Exam question C&G level 2

[richardindorset]

Any thoughts on this question:

Which one of the following devices will disconnect in the fastest time if a short circuit current of approximately 300A occurred on a circuit?
a)	32A/30mA Type C RCBO
b)	32A Type B circuit breaker
c)	32A Type C circuit breaker
d)	32A Type D circuit breaker

As this has not been covered in the course notes, I assume we would be expected to look up the curves in the Regs book?
I was wondering if what they are getting at is that 9.3X rated current on a type D and C would be in the region where the thermal trip will operate but for a B type, the magnetic trip would operate, so type B would be fastest?

 

[Adam_151]

Its not the best question in the world, an honest answer would be "defintaly *not* D" and you are on the right lines. With less than 10x In, you'll not hit the mag trip point on D curve (between 10-20x) so it will have to go out on the thermal trip which won't be as fast.

Now you cannot guarentee it will hit the magnetic trip of the two C curve devices (mag trip between 5-10x) but with 9.3 X there is a bloody good chance that you will, of course you cant be sure.

with the B curve you definatly will hit the magnetic trip point (between 3-5x) so you can be sure it will be going out instantly.

The RCBO will be going out on the RCD element as well as the overcurrent trip (which you will likely trigger the mag on for the C curve - see above) and it will operate in less than 40ms which is faster than we generally assume for magnetic operation (0.1s - 100ms) however in reality it'll be all over in less than a half cycle (10ms) so its actually much slower than the actual magnetic operation.

I don't beleive that when you hit the instanateous magnetic operation that a type B is any faster than a type C, the I²t is lower, but there is more to that than speed of operation, the inductance of the trip coil has got to be significant in limiting that, operation time on both is likely to be as past as the contacts can physically open once triggered (its probably best we don't start thinking about time to quench the arc or we might be here all night!)

TLDR: I suspect they want you to write "B" but it would probably be more correct to write "Could be any of them, except D which it definatly won't be"

 

[richardindorset]

Yeah, I think the activation of the magnetic trip rather than the thermal is what they are getting at in a cack handed way.

Given that we won't have the curves to hand and that the exam is done on a computer so you can't say, its type B and C, I'd go with type B.

There are some other 'questionable' questions that I'll post too...

 

[ericmark]

Has to be B) with the type B curve, it needs 160 amp to trip magnetic part, the type C takes double that so 320 amp is over 300 amp so they will not trip on magnetic part, actually they may as it says between 5 and 10 times, but we have to go by upper limit plus a margin for error. It clearly states short circuit current not leakage to earth.

 

[richardindorset]

Has to be B) with the type B curve, it needs 160 amp to trip magnetic part, the type C takes double that so 320 amp is over 300 amp so they will not trip on magnetic part, actually they may as it says between 5 and 10 times, but we have to go by upper limit plus a margin for error. It clearly states short circuit current not leakage to earth.

That was my thinking too, but as you and other said there is certainly a possibility of a type C tripping magneticially too, so only type D is going to be the slowest to trip.

I mean this chart doesn't really answer the question does it: