Surge protection, fuses and MCBs in loft from PV array.

[JohnMo]

12 minutes ago, Beelbeebub said:

My inverter has instructions to turn the inverter off

ALL inverters need the AC disconnect, prior DC. Big bangs likely in full sun if you don't.  Likely to need a new isolator switch if your lucky, if your not, big volt DC isn't good for ones health.

 

[jimseng]

Quote

if you were desperate to have a "disconnection" point on the loft

I don't think it is really about having a disconnection point in the loft. I started this thread wondering if there was any point in having the SPD/Fuse/MCB box in my first post. I have concluded that it isn't as @Dillsue pointed out, what is the point of a fuse given the short circuit current of the panels? So I think I will stick with MC4s and buy a cheap crimper for the 8 terminations or so I will have to do. This seems the safest way to make the connections over any period of time, day or night when I come to it.

[JohnMo]

Just looking back at the original post on this thread the box shown is a combiner box, so takes two parallel strings and brings them into one. So you double the amps. So may not be suitable anyway.

 

Your other question is are you have string fuses? If so have them in a DIN enclosure in loft or anywhere as an easy place to terminate the strings. Terminate the other side of fuse and take to the inverter.

 

 

[Beelbeebub]

26 minutes ago, JohnMo said:

ALL inverters need the AC disconnect, prior DC. Big bangs likely in full sun if you don't.  Likely to need a new isolator switch if your lucky, if your not, big volt DC isn't good for ones health.

 

There is a whole shutdown sequence including shutting down the bms etc etc. My point being it's fairly easy as all the isolators (AC and DC) switches and buttons are in one place. 

 

If there is a loft isolator, before it can be turned the operator has to go down stairs do the various operations before coming back up. 

 

There is, unless the isolator is locked on, a fair chance someone will just turn the knob possibly under full load. 

 

Much less likely if the isolation requires opening a box (with a tool) and then using another tool to disconnect the MC4s

[Alan Ambrose]

Is there a vague 'requirement' for a fire-brigade-accessible PV-shutdown switch. I'm thinking a big red stop switch in an obvious place, probably wired to the inverter or a DC-contactor? Would this be sensible or is it completely OTT?

Edited December 9 by Alan Ambrose

[DannyT]

On 09/12/2025 at 19:39, Alan Ambrose said:

Is there a vague 'requirement' for a fire-brigade-accessible PV-shutdown switch. I'm thinking a big red stop switch in an obvious place, probably wired to the inverter or a DC-contactor? Would this be sensible or is it completely OTT?

I did this for that very reason. Sealed weather proof unit. A breaker switch for each string between panels and inverter. Clearly labelled solar power shut off.

Mine was a DIY install so wanted to go beyond the normal standard.

[JohnMo]

On 09/12/2025 at 19:39, Alan Ambrose said:

Is there a vague 'requirement' for a fire-brigade-accessible PV-shutdown switch. I'm thinking a big red stop switch in an obvious place, probably wired to the inverter or a DC-contactor? Would this be sensible or is it completely OTT?

Make sense to have something obvious like a DC isolator to kill the PV at ground level outside - I have a DC isolator outside near the front door. Isolator hidden in the fabric of the building or in a loft not easy for anyone. If switching under load kills the inverter who cares.

[Onoff]

Just jumping in here ref the SWA thing. I know zero btw about PV.

 

Just upgraded the house to garage supply from 4 to 16mm² SWA. Gone from overhead to ducted/underground. I sized it for 1% voltage drop with a nod to future PV. 

 

The plan is PV panels on the garage, with the inverter & batteries there. 

 

Presumably the normal SWA between garage and house will be fine as that will only have AC on it?

[Marvin]

24 minutes ago, Onoff said:

Presumably the normal SWA between garage and house will be fine as that will only have AC on it?

 

I used SWA. The energy can go both directions remember...

 

 

[Beelbeebub]

39 minutes ago, Onoff said:

Presumably the normal SWA between garage and house will be fine as that will only have AC on it?

Yes, that's fine. It"s what my set up uses. 

 

If my panels were on the garage and my inverter in the house, then I woikd need the specialist DC SWA to carry the DC between garage and inverter. 

 

My understanding is the issue with carrying DC along standard SWA, it that DC needs double insulated conductors and standard SWA is single insulated. 

[Dillsue]

49 minutes ago, Beelbeebub said:

My understanding is the issue with carrying DC along standard SWA, it that DC needs double insulated conductors and standard SWA is single insulated. 

AFAIK std SWA is rated for DC as the SWA I've seen includes ratings for DC as well as AC.

 

I think the issue with DC PV is that the short circuit current(Isc)is limited by the panels to a value only slightly higher than normal generating current(Imp). This makes it virtually impossible to cost effectively fit fuses or circuit breakers that will detect the difference between Isc and Imp and trip on Isc but not trip on Imp. As you can't then reasonably protect against short circuit between the 2 conductors, double insulation is specified to reduce the chance of a short circuit between the conductors. That's my understanding but happy to hear other takes on it??

[Beelbeebub]

The cables have a DC rating because you can stick DC down any conductor. 

 

But the UK regs say that cables that carry DC must meet certain requirements around insulation etc. 

 

Specifically the conductors need to be double insulated whilst they can be single insulated for AC. 

 

Here is an armoured AC cable. 

 

Notice you have:

Copper conductors

Layer of insulation (the coloured bit) 

Packing material (beige bit) 

Armour 

Outer sheath. 

 

Here is an armoured DC cable

 

Copper conductors

Layer of insulation (white) 

Layer of insulation (the red/white bit) 

Packing material (beige bit) 

Armour 

Outer sheath. 

 

Notice the two layers of insulation around each conductor 

 

You could use the top cable, it would physically work, in the same way you could just bury an extension flex in your garden to power your garage.

 

But it wouldn't meet the regulations. 

 

For that you need the specialist cable. 

[Dillsue]

18 hours ago, Beelbeebub said:

The cables have a DC rating because you can stick DC down any conductor. 

 

But the UK regs say that cables that carry DC must meet certain requirements around insulation etc. 

 

Specifically the conductors need to be double insulated whilst they can be single insulated for AC. 

 

Is that for all DC applications? I get that there's been changes in the last few years over the use of SWA for DC PV and the Ultra PV SWA has been developed to meet the specific requirement but not necessarily for all DC applications??

[Beelbeebub]

3 hours ago, Dillsue said:

Is that for all DC applications? I get that there's been changes in the last few years over the use of SWA for DC PV and the Ultra PV SWA has been developed to meet the specific requirement but not necessarily for all DC applications??

I'm not sure "all dc" would include low voltage stuff like door bells. 

 

I think (and am happy to be corrected) that under 50v DC is low voltage and different regs apply - so single insulated is fine. This would prob apply to the single panel type "camping" systems people stick on vans and the like.

 

But for the high voltage DC you get with large arrays a different set of regs apply and they say DC must be double insulated.  Hence using special solar cable rather than regular 4 or 6mm 2 core. I imagine you would need similar if you had another high voltage DC application eg some sort of machine tool or something. 

[Alan Ambrose]

My reading is that (with the possible exception of bathrooms) that low voltage dc e.g. 24V led strips (or ac come to that) is outside Part P. Similarly PoE which is usually 24V or 48V dc over ethernet cables. 

[Nickfromwales]

SELV covers up to 50v iirc.

[Dillsue]

50vac or 75vdc for ELV. I assume part P applies above those voltages??

[Nickfromwales]

2 hours ago, Dillsue said:

50vac or 75vdc for ELV. I assume part P applies above those voltages??

My books are all still wrapped in the cellophane as I didn’t bother going for my NAPIT in the end; Covid put that on ice, and a change in career path (coming away from the coal face) put it to sleep. 
 

Maybe I should give them a read lol.