Underground T&E?

[westie101]

Looking at this from another angle, the mechanical protection of the T&E in the concrete encased duct would seem adequate. But taking into account best practice it would make sense to use SWA or even a concentric cable as this would ensure that the live conductor is surrounded by neutral/earth.

From my side of the industry all we are required to do is to have that neutral/earth around any live, so can lay a concentric at .45m with marker tape.

Though from a degree of risk point of view, it would be far safer to hit a piece of T&E (or any other type of cable) protected by a 30mA RCD that a cable protected by a 400A HRC fuse. But that is what designing and assessing an installation is all about, rather than strict adherence to a set of guidelines!

 

[JohnW2]

Looking at this from another angle, the mechanical protection of the T&E in the concrete encased duct would seem adequate. But taking into account best practice it would make sense to use SWA or even a concentric cable as this would ensure that the live conductor is surrounded by neutral/earth.
....Though from a degree of risk point of view, it would be far safer to hit a piece of T&E (or any other type of cable) protected by a 30mA RCD that a cable protected by a 400A HRC fuse.

I cannot disagree with any of that. The two sides of the industry are obviously different in their approaches, not the least because of the different levels/natures of risk such as you mention. Presumably for this reason, when BS7671:2008 wanted to include a new requirement for protection of cables buried <50mm deep against mechanical attack, they built in a requirement for RCD protection OR for the live conductor to be surrounded by earth, not just the latter.

Despite what some people think, I am not (on this occasion!) talking about contravening regulations or even making iffy interpretations of them. I may, of course, be wrong, but I sincerely believe that T&E in concrete-encased ducting is actually fully compliant with BS7671, for the reasons I have explained in this thread. Interestingly, no-one has really siad that the believe it would be non-compliant, merely that "I should use SWA" (which I probably will, if/when I put a power cable into that duct).

Kind Regards, John


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[securespark]

Apply lubricant liberally, of course.

And don't forget to use some to aid the progress of the cable pulling-in.

 

[ban-all-sheds]

I have a run of about 25m of ~100mm plastic conduit.

Does it comply with the IEC 613862-24 standard for underground conduit?

Interestingly, no-one has really siad that the believe it would be non-compliant

134.1.1?

 

[JohnW2]

I have a run of about 25m of ~100mm plastic conduit.

Does it comply with the IEC 613862-24 standard for underground conduit?

This is starting to get very silly; it's all too easy to see why regulations (and the Mr Jobsworths who administer them) get such a bad name.

The short answers to your question are (a) I haven't got a clue and (b) unless that standard was in force in the mid-80s (when the duct was installed), if it were compliant it would only be by accident/luck.

As for the longer answers, for a start, the conduit only really exists as a former for for the concrete, which is the primary protection. I'm not sure whether it was 100mm or 4". It was pretty thick walled, fairly stiff and came in very long (at least 5m) interlocking lengths, and the contractor who 'provided' it (I didn't ask tool many questions!) assured me that it was "the stuff wot they use for motoway lighting"

Although neither I nor Mr Google have even heard of it, you are clearly conversant with IEC 613862-24, so perhaps you could tell me something about what it says?

Interestingly, no-one has really siad that the believe it would be non-compliant

134.1.1?

That clause about 'good workmanship' is far too vague, and hence needing of interpretation, to be a useful answer to any question! In any event, I would have thought it was clear that I was talking specifically about compliance with 522.8.10 of BS7671:2008. Is anyone here prepared to say that they believe it would be non-compliant with that clause?

Kind Regards, John

 

[ban-all-sheds]

Although neither I nor Mr Google have even heard of it, you are clearly conversant with IEC 613862-24, so perhaps you could tell me something about what it says?

Can't tell you anything about it.

Apart from the fact that 522.8.10, which you claim compliance with, notes that it is the standard for underground conduit.

But without the typo (which you didn't spot either), Google does know about it.


The concrete is irrelevant - no matter how deep, it is the conduit which has to provide equivalent mechanical protection, not the concrete.

You may think it's probably OK, but you have to be able to sign to say that you, having exercised reasonable skill and care when carrying out the Design, Construction, Inspection & Testing, hereby CERTIFY that the said work for which you have been responsible is to the best of your knowledge and belief in accordance with BS 7671:2008....

You want it to be, but you asked here because you are not certain, and yet you seem to only want to hear things which agree with what you want.

You've said, or hinted, more than once that you think that some of the regulations are silly. Well fair enough, but I'm afraid that your assessment of their silliness does not mean that you can ignore ones that are inconvenient to you and still claim compliance with BS 7671.

That clause about 'good workmanship' is far too vague, and hence needing of interpretation, to be a useful answer to any question!

It's not too vague for me. I don't consider that using T/E in that underground pipe constitutes good workmanship (nor am I the only one).

You observed that no-one had really said that they believe it would be non-compliant, but when I did you dismissed it as not useful.

You may think it a vague requirement, but it is a requirement of BS 7671, and someone has now said that they believe your use of T/E would be non-compliant.

In any event, I would have thought it was clear that I was talking specifically about compliance with 522.8.10 of BS7671:2008.

You have to comply with all of the relevant regulations if you want to sign an EIC, not just some of them.

Is anyone here prepared to say that they believe it would be non-compliant with that clause?

I'm prepared to say that I could not certify that to the best of my knowledge and belief that it was compliant.

And BTW - your IR tests of those SELV cables was interesting, but it doesn't make them Band II.

 

[JohnW2]

The concrete is irrelevant - no matter how deep, it is the conduit which has to provide equivalent mechanical protection, not the concrete.

As a matter of interest, where do the regs say that? 522.8.10 refers to "conduit or duct which provides equivalent protection" and Part 2 defines 'duct' (after re-directing one to 'cable ducting') as "an enclosure of metal or insulating material ... intended for the protection of cables". In what way would my concrete 'duct' not satisfy that, even if the plastic pipe were not there?

The I'm prepared to say that I could not certify that to the best of my knowledge and belief that it was compliant.

Thanks. Somewhat short of a statement that you don't believe it would be compliant, but at least an opinion.

And BTW - your IR tests of those SELV cables was interesting, but it doesn't make them Band II.

Yes, interesting, but not really surprising - PVC is a very good insulator, even when very thin ( IIRC, its dieletric strength is something like 20 kV/mm). My tests obviously do not elevate its official rating to Band II but, if the sample I tested was typical (which I imagine it is), my results suggest that the product could probably pass functional tests appropriate to a much higher voltage rating. However, as I said before , what I don't know is whether the relevant standards require, say, certain minimum thickness of insulation and/or sheathing for higher voltage ratings (as well as passing functional tests) - do you happen to know? And BTW, you don't seem to have picked up on my deliberate mistake in that post - the CSA of 7/0.02mm conductors is about 0.21mm², not 0.03mm² (I forgot to multiply by 7)!

Kind Regards, John.

 

[JohnW2]

iirc you can run band 1 and band 2 in close proximity if the band 2 cable is screened, the screen needing to be at least the CCC of the live conductors in the cable.
You'll have to check the regs as I don't have them to hand though!

Having just looked, I presume you are thinking of 528.1(vi). However, if I'm reading this correctly, I think it refers to an earthed screen separating Band I from Band II conductors within the same multicore cable or cord. Do you agree?

For other situations, if the Band I cable is not insulated to Band II ratings (528.1.(i) and (ii)), the only other options offered by the regs seem to be physical separation (528.1.(iii),(iv) & (v)). In fact, apart from (vi) (which I think applies only to Band I and Band II within the same multicore cable/cord), none of the options in 528.1 appear to relate to an earthed screen bewteen Band I and Band II circuits.

Without much comment, I would observe that this may well be one of the most common ways in which existing installations fail to comply with current regs. It does seem a little surprising that it is considered safe for me to touch an insulated and sheathed Band II cable, or for two such cables to touch one another, but not for a Band I cable to touch a Band II cable.

Kind Regards, John.

 

[JohnW2]

This is ‘technical’ stuff, not of interest/relevance to most DIYers – and definitely a case of “don’t try this at home” ....

Another chain of thought, how do they react to 60v> AC?

Good question; watch this space.

Those who decided to do IR testing with DC were not stupid - this is far less than straightforward!

The ‘alarm’ cable I was testing has a capacitance between cores of around 85 nF per metre (measured at RF). At 50 Hz, that corresponds to a reactance (= impedance, in the absence of any resistive leakage) of about 0.37 M&#937; for 100m of cable.

Testing between two cores of 100m of cable at 240V AC, I get a measured impedance of about 0.34 M&#937;. Particularly given that there were some assumptions in my measurement (notably those relating to the input impedances of measuring devices), that is obviously close to the impedance I expect from the capacitive reactance alone, but it is so relatively low that there could be a seriously significant (in IR terms) resistive component (i.e. leakage) in there without my being any the wiser. Even if I tested a much smaller length, say 5m, the capacitive reactance would still be only about 7.4 M&#937; – so if I got an impedance result around that figure, it would still be impossible to be sure that there wasn’t a significant resistive leakage component present. If I tested a very short length to avoid that problem, I would be reducing my chances of picking up any deficiencies in the insulation - which are obviously not necessarily consistent throughout a length of cable.

Short of laying my hands on some more sophisticated kit, the only way I could attempt to resolve this would be to try to measure phase angles with Lissajous figures on an oscilloscope – but I doubt that approach would be accurate/sensitive enough.

The one thing I can obviously say is that nothing dramatic happens with 240V AC between cores (on the sample I have tested), but that’s not much of a surprise given that 1000V DC was OK.

Kind Regards, John.

 

[JohnW2]

The concrete is irrelevant - no matter how deep, it is the conduit which has to provide equivalent mechanical protection, not the concrete.
....
I don't consider that using T/E in that underground pipe constitutes good workmanship (nor am I the only one).

As something else to throw into the melting pot, I've just noticed with interest that Table 3A of the OSG appears to indicate that BS6004 PVC T&E is suitable (amongst other things):

• "For use underground in conduits or pipes"
  and
  "For use in in building voids or ducts formed in-situ"

...either of which could be said to describe the situation I was talking about. Although the regs themselves are less explicit than this, the OSG comes from essentially the same horse's mouth as BS7671 and appears to be condoning what I was talking about.

Any comments?

Kind Regards, John.