DC cabling

[pvconnections]

I'm sure you would potentially create problems if the cable was mechanically damaged if the armour was earthed. A bit of DC injection on the local network ? I'm sure someone far brighter than me will be along shortly..... lol !

 

[yellowvanman]

The whole point of earthing the SWA is that if the cable gets damaged it would trip an MCB or RCD. With DC flowing that wouldn't happen, in fact you could liven the SWA to a high dc voltage but wouldn't necessarily get a shock unless you touched the other polarity of the dc, so other than mechanical protection I don't see what the SWA does! Even if the + and - got short circuited nothing would happen - you would just get Isc flowing with 0V present.

Thats one reason for not earthing the SWA, and another reason for not using dc in an SWA cable, but I wouldn't say this is conclusive!

 

[malcolmsanford]

I would agree to a point, though the armour of a SWA is classed as an exposed conductive part as there is only basic insulation on the conductors, there are 2 sheaths, but basic insulation, and of course if your using the armour of the SWA as a CPC then it's a conductor. Reg 411.6.2 tells us all exposed conducive parts shall be earthed.

Also if the cable is going to buried you again have to earth it as per reg 522.8.10.

Edexlab treat this as you would say protecting the SWA at a TNC-S supplying a building site. You would run your SWA as a sub main say and earth it at the TNC-S. Then when you get to you sub DB and you would TT that. When connecting the SWA you would make sure that you terminated it so that it was insulated from the system and you can get insulated glands for this, or use an insulated enclosure. This way your not transferring different potentials into the installation, the same with your frame, you would terminate the SWA so it was insulated from the metal work.

 

[edexlab]

I understand what you are suggesting ,and have done this many times on outbuildings /sites etc

I can't see how this example relates to a typical Pv system setup ,and I think that this is one case where the regs are'nt going to give an answer, the question becomes should you apply regulations which work on circuits which have measures such as cpc/overload/fault protection to these circuits which don't!
This could result in an otherwise safe setup becoming less so, as mentioned in previous posts

 

[PeteFT]

Most of this thread is going over my head, but seems the right place to ask for advice. I had a PV system installed last week, 21 x 190 Schott in three strings to an SMA 4000tl inverter. 2nd day it stated tripping out, and the Sunny beam showed grid disturbance. research on other threads suggested the problem may be wiring from inverter to the CCU. Lo and behold it was 2.5mm, which the installer insisted was ok. I pointed out that the SMA manual says 6mm and the installer's electrician (subby) is now going to rewire. If this was done to save money, how can i check that the DC cable is correct?

 

[malcolmsanford]

The majority of the time if the panels are roof mounted, the DC cables will be a black rubbery type cable that is about 4mm in CSA with a double sheath/insulation to it, these go into your DC isolator.

Quite often the cables wired into the panels are not long enough to come into the roof area and into an isolator, and so are joined. Most companies join though using exactly the same type of cable that is wired into the panels.

 

[PeteFT]

The majority of the time if the panels are roof mounted, the DC cables will be a black rubbery type cable that is about 4mm in CSA with a double sheath/insulation to it, these go into your DC isolator.

Quite often the cables wired into the panels are not long enough to come into the roof area and into an isolator, and so are joined. Most companies join though using exactly the same type of cable that is wired into the panels.

Malcolm
Many thanks for this, you have described them cables exactly.
regards
Pete