Cable in Buried Conduit

Look here

https://www.prysmiangroup.com/sites/default/files/business_markets/markets/downloads/datasheets/TECSUN%28PV%29%20H1Z2Z2-K.pdf

At the risk of sounding a bit obvious, the exam question should be "Is the cable adequately protected against the likely environmental influences ?"


Do not assume that the annex of suggested cable ratings is some sort of list of approved installation methods from which you cannot deviate - you can very much roll your own,  but then  you are on your own to work out what the current ratings and so forth should be.


So, as an extreme example of something  not in the annex to BS7671, a bit of arctic flex cable under a concrete path  in a bit of old hosepipe, it could be argued, is probably safe from damage by gardening tools, but it may flood,  so we then  ask  is the cable OK if it gets wet. Usually PVC is fine, but rubber insulations are better if long term immersion is expected.  Now that example would be dog rough, but not immediately dangerous. Under a flower bed unsuitable, and unwise for new installation in all cases. Considering that BS7671 has a scope that includes doorbells running at 8V on one hand and 3 phase bare overhead supplies at the other,  and you may see that some adjustment with  the application is required. (for example I'd be OK with the flex feeding a doorbell, less so for mains..)


So what are the killer factors here ?

What is the particular aspect of these buried ducts, as opposed to say similar ducts on the surface,  that worries you - will  it be waterlogged, will there be rats, will there be other cables pulled in beside it to do damage ? If yes to any of the above, then you are right to flag it up.

Actually the  XPLE AWA is probably OK  against those, being quite tough,  but the H1Z2Z2  - you do not say the size, but I imagine this is  like the singles used for connecting solar panel hookups, about 3/8"  in diameter with a thin reinforced insulation ?  This could be damaged by drag- but it all depends how it is/was installed. I note the datasheet linked above says OK for direct burial...

Mike.

+1 for not reading too much into the INFORMATIVE appendixes of BS 7671 - by definition they don't form a definitive part of the standard, and appendix 4 in particular is renowned for only talking about a sub-set of possible installation methods (probably constrained by what they actually have some comprehensive data for).


For cables buried in the ground, 522.8.10 is probably a good starting point.


   - Andy.

Thanks very much for the responses.

I had fallen into the trap of thinking of confusing Informative and Definitive. You are correct, the H1Z2Z2 cables are used to connect solar arrays and have an OD of about 6mm. They are is a separate duct so there is probably a reduced risk of damage during installation. I still have a couple of concerns: -

• I do not have a specification for the XLPE/AWA and it is being used at 1500VDC, which is worrying me as I can't find a 1c XLPE/AWA cable at that voltage rating.

• The site is very wet, with the cable trenches filled with water to almost ground level in the photos I have seen. The ducts seem to be sealed ok and there are some H1Z2Z2 cables that are rated for immersion in water. 

• Technically the BS6724 AC cables (direct buried) are not rated for immersion, although from experience this is not likely to be so much of a problem unless the sheaths have been damaged.

I think I need more information to see if there is a problem or not.

It is certainly not great that the ducts are flooded - while it means you need have no concerns about cable ratings (!) ; most cable designs are only designed to get wet, not for long term deep immersion, and as you say any jacket damage becomes very  serious, as armour will begin to corrode. Presumably all joints are above the high tide mark.

There are higher voltage single core AWAs (example) but if your cable just has BS6724 stamped on it then it probably isn't one of them.

Does the 1500V cable have any kind of insulation monitoring, or would the first sign of serious trouble be hot water or the ADS tripping off ? also is it 1500V between core and armour, or between a pair of singles, so the stress on any one core is half that total?

and  in this application is it part of the DC return path- what would happen if it went a bit high resistance at a point ? Trying to work out how serious the consequences of the most likely failure event would be.

I have looked for sheath damage on shielded cables in the past using an earth electrode and in effect testing for isolation to the armour of the affected cable, but that is not practical in a system that is already up and running if such testing was not designed in.

Mike.

OK, well look and see what is buried elsewhere. HV underground 1 core cable (modern red XLPE/AWA) is direct buried all over the place and works fine at 11kV AC.  Why the question about the Solar cable, I presume that a ground-mounted solar array might be this, but as these are usually large, the red HV cable would be more suited to the high current and voltage expected? The thin stuff used for domestics will not be rated at more than a few Amps, and I don't have the specification for the insulation type although it is probably polythene itself or cross-linked polythene (XLPE) which is an excellent insulator. I am sure it would survive in a flooded duct, whether ideal is a different question.

I not not particularly concerned about the installation method of the AWA cable. It is not ideal that it is flooded, but as has been pointed out, there are a lot of buried armoured cables out there. I would like more information on the voltage rating as it runs between the DC Connector Box and the Inverter, an ungrounded circuit that should be rated at 1500V. The inverter provides ground fault monitoring, although not when the PV array is in operation. There is also BS6724 SWA AC direct buried cable between the inverter and the switchboard in the same sort of ground conditions. Again, not ideal but in practice it should be ok.


The solar cables are double insulated, rated at 1.5kV (up to 1.8kV unloaded) and buried in ducts. The site has some very wet places which would mean the ground around the duct would be waterlogged. I know there have been problems on other sites with earth faults in the DC array cables which were flooded.


Thanks for everyone's help. I think I have a clear way forward now and know what information I should be asking for.