SWA on DC side of PV system

I won't make a statement regarding suitability of any particular cable in this thread, just point out that:

• Reference: Reg 712.410.102. Double or reinforced insulation is required on the DC side where the inverter has no protective separation between AC and DC sides (i.e. SELV or PELV can't be met), because, if the inverter has no separation between AC and DC sides, effectively you're "shaking hands" with the grid under some conditions as the DC is switched to the AC (quite literally) by the electronics in the inverter (perhaps though inductances etc.). Hence, it could not be
• Reference: Reg 712.521.101. Double or reinforced insulation is required on the DC side to help separate L+ and L- conductors to minimize risk of short-circuit, in parts of the DC side without overcurrent protection (i.e. because there are no fuses at the array). Required to protect against fire, and also possibly in line with 712.410.102 electric shock. The methods exemplified in the standard demonstrate how conductors need to be arranged (each insulation, and then each enclosed either in a sheath or another run of non-conductive containment.
• Regulation 412.2.4.1 is the requirements for wiring systems used where double or reinforced insulation are employed.
• In an SWA cable, there is no insulation or sheath between the armour and the insulated conductors. If it can [ever] be touched it's therefore an extraneous-conductive-part. In an SWA cable.

It's necessary to consider all of the above points when looking for a suitable wiring system for the DC side.

Doncaster Cables now produce DC rated SWA cable.  Interested to know your view on this new cable?  https://www.doncastercables.com/uploads/PV-Ultra_Datasheet_.pdf  

It seems it double insulation (or insulated + sheath if you prefer) around each core - so would seem to meet requirements.

But it presumably there's no BS (or equivalent) for that cable type - so it's use would be on the head of the designer and would have to be recorded as a departure on the cert. (133.1.3).

  - Andy.

Had a few discussions about it already.  Likely to be used where damage to cable may occur, what happens if the live conductors faulted to the armour?  And what happens if the armoured is earthed through to the AC side and you end up connecting the DC side to the AC side.  Lots to consider here.  

My PV system is a switched alternative supply to the grid connection. The Victron inverter charger has transformer ac/dc isolation. Victron recommend connecting the negative of the battery to the ac installation MET. I have a TT arrangement anyway. The PV array has string fuses. So given that the mppt controller does not galvanically isolate the PV from the battery, the negative of the PV will be earth referenced.  Given this scenario, would you consider SWA a permitted cable for from the array to the inverter?

Is the battery ELV (e.g. 12, 24 or 48V)? (as seems common with Victron setups) Is the PV d.c. side also ELV? If both are, then the need for double insulation on the d.c. side quite possibly goes away, in which case so does one of the major objections to using SWA.

I'd almost turn the question around and ask why use SWA though - you won't get ADS on the PV side (with a fault current hardly likely to exceed 1.2x normal current even if the sun happens to be shining brightly at the appropriate moment so the PV fuses won't blow). If it's just for mechanical protection, alternative approaches (e.g. insulating conduit) might be preferred.

I'm not sure that the lack of galvanic isolation between PV and battery means the -ve sides are necessary solidly connected together - there's likely to be some DC-DC conversion going on to match the panel output to the battery voltage, which may or may not have a simple solid connection on the -ve side - it all depends on the individual design. Maybe Victron have stated that theirs do?

  - Andy.

Thanks. The string voltage is about 240Vdc, so not ELV. Not clear what the electrical pathway is between the mppt controller input and output, The regs advise against earthing the negative side of the PV, but unless there is electrical separation between the input and output of the mppt controller this cannot be guaranteed with the battery negative earthed as required by the manufacturer. The problem with using PV ultra double insulated cable is that it comes in 6mm max and I need 25mm to avoid excessive volt drop. Paralleling up the PV ultra would be very expensive.

Update on my previous reply. It seems that the mppt input and output negative terminals are tied together, so PV negative is effectively earthed to the battery negative and installation MET.

MFR58 said:

The PV array has string fuses. So given that the mppt controller does not galvanically isolate the PV from the battery, the negative of the PV will be earth referenced.  Given this scenario, would you consider SWA a permitted cable for from the array to the inverter?

BS 7671 still requires double or reinforced insulation on the DC side. There are a number of reasons for this.

The negative of the PV array will only be earth referenced until the system is partly disconnected for maintenance, and the array cannot be switched off.

MFR58 said:

The PV array has string fuses. So given that the mppt controller does not galvanically isolate the PV from the battery, the negative of the PV will be earth referenced.  Given this scenario, would you consider SWA a permitted cable for from the array to the inverter?

BS 7671 still requires double or reinforced insulation on the DC side. There are a number of reasons for this.

The negative of the PV array will only be earth referenced until the system is partly disconnected for maintenance, and the array cannot be switched off.