18th edition exam question..

[Colin33]

Booked in for my 18th edition exam, so I've been doing some mock exams online. Can anyone explain this one? There's me thinking an RCBO protects against over-current and earth fault current?

 

[Pretty Mouth]

Hi guys,
Looking at this question in detail, I would like to add my two pennyworth. I think the question was sound and correct. It is peoples mis understanding of technical terms that leads to questions being interpreted as not being correct. In part two (definitions) of the regulations it clearly states what an RCBO is designed to do. However could I deflect your attention to the table A53.1 on page 188 of the regs book. There you will see that an RCBO has functions to protect against:
Overload, Short circuits and Residual Currents
whereas an RCD (as one of the RCCB family) only protects against Residual Currents
and Residual currents are again described in Definitions part 2.
Therefore both protect against Residual Currents, but of the two only the RCBO protects against short circuits
Enjoy the Bank Holiday, I need to get out more.
Regards
JohnC

I disagree. Nothing to do with technical terms, it's just really badly put together. The question (or statement if you like) has 4 answers, 3 of which are correct: 1,2 and 4.

 

[timhoward]

I'd have liked to see the word "includes" in the question, as to me that makes it much fairer.
"An RCBO device includes protection against"...
As it stands there is a hint of the absolute and a natural tendency to not want to choose the correct answer because it omits residual currents completely, which will also be in the top of the candidates mind as their unique feature.
At the end of the day the exam should test what people need to know in an uncomplicated way without trickery.
EDIT to add I do also agree that answer 1 and 2 are arguably correct, even if not using strictly BBB terms. (I believe it refers to residual current and short circuit conditions, but would have to check)
Bottom line it's just a naff question!

 

[Colin33]

Another question I'd like to take issue with! Regulation 411.3.2.3 clearly states a maximum of 5 seconds. The question clearly asks for the 'maximum' What am I missing?

 

[Colin33]

Oh dear, the cow has just keeled over and died following a 'fault protection'...

 

[pc1966]

Another question I'd like to take issue with! Regulation 411.3.2.3 clearly states a maximum of 5 seconds. The question clearly asks for the 'maximum' What am I missing?

That is also a big dodgy.

For a TN supply it is 5s sub-main or 0.4s for a final circuit. But the one you ticked was 0.4ms which is not feasible for a 50Hz supply!

 

[Colin33]

That is also a big dodgy.

For a TN supply it is 5s sub-main or 0.4s for a final circuit. But the one you ticked was 0.4ms which is not feasible for a 50Hz supply!

I chose 5 seconds, but the 'correct' answer is 0.4ms apparently...

 

[pc1966]

I chose 5 seconds, but the 'correct' answer is 0.4ms apparently...

Well they have a mistake in the list!

 

[DPG]

That livestock one is definitely wrong.

 

[Colin33]

I passed the exam today, only got 85% so got a handful of questions wrong. I have to admit, there were a few questions which evaded my knowledge and couldn't even find the answer in the BBB! Overall I'm happy though, considering I only booked the exam last week and purchased the BBB less than a week ago