Mock exam question

[rich1]

Not entirely sure what they are getting at with this question:

A circuit has been poorly desgined for selectivity between an appliance fuse and a MCB protecting the entire final circuit.

What is the most likely outcome should a single fault occur on the appliance?

a: A fault would disconnect the appliance fuse only.
b: A fault would cause the loss of all equipment on the circuit.
c: A fault would cause no loss of any equipement.
d: A fault would cause the entire supply system to disconnect.

I assume they mean that the fuse sizing and MCB rating are at odds with each other? Not sure which way though. Fuse rating higher than MCB? Or fuse and MCB rating that allows a fault condition to occur without either the fuse of MCB blowing/opening?
Does 'Loss of all equipment' mean loss of power or permanent loss of the equipment - as in going bang?

It doesn't say what sort of fault either.

 

[Andy™]

selectivity is nothing to do witht he fuse ratings being higher than than what would protect the cable. its making sure the fuse ratings will generally cause the one closest to the fault to go first. so if both are the samt hen either or both will blow. so go through the options again and see which match what will happen if both were to blow

 

[rich1]

selectivity is nothing to do witht he fuse ratings being higher than than what would protect the cable. its making sure the fuse ratings will generally cause the one closest to the fault to go first. so if both are the samt hen either or both will blow. so go through the options again and see which match what will happen if both were to blow

OK, so if the MCB and fuse are rated such that both open together when the fault occurs at the appliance, the whole circuit goes off. Answers b: loss of all (power to) equipment on the circuit.

 

[NozSpark]

IMHO the question should say “could” instead of “would”

 

[Andy™]

IMHO the question should say “could” instead of “would”

the entire question is a bit crap. id expect most socket circutis to be 16a or greater MCB. whatever fuse is in the plug of the appliance isn't really my problem. and we all know in a short circuit a B32 will probably trip before a 13a fuse blows anyway

 

[SPECIAL LOCATION]

B.

 

[C15JON]

Having spent a couple of years writing technical exam questions, I can assure you it's not an easy thing to do.

 

[Captain Kirk]

The question is very poor and looking a previous papers - "selectivity" should be "discrimination"
Basically what would happen if the appliance fuse was a higher rating than the MCB.

The answer is that the MCB would trip first causing the loss of the whole circuit instead of the appliance only, in the event of an appliance fault

 

[Andy™]

The question is very poor and looking a previous papers - "selectivity" should be "discrimination"

best keep up with current standards. its been selectivity since 18th edition in 2018...

 

[SPECIAL LOCATION]

The question is very poor and looking a previous papers - "selectivity" should be "discrimination"

Factually incorrect information is probably not much help to anyone new to the industry trying to pass exams which are based on the current edition of wiring regulations NOT historic issues of BS7671..

Familiarity with, and 'usage of, correct current terminology can be critical when answering C&G exam questions.. And as Andy points out.. "Discrimination" is no longer a valid definition, as per part 2 of BS7671.

e.g. although page 34 of the Yellow book, (BS7671:2008, Amd 3:2015) for "Selectivity" states see "Discrimination", page 27 of the Blue book, (BS7671:2018, and onward), for "Discrimination" states see "Selectivity"!

Using obsolete terminology could add an element of confusion and miss-understanding to anyone who is a student trying to get their head around the wording of BS7671. .

 

[rich1]

Selectivity as a topic doesn't seem to be in the course material (not mentioned even once) which is why the question confused me a bit. Fully understand now though and passed the exam with a 100% score!

Oddly, the term descrimination is mentioned in a section about RCBO's, saying that multiple RCBO's allows discrimination, so only the circuit with the fault is interrupted. Nothing to do with descrimination on a circuit between a fuse and a breaker!

Thanks for all the help!