Next one... does anyone know exactly what the "N750" designation for underground ducts mean? So far I've only gathered that 750 means 750 Newtons for crush resistance (over what sort of area I can't tell - if it were per mm² like concrete blocks that would be quite impressive, if it's over a much larger area, then maybe it won't necessarily cope with even me standing on it with one foot). "N" seems to be impact resistance of "normal" ... which doesn't tell much much as all.

As you might have noticed there's a suggestion in 522.8.10 that N750 duct will provide the equivalent protection as earthed armour ... which I'm a little doubtful of so far,

   - Andy.

Andy,

Im sure you know some of this already, N450 Ducts under footpaths etc, N750 under roadways. Standard would be BS EN 61386-24:2010 and from what I can find testing is carried out and marking of duct as per the following;

" Corrugated cable conduits 450N and 750N are manufactured and tested in accordance with EN 61386-1 and EN 61386-24. With reference to these standards, each European country issues its own product marking:  Cable Conduit Pipes are manufactured in accordance with the “Directive 2014/35/EU relating to electrical equipment designed for use within certain voltage limits” and CE-marked, so they do not fall under the ‘Construction Products Regulation’ nr. 305/2011.

Cable conduits must be identified with a durable and legible marking applied to the surface of the conduit every 1 to 3 metres.

The marking in accordance with EN 61386-1 and EN 61386-24 must contain the following information:

• manufacturer’s name
• nominal diameter
• the letter N (normal type)
• the normative reference
• the certification body marking (e.g. IEMMEQU)
• date of production
• crushing resistance (450N or 750N)

The tests required by the above-mentioned standards and often stated in the corrugated pipe data sheet are:

• compressive strength (450N or 750N);
• impact resistance (L or N series);
• resistance to bending: foldable or flexible;
• resistance to penetration by solids and liquids: IP protection rating according to IEC EN 60529.

The main characteristics for the choice of corrugated pipe for regulation electrical installation are resistance to compression and impact.

Compressive strength is the most important characteristic since the pipe is buried and consequently subjected to the overlying static load. On top of this there may also be any load due to above-ground stresses such as the weight of vehicles. As defined by the standard, the corrugated cable conduit sample is crushed between two plates so as to deform the inside diameter by 5%; the force required to achieve this crushing must exceed 450N or 750N.

 While compressive strength is a property that accompanies the Cable Conduit during its life, the impact test, on the other hand, serves to guarantee the duct during installation. The test simulates the accidental stresses caused by stones in the ground falling onto the surface of the structure during the burial phase. Therefore, the soil layer adjacent to the cable conduit should be free of stones larger than 80-100 mm in diameter. To test this characteristic, the sample, conditioned for 2 hours at -5°C, is hit by a dart with a fixed weight of 5 kg from a variable drop height depending on the nominal diameter of the corrugated pipes (300 to 800 mm height). At the end of the test, there must be no gap allowing water to pass from the inside to the outside."

Hope that helps a little.

Cheers GTB

AJJewsbury said:

As you might have noticed there's a suggestion in 522.8.10 that N750 duct will provide the equivalent protection as earthed armour ... which I'm a little doubtful of so far,

Also, to be pedantic 'equivalent protection against mechanical damage' is different to plain 'equivalent protection' when talking about the use of earthed armour.

It's also worth considering that the circuit may well be for an SELV system, or perhaps occasionally full electrical separation, where the earthed armour adds nothing even when considering 'equivalent protection' (without just the mechanical).

AJJewsbury said:

"N" seems to be impact resistance of "normal" ... which doesn't tell much much as all.

For the 'N' and 'L' classifications, these are tested using falling hammer test on conduit pre-conditioned at -5 deg C. Test mass as follows:

Pass criteria as follows:

AJJewsbury said:

As you might have noticed there's a suggestion in 522.8.10 that N750 duct will provide the equivalent protection as earthed armour ... which I'm a little doubtful of so far,

Just to clarify in terms of 'standards speak' what is going on with this proposed change:

• The NOTE to 522.8.10 clarifies the intent. Without the note, one is left to wonder what 'equivalent protection' actually means.
• There has been a note in 522.8.10 regarding the use of underground conduits for some time. Since the standard referenced has options for mechanical protection classification, in BS 76761:2018+A2:2022, there is still a question of "which classification is equivalent".
• The DPC wording proposes a clarification as to what is intended.

It would be OK to comment that the note ought to be removed, BUT it's worth remembering that, without that note, there would be no way conformant with BS 7671 to bury any cable (including, notionally, fibre) that doesn't have 'an earthed armour or sheath suitable for use as a protective conductor' ... and this is not possible for some systems because there are not suitable constructional standards for relevant cables.

Thanks guys - these extracts do help!

Also, to be pedantic 'equivalent protection against mechanical damage' is different to plain 'equivalent protection' when talking about the use of earthed armour.

Fair point. I'll think about that. I guess I'd read the original as 'providing equivalent protection (to people, property animals etc, from shock, fire etc) from the results of mechanical damage to the cable" rather than simply providing protection against mechanical damage to the cable itself, as the concentric c.p.c. approach doesn't really do anything to prevent mechanical damage - even the slightest damage will likely cut through the outer sheath and expose the armour to corrosion, and the other option 'metal sheath' (not necessarily armour) even less so.

I quite agree that there is a problem here (esp. with SELV, DC, etc) that needs looking into (I said a long time ago I thought 522.8.10 was mis-placed and might be better in each of the chapter 41 sections, with something appropriate for each different approach to protection against shock as the earthed concentric c.p.c. really only makes sense for ADS.

