D.C. Cables

[Elysium]

We have been asked to price a 4kw Pv Installation on a Barn conversion with no loft space for the inverter.
The D.C. cables will have to be installed in the fabric of the building with the inverter installed in a downstairs cupboard, what are the requirements for this? being an electrician I know what the regs are for the A.C. side. Has any one had a similar installation?

 

[Markc]

Voltdrop of no more than 1%

I would also mechanical protect the DC's with conduit for the entire length and they must be labelled up to indicate high DC voltage.

 

[Elysium]

Is it Ok to install the cables under the dry lining? you mentioned protecting the cables with conduit would that be pvc or galv ? ( if galv would this need to be earthed? ) sorry for so many questions but we want to get it right!

 

[yellowvanman]

Need to treat dc cables like any other, safe zones etc. however you can't protect the dc with an RCD (I don't think), so that leaves you with mechanical protection only.

Earthing the glav conduit won't help, because it won't cause a breaker to trip.

 

[edexlab]

If as you say you have to bury the Dc I would suggest you put + and - in separate conduits and try and space them apart to reduce any risk of a short circuit in the event of any damage, ie if they are apart then its more unlikely that they will be penetrated by anything at the same time ,personally I think if the customer won't allow trunking surface or flush with just the lid showing or having it boxed in then I would install some flat steel plate of a decent thickness over the conduit (as extra mechanical protection) under the drylining and make sure you note this clearly on schematic/handover pack
This might be one to ask a scheme provider like the Niceic for their advice

 

[Worcester]

@edexlab,

I've seen two opposing agurments for putting and NOT putting DC cables in the same conduit.

One is the theoritical possibility of short circuit, even though the are double insulated.

The other argument is that when kept separate, the problem of creating inductive loops and being an lightning attractor.

If you saw those pics of the big array :
http://www.a-sunenergy.com/wp/wp-content/gallery/kw-solaar/021-Inverters-external-area.jpg
http://www.a-sunenergy.com/wp/wp-content/gallery/kw-solaar/022-Inverters-switchers.jpg
http://www.a-sunenergy.com/wp/wp-content/gallery/kw-solaar/027-Cabling-arrays.jpg
http://www.a-sunenergy.com/wp/wp-content/gallery/kw-solaar/028-Cabling-string-arrays.jpg

It looks to me as if they are keeping them as close as they can.

Why shouldn't / can't they be put together in the same conduit?

 

[edexlab]

Yes you're right about the inductive loop that is something to think about if the conduits are say 1ft apart then this would be no worse than the loops between module cables,
I was concentrating on giving them some mechanical protection and reducing the risk of a short circuit but you can use screened Pv cable (prob expensive) or single swa's ( they do exist but little demand means its hard to source)

double insulation won't stop mechanical penetration ie nail / screw bolster chisel etc

on a large system they should be using string fuses (3 strings or more) which should give some short circuit protection assuming there is the irradiation/ current flow

I've seen a sheet VDE German standards/regs which says you should'nt use a twin cable and advises laying them apart (I'll look for this again)

this is one of those situations where there is little guidance ,and its upto the installer
I would'nt be too comfortable about burying Dc and always tell the customer it'll be surface, with the exception of one new build job we've done where we've used separate conduit laid along the steel joists which will prevent any damage to cables from pentration or rodents etc
Pv cables being a lightening attractor is not something I've heard about ?

I would think that the risk of short circuit is more than a theoretical possibility

 

[Elysium]

Thanks to all that replied to this question

 

[Markc]

Lets fall back to BS7671 for this a minute.

If embedded in a wall at less than 50mm regs require RCD protection OR Earth metallic mechanical protection not both. So if DC's are in the wall and RCD protection can not be offered (this needs checking out as ships etc run DC and have RCD's but I don't know more on this than that!) then metallic conduit is suitable. 522.6.8.

 

[edexlab]

I think the main question is, if earthed then where to , I'm assuming any earth on DC side would be connected to the array to allow the Rcmu to operate ,this then gives the problem of two different earth potentials within a building ie between the array spike and the AC earthing/ bonding and the extension of the equipotential zone outside in a TNC/S system
or is there another option or am I thinking about this the wrong way?

 

[Markc]

I think the main question is, if earthed then where to , I'm assuming any earth on DC side would be connected to the array to allow the Rcmu to operate ,this then gives the problem of two different earth potentials within a building ie between the array spike and the AC earthing/ bonding and the extension of the equipotential zone outside in a TNC/S system
or is there another option or am I thinking about this the wrong way?

This is where the Regs let it down as well. For protection of the cables embedded in wall then earth back to MET in some manner but you are correct it does not operate the AC protection device. So is a better answer to embed at greater than 50mm? No Mr Building inspector could cause an issue over the depth of chase? Loose loose either way.

Looking at the issues it's a judgment call to see which way is practical to do and in the opinion of the installer the safer. As BRB for reg 522.6.8 refers to "A cable" then metallic earthed protection complies regardless of AC or DC. The other option would be a surface route in cupboards or alike and mark the trunking up as DC cables enclosed. Saves all these issues.

 

[Elysium]

Got no option with this as there is no attic or cupboard on first floor to install inverter ( I am sure some of you out there are going to have this problem at some time)
We will have to install the D.C. cables in conduit - depth should not be a problem as they are only at first fix stage and are going to stud the wall out.

 

[TJLinks]

Is it ok to run four dc cables (two strings) in one conduit from the array down to the inverter??

 

[Elysium]

Not sure with that. I was going run 3 no 25mm conduits down with 1 string in each, but are going to get in touch with niceic before we carry out the work, I know that with A.C. you cannot run Live & Neutral in seperate conduits as it causes eddy currents - not sure with D.C. its all a learning curve at the moment!

 

[daytona600]

use good european pv low power loss dc cables 4/6mm2
most are not very good quality look for tuv specs and designed for 25/30years lifespan
keep dc runs short as possible 10metre is good practice
you could try mounting the inverter & dc isolator outside most goods ones are waterproof IP65