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?

 

[Wilko]

There's me thinking an RCBO protects against over-current and earth fault current?

Sounds about right to me.
I think it’s just a poorly worded example question and for what it’s worth, I would’ve picked your answer too

 

[timhoward]

It does, and IMHO it's a really bad question. I remember that one from my exam and I'm now fairly sure I got it wrong!
I don't have BBB here at home but in the definitions section at the beginning does it have "earth fault current" listed?
It might be that "earth fault current" is more associated with "prospective earth fault current" in the regs, and answer one isn't technically using correct terminology in the mind of whoever wrote the question, in which case answer 4 is the closest.
But it's a definitely a poor question!

 

[Colin33]

Sounds about right to me.
I think it’s just a poorly worded example question and for what it’s worth, I would’ve picked your answer too

I think it's a genuine exam question, one of perhaps a few which the 'experts' have wrong. I can remember a similar situation with the 17th exam, where I stated the obvious answer but it was 'wrong' apparently. Even the course tutor agreed. However, this time I've skipped the course and just booked the exam so I have no tutor to back me up

 

[Colin33]

It does, and IMHO it's a really bad question. I remember that one from my exam and I'm now fairly sure I got it wrong!
I don't have BBB here at home but in the definitions section at the beginning does it have "earth fault current" listed?
It might be that "earth fault current" is more associated with "prospective earth fault current" in the regs, and answer one isn't technically using correct terminology in the mind of whoever wrote the question, in which case answer 4 is the closest.
But it's a definitely a poor question!

From the BBB: "A residual current operated switching device designed to perform the functions of protection against overload and/or short circuit"

So it doesn't protect against earth fault currents??

 

[timhoward]

From the BBB: "A residual current operated switching device designed to perform the functions of protection against overload and/or short circuit"

So it doesn't protect against earth fault currents??

The "residual current operated" bit means it detects imbalances between current flowing in and out of it, and any imbalances "switch" it, and therefore that includes any leakage to earth/ fault currents.

So RCBO's are a residual current operated switch that also provide overload protection.
I do believe you had this all correct in your mind before that question threw you!

(Tangent, but the question I got hung up on in exam related to:
463.1.2
"A single-pole switching device shall not be placed in the neutral conductor except for the connection of the control device for lighting circuits as shown in Fig 46.1"
The question was basically " when can you switch the neutral conductor in a lighting circuit" and I was sitting there thinking "never?!"
)

 

[Colin33]

(Tangent, but the question I got hung up on in exam related to:
463.1.2
"A single-pole switching device shall not be placed in the neutral conductor except for the connection of the control device for lighting circuits as shown in Fig 46.1"
The question was basically " when can you switch the neutral conductor in a lighting circuit" and I was sitting there thinking "never?!"
)

I'd think the same. But I still can't work out what a 'single pole switching device' (switching neutral only) for control of lighting circuits actually is?

 

[timhoward]

I'd think the same. But I still can't work out what a 'single pole switching device' (switching neutral only) for control of lighting circuits actually is?

Basically a contactor/relay, because the switching circuit that energises the coil is not connected to the lighting itself, so switching the N of the coil doesn’t leave the lights live. Never used that fact since!

 

[Mike Johnson]

Does anyone else think these are real smart a**e questions put there just to catch candidates out and not necessary to be there at all?

 

[pc1966]

It is a somewhat misleading set of answers as really a RCBO protects against:

1. Earth currents (well below normal operating levels, let alone fault conditions)
2. Overload (usually, though fixed loads such an immersion heater may not need it)
3. Fault (i.e. short circuit style events) which could be L-N or even L-E so also an "earth current".

So the last answer in the multiple choice is actually correct as it is a sub-set of what it does (2 & 3 above). However, as @Mike Johnson points out the a**hole creating it has decided that wants to see 1 & 3 since 2 & 3 are also met by a MCB/fuse where as 1 is not.

Similarly a RCCB meets 1, but not 2 or 3.

 

[pc1966]

Basically a contactor/relay, because the switching circuit that energises the coil is not connected to the lighting itself, so switching the N of the coil doesn’t leave the lights live. Never used that fact since!

I was going to do that to save conductors for some relay-controlled floodlights but the guy we used for our power feed advised against it as it is so rare it is likely to bite someone else in the future who did not expect it and did not read the documentation (or documents got lost, eaten by local dog, etc).

 

[Lucien Nunes]

Does anyone else think these are real smart a**e questions put there just to catch candidates out and not necessary to be there at all?

Quite possibly, but equally likely to be dumb-a**e questions set by someone who was not paying attention and didn't check that the answers made sense.

IMO bad questions with ambiguous answers are doubly unfair to strong candidates who know the subject. Not only can they lose marks needlessly, but can be sidetracked into wasting valuable time and stress on trying to understand and resolve or see past the problem with the question, compared to someone who doesn't spot the problem, picks an answer and moves on.

 

[timhoward]

I think 58 of the 60 questions made sense when I did it, about 25 were so easy book not needed, another 20 it was fairly obvious where the answer would be in the book, leaving 15 to hunt for.
All the best to the OP for the exam.

 

[padawan]

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?

RCD's and MCB's are designed with a purpose
when an RCD and a MCB go on a date, you get a little RCBO

 

[jc_150]

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

 

[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!