Direct Buried cables within a controlled area and what constitutes mechanical protection

I guess they're thinking of reg 522.8.10 of BS 7671 which requires cables buried in the ground to have an earthed metallic sheath or earthed metallic armour or be in a duct or conduit that provides equivalent protection. That might well be a peculiarly UK requirement (I've seen buried cables on the continent without any armour.)


There is a difficulty for circuits that aren't protected by ADS in that an earthed  armour/sheath then doesn't in fact provide much protection from shock but BS 7671 seems to overlook that.


If you feel that the arrangement you have provides at least the same level of safety as a compliant one, you always have the option of a 'deviation'.


   - Andy.

Is it supposed to meet BS7671 ?  Or are there the equivalent of makers instructions that over-rule it ?

In practice it may be more pertinent to ask are the cable routes out of the way and are they  well marked/ recorded  or are they likely to be ploughed up?

Given the insulation monitor and ADS, assuming the cable has a jacket that will survive burial, I'd be more worried about any buried joints and what happens with water ingress.


Also, when the inverter is cut off, how is (or just 'is') the DC string isolated ?  - If the sun us shining, surely there could well  still be dangerous voltages on the DC wires despite inversion ceasing?

Mike

client stating that a PV solar Generation site built on agricultural land is non compliant


Did the client require/specify a BS7671 compliant installation and left it to the designer/constructor/inspector&tester to achieve this?


Regards


BOD

Thanks all for your replies

mapj1

Yeh all mapped out and a controlled site with clear Rams in place 

Disconnecting the load (inverter) will greatly reduce the current flow. 

Mechanical protection is in place on all cables exiting ground

Some thoughts:

Disconnecting the inverter will not reduce the current flow in the event of a cable short circuit in the buried section...I think Mike was asking whether there are disconnectors on the array side of the buried cables so they can be isolated from the source of supply. If not there might be issues - and indeed danger if done wrong - dealing with any faults that may arise. String connectors ("MC4"s) are not for on-load disconnection and may be damaged if so used. This isn't of course specific to this particular install but does affect the risk assessment.

How have the string cables been laid? Are pos and neg run separately or have they laid in pairs? Are the cables laid flat or bunched?

Do take a good look at the cable specs. Are they proof from damp and might there be issues with low IR causing startup trouble? The subarray size, DC operating voltage and AC arrangement might all affect this.

What protection is there from rodents, badgers and the like?

Might maintenance vehicles / tractors drive over the cable route (PV maintenance is generally in the winter because that's when the array won't be generating so much, so the ruts can get quite deep if its a "main" route, particularly where the ground is soft over a cable trench)?

What about mechanical damage from groundsmen strimming (or sheep), at the transition from under- to above ground?

PV d.c. side can be interesting. Usually fault currents of themselves aren't too much of a worry since the supply is limited by what the panels can produce - sizing the cables to (from memory) 125% of the panel's maximum nominal current usually provides all the fault protection the conductors need.


If the inverter disconnecting from the d.c. side also means that the d.c. side is isolated from earth, then you might have shock protection (by means of a separated system) even in the case of a cable being spiked and the d.c. cables still "live". But that of course depends on the d.c. side not being deliberately earthed before the inverter and how well you can reliably insulate it from Earth in general (much harder with larger systems).


   - Andy.

Yeah Disconnectors at the SCBs. If there were late starts and R<iso on a regular basis then yes I would suggest full protection using twin wall duct etc. I have been within the Photovoltaic engineering sector since 2005 moving into large scale in 2011 with first hand knowledge of the majority of UK large scale solar farms and have seen multiple failures of direct buried cables usually down to poor installation. Usually faults appear once the ground starts too settle back after excavation. 

This particular site has been running like clockwork for almost 4 years and not a single incident its also an incredibly wet site. Cable routing was done properly to the manufactures guidelines laid with cable sand and plenty of segregation between poles. Cable manufacturer specifics states suitable for direct burial as double insulated also.

I have seen multiple failures when cables are pulled through ducts and then ducts get full of water rising to insulation failures leading to dead shorts between DC poles sometimes causing arcing. 

A really good quality EPC constructed the site. I mean after almost 1700 site visits to solar farms in recent years I know the good bad and ugly!! 

In my opinion the consultant who has raised this issue does not know Photovoltaics very well and not asking about any protection measures in place which monitor the insulation values constantly and if detected will shut down inverter. usually all these sites are built using IT floating earth method so monitoring  GFDI protection devices are essential.

Hi RE, welcome

This is not an installation that comes under BS7671 is it? This is the problem with feature creep of standards. This is not a bit of domestic "top-up" electricity, in reality, it is a power station. It is difficult to see why the "Consultant" concerned is reading a standard that is not aimed at the site. Whilst I agree that the general principles are similar to domestic or small-scale PV, things like IT installs are aimed at hospitals and medical facilities, whereas these sites have a completely different set of risks and challenges. Cable protection is obviously a minor issue, the most likely damage will probably come from badgers and similar wildlife tunneling (which they do quite deeply and would deal with the cable in the same way they do a tree root, and knaw through it), clearly, chisel ploughing would not happen on-site as it would damage the panel mechanical stability. Conventional power stations may have some areas built strictly to BS7671, but other areas will not comply. This is just as DNO and National Grid manage installations. As we have discussed elsewhere, DNO supplies may well not be BS7671 compliant, they should once past the meter. I suspect these sites are the same, just before the connection to the Grid metering.

Hi Dave 

and thanks 

Exactly my thoughts These sites come under ESQCR as they are Power stations and the owners are in effect are electricity supply generators supplying directly to the grid. 

EAW 1989 H&S is covered by the Bender ground fault detection system which injects current into the DC strings every morning. They are set at >1M ohm 


Thanks Dave