ABB has a product that explodes contacts apart and is supposedly the only such device that can truly limit fault current.

I don’t know about magnetic repulsion. Seems like it has the same flaw as current limiting fuses.

The lowered pickup settings your thinking of is in regard to arc flash energy. That depends on arcing time and can be reduced by tripping on instantaneous. Even so, the arc will take several cycles to interrupt.

Current limiting fuses were supposed to break the fault before the current reached the peak of the cycle.