I'm still thinking of my garden fork experiment, where the strands of SWA were easily parted by one of the tines and I dare say would pierce a bit of twinwall just as easily  -

No matter how deep we originally bury them, cables are going to get damaged or pierced (in all sorts of ways), and in that sort of situation ADS is likely to provide much better results than a bit of twinwall containing the equivalent of a bit of 2-core flex.

Certainly alternatives are needed, and in some situations (e.g. PV farm) where digging etc can be controlled, ducts may well be perfectly adequate. Ditto for SELV. Maybe unearthed armour connected to the c.p.c/bonding conductor for separated circuits. I'm less comfortable with a blanket 'N750 will always be sufficient in all circumstances' approach though.

   - Andy.

AJJewsbury said:

I'm less comfortable with a blanket 'N750 will always be sufficient in all circumstances' approach though.

SWA isn't sufficient either, which is why depth of burial is very important. (See this article, and especially Table 1: https://electrical.theiet.org/wiring-matters/years/2024/101-july-2024/buried-conduits-and-ducts/)

With BOTH a suitable depth of burial,, AND either SWA or a suitable duct, things ought to be fine. (Both are a requirement of Regulation 522.8.10).

... AND, just because a duct is struck, doesn't mean the cables contained within it are damaged !

With respect to accidental cable strike, this is most likely in a workplace situation, and there are suitable procedures to help vastly reduce the risk of cable strike. Who forgot the marker tape?

SWA isn't sufficient either,

But at least should disconnect if the foreign conductive object gets as far as a line conductor.

depth of burial is very important.

It's a help, but can't always be relied upon. Landscaping changes surface levels, other works can loose shallow marker tapes. Some operations (e.g. driving in a fence post spike (e.g. https://www.wickes.co.uk/Wickes-Wedge-750mm-Support-Spike-for-Fence-Posts---100-x-100mm/p/540564)) wouldn't cause any warning tape to be visible in time anyway. And whatever depth you choose for a cable, a later operation to install another cable in the same area will have to dig to the same depth.

With respect to accidental cable strike, this is most likely in a workplace situation, and there are suitable procedures to help vastly reduce the risk of cable strike. Who forgot the marker tape?

I agree safety procedures can mitigate a lot of risks, but we can't ignore the other situations - e.g. domestic. Some gardens have quite a lot of buried cables these days (from gate openers and lighting to pond pumps and outbuildings) and  will inevitably be subject to DIY or semi-professional excavation.

Other thought was to compare with 522.6.202 (cables concealed in walls) - if no concentric c.p.c. and conductors hazardous live (e.g. non-SELV etc) then maybe some form of additional protection (maybe 30mA RCD, maybe concrete/brick cable cover tiles) in addition to the twinwall.

   - Andy.

The question comes as to whether we are overthinking the risk, and to what extent it's attributable to poor installation (insufficient depth for the application) vs

It is worth remembering that many other countries that use HD 60364 (on which BS 7671 is now based) permit direct burial of cables without armour/screen , and internationally, cables are available without armour/screen etc, that are marketed for direct burial.

AJJewsbury said:

I agree safety procedures can mitigate a lot of risks, but we can't ignore the other situations - e.g. domestic. Some gardens have quite a lot of buried cables these days (from gate openers and lighting to pond pumps and outbuildings) and  will inevitably be subject to DIY or semi-professional excavation.

And of course, many of those applications can continue to use direct-buried SWA which would provide the safest approach provided the armour is properly terminated. However, as previously discussed, the ADS safety mechanism of 'earthed armour' doesn't apply for all supply/source arrangements and cable types.

H&S legislation doesn't consider 'semi-pro' or 'voluntary work' as anything different to 'pro', and as far as I know it's still the legal precedent that anyone doing DIY has to exercise the same skill and care as a professional doing the same job.

For right or wrong, there are those in the industry who now view the armour as being an unreliable cpc because of corrosion and termination problems. (I am only sharing this perspective, it is not necessarily mine .... and should also be considered with the backdrop that BS 7671 requires 'good workmanship' and also electrical equipment to be selected appropriate for the appropriate external influences.)

Overall, I don't think it's quite as simple a discussion as it would first appear.

H&S legislation doesn't consider 'semi-pro' or 'voluntary work' as anything different to 'pro',

But the Health & Safety at Work Act (and regs made thereunder) only covers people at work... DIYs (at home) are sort of ordinary persons. BS 7671 has to be reasonably safe for all applicable situations, not just work ones.

It is worth remembering that many other countries that use HD 60364 (on which BS 7671 is now based) permit direct burial of cables without armour/screen , and internationally, cables are available without armour/screen etc, that are marketed for direct burial.

I was aware that some of out continental cousins (esp the French) didn't do earthed armours (I've also seen copper screened underground types from elsewhere (might have been Denmark?). Mind you there might be a degree of culture that goes along with that sort of thing - like sockets and wall switches in bathrooms - from what I've seen of French roadworks they seem to like to backfill their duct trenches with concrete (but whether that's for the benefit of the road structure or the protection of the duct, I can't be sure).

Overall, I don't think it's quite as simple a discussion as it would first appear.

Naturally, they never are. Hence the value of the Forum...

   - Andy.

AJJewsbury said:

DIYs (at home) are sort of ordinary person

That clearly can't be the case. For example, the internals of electrical equipment would have to meet requirements of BS 7671 they clearly can't meet when screws are removed.

AJJewsbury said:

DIYs (at home) are sort of ordinary person

That clearly can't be the case. For example, the internals of electrical equipment would have to meet requirements of BS 7671 they clearly can't meet when screws are removed.