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.

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.