Minimum depth underground cable

[securespark]

Just got to my old regs.

The 13th Edition (1955) and 14th Edition (1966) both state that cables should be buried at a depth of at least 18 inches (457mm).

The 15th Edition (1981) says cable shall be buried at a sufficient depth to avoid their being damaged by any disturbance of the ground reasonably likely to occur during the normal use of the premises.

The 16th Edition (1991) says cable shall be buried at a sufficient depth to avoid being damaged by any disturbance of the ground reasonably likely to occur.

The 17th Edition (2008) says cables shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground.

 

[JohnW2]

The 13th Edition (1955) and 14th Edition (1966) both state that cables should be buried at a depth of at least 18 inches (457mm).
The 15th Edition (1981) says cable shall be buried at a sufficient depth to avoid their being damaged by any disturbance of the ground reasonably likely to occur during the normal use of the premises.
The 16th Edition (1991) says [much the same] .... The 17th Edition (2008) says [much the same]

... and the 18th Edition (2018) also "much the same".

Thanks. So, prior to 1981, there was an explicit ≥18" (≥457mm) requirement, but for the last 40 years or so it has been "as deep as needed"

Kind Regards, John

 

[blup]

Do I take it that the expected/'correct' answer was (c) ?

Kind Regards, John

If (c) is the correct answer, that is concerning because the student is not given the opportunity to demonstrate that he/she understands the thinking behind the regulations. As deep as it needs to be is meaningless. 450mm is a better answer.

Blup

 

[RickH]

So BS7671 (oft quoted) does not call for any particular depth .... the 450 mm maybe a usual practice, but its not a 'regulation'

Related Q .... when the cable reaches the outbuilding it will have to go vertical, does it need to be covered, or are there clop on 'shields'. The proposed entry is as per the sketch attached. I could grind the corner off concrete if neeedbe to avoid sharp corner on cable.

 

[securespark]

TBF, the earlier reg says "at least 18" "

A guy I used to work with in 1983 who started on the 13th Edition used to count two of his spade depths. His spade was 11" deep. He would round it up to a foot, then double it. So his cable trenches were 24" deep (around 600mm).

His thinking was a gardener might dig two spits deep, so that would leave a couple of inches spare.

 

[RickH]

I'll go with 450 (less spade work) but will also put in a warning tape 150 above it.
It's under my prize lawn .... so no digging

 

[DetlefSchmitz]

Do I take it that the expected/'correct' answer was (c) ?

Sorry missed this. I guess it's obvious by now, but yes.

If (c) is the correct answer, that is concerning because the student is not given the opportunity to demonstrate that he/she understands the thinking behind the regulations. As deep as it needs to be is meaningless. 450mm is a better answer.

"450mm" is incorrect because it is not what the regulations state. The paper is designed to test if the student can read what the regulations actually say.
The wiring regulations paper is not designed to determine a student's understanding, but their ability to read, or to be able to use an index efficiently. In practice the wiring regulations are not about knowing the reasoning behind the rules, which is rarely explained, but simply knowing what the rules are. The long discussions on these pages demonstrate this perfectly. Not saying this is good, but that's just the way it seems to me to be.

 

[JohnW2]

If (c) is the correct answer, that is concerning because the student is not given the opportunity to demonstrate that he/she understands the thinking behind the regulations. As deep as it needs to be is meaningless.

As has been said, the nature of the exam is such that one is not being asked how deep a cable should be buried but, rather, what BS7671 says about how deep a cable should be buried - and "as deep as it needs to be" is the closest of the answers on offer to what BS7671 says.

450mm is a better answer.

Even if one were being asked how deep a cable should be buried (rather than what BS7671 says) "450mm" would, without qualification, really be incorrect (and potentially 'dangerous') - since 450mm would probably be a lot less deep than it would "need to be" in, say, agricultural land. If "at least 450mm" were on offer, then that would be better although, without qualification, it would still be misleading (and still potentially dangerous). The 'ideal' answer would probably be "as deep as it need to be {in order.....) AND at least 450mm".

Of course, what I've said in the previous paragraph assumes that how deep "it needs to be" is never less than 450mm - and there might be scope for some debate/discussion about that.

Kind Regards, John

 

[plugwash]

Related Q .... when the cable reaches the outbuilding it will have to go vertical, does it need to be covered, or are there clop on 'shields'. The proposed entry is as per the sketch attached. I could grind the corner off concrete if neeedbe to avoid sharp corner on cable.

It doesn't necessarily need to be covered, but it should be run in a way that minimises risk of damage and respects the bend radius of the cable.

If the concrete has not yet been poured, then IMO the best option is to run a duct through the slab and enter the building from underneath.

 

[blup]

Saying a cable needs to be buried as deep as it needs to be doesn't tell the examiner anything about the students understanding of the regulations as this is presumably a multiple choice question: if some further explanation is required then yes, it would make sense.

Blup

 

[DetlefSchmitz]

Saying a cable needs to be buried as deep as...

You should probably be complaining to the IET.

 

[JohnW2]

Saying a cable needs to be buried as deep as it needs to be doesn't tell the examiner anything about the students understanding of the regulations ...

It certainly doesn't - but, as said, daft though it may be, an exam of that type is not designed to ascertain what the candidates understand but, rather, only to ascertain how well they can remember what some 'book' says (even if they don't understand it).

as this is presumably a multiple choice question: if some further explanation is required then yes, it would make sense.

MCQ exams were just starting to show their face towards the end of my fairly extended period of education - and they frustrated me beyond belief. So few things in life, in any discipline, are totally clear-cut, that I was forever pulling my hair out because there was no way that I could qualify my answers on the true basis of my 'understanding' of the subject.

Not only that, but such exams are not really a good indicator of how people will be able to perform/behave in the real world - in which one does not benefit from being prompted with a short list of possible answers!

Kind Regards, John

 

[EFLImpudence]

It certainly doesn't - but, as said, daft though it may be, an exam of that type is not designed to ascertain what the candidates understand but, rather, only to ascertain how well they can remember what some 'book' says (even if they don't understand it).

It's not even the remembering actually and not meant to be.

The exam determines that a person knows how to use BS7671 so that they can find the answer (in a relatively short time).


A lot of people think the exam belittling (is that the right word?) but as I say when the subject arises; give someone who knows absolutely nothing of electrics and ask them to find which of the four choices is the maximum Zs of a C32A MCB and see how long it takes them - along with the fifty-nine other questions.

 

[JohnW2]

It's not even the remembering actually and not meant to be. The exam determines that a person knows how to use BS7671 so that they can find the answer (in a relatively short time).

Yes, I'd forgotten that - but that really only makes the situation 'worse'.

A lot of people think the exam belittling (is that the right word?) but as I say when the subject arises; give someone who knows absolutely nothing of electrics and ask them to find which of the four choices is the maximum Zs of a C32A MCB and see how long it takes them - along with the fifty-nine other questions.

Sure, but in cases such we are talking about, even if a person were able to 'instantly' find the right bit of BS7671 to look at, there would still only be able to say which of the four choices corresponded with what BS7671 says about cable depth - without necessarily having any idea as to, in practice, how deeply (in inches or mm) they should bury a cable!

Kind Regards, John

 

[EFLImpudence]

Yes, I'd forgotten that - but that really only makes the situation 'worse'.

Well, yes but not with regard to the exam in question.

It is not the only thing people are supposed to do. They are supposed to be competent in the job as well.

Sure, but in cases such we are talking about, even if a person were able to 'instantly' find the right bit of BS7671 to look at, there would still only be able to say which of the four choices corresponded with what BS7671 says about cable depth - without necessarily having any idea as to, in practice, how deeply (in inches or mm) they should bury a cable!

I suppose there is no depth that would cover all situations -

"shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground"

is what it states.