@transparent I have a question about the electrical connection for my micro-inverter. It's designed to be a plug-in device, but I'd rather make a properly protected permanent connection to the mains than just plug it in.

The outbuilding is fed by a SWA cable from the main CU in the house on a 32A MCB protected by an 80A (30mA) RCD. I very much doubt the RCD in the main CU is bidirectional.

There is a CU in the outbuilding fed by the SWA with individual MCBs for the chest freezer, lights and sockets. These are only protected by the RCD in the main CU that feeds it. If connecting the micro-inverter into this secondary CU, what type of RCD or RCBO should I specify to best protect that 800W micro-inverter?

Would you recommend we also install a SPD in the outbuilding CU as there in no surge protection at the main house CU?

@transparent Hi, so I have already bought the splices, but the hole you refer to is actually a nipple! 🙂 (Bugger, there go the midwifery students again). I would have to drill the section to insert any fixings, so Im guessing its not meant to be fixed. They do supply a much longer rail though so I suppose fixing it isnt going to be the end of the world.

I watched a very interesting youtube video with two sparkies discussing the earthing conundrum. As you say its not mandatory but they did come up with some plausible scenarios where there could be an issue. One of them actually had an arc from his forehead to a scaffold when he bent down to pick up some wet leaves from the gutter! They blamed parasitic coupling (???)

One other point against earthing it is the fact that I cant actually source the correct renusol item, to connect the earthing wire!

@old_scientist I got them from Tradesparky. In the end after much debate with ChatGTP, I plumbed for these hooks. The height adjustment is quite useful, and one up from bottom is perfect for my tiles.

Let's deal with mounting systems first.

You don't have to use a proprietary brand of aluminium rails. The shipping costs can be prohibitively expensive for long items, especially if you only want to put a couple of panels on a shed roof.

Each profile for aluminium extrusions is registered. That too can add to the cost. Once you've bought into a system such as Renusol, then there's only one manufacturer with the licence to produce it.
In contrast, aluminium c-section is ubiquitous, and might be available from a local supplier.

However, even if I do start with a Brand-named rail, the C-section parts of a profile are often the same size. So I might be able to use a cheaper/generic clamp to hold the panels themselves.

Here's an installation using Fastensol rails.

and here's the same panel clamps being used on generic C-section.

In both cases, the inner dimension of the C-channel is 20mm.

That means I can buy ready-made T-nuts to suit it, selecting whatever thread-size I want.

Or I can use aluminium flat bar to make my own captive sliding nuts, and interconnect with other generic mechanical structures, such as 50mm dia scaffolding.

That last photo shows an aluminium half-coupler truss-clamp, usually sold for lighting and exhibition work.

And that's the other benefit of staying generic. You can often find private listings on Ebay where someone has 10 panel-clamps for sale, for example.

As for the 20mm C-channel itself, many of the online metal suppliers have it in stock, even if it's not listed on their main website. Sometimes it's because they've incorrectly identified it, calling it "slide channel" or placed it amongst the U-channels.

C-channel is also readily available from companies who supply component parts for sliding (patio) doors, garage doors or stage lighting. The price can vary a lot, depending on the market they're aiming at.

I once bought 80-metres of C-channel which had been sitting on a stockist's shelf for more than 6-months. They'd simply mis-identified it, meaning that prospective customers were unaware it existed.

A couple more points about placing solar panels at low level:

Unlike an installation on a high-roof, panels on a shed or garage are less likely to be subjected to wind-shear.
that means you could allow them to overhang the end of a roof in order to maximise space:

The bracket supporting the C-section rail was bought for a main-roof kit for fixing into a rafter. However, it's been repurposed and provides perfectly adequate support for the panel overhanging the end of the lower roof.

Another opportunity exists to place panels onto low roofs which have a wall behind them (a lean-to roof).

Instead of using roofing brackets throughout, the upper end of a rail can be directly attached to the back wall:

In this case I've used an eye-bolt fixed with a shield anchor (aka sleeve anchor).

The wall bracket is made from 5mm thick aluminium angle, and the entire assembly is angled at 20° using a plastic wedge through which the eye-bolt is screwed.

As you can probably see, there's a lot of scope for ingenuity.

Posted by: @old_scientist

↑

I'd rather make a properly protected permanent connection to the mains than just plug it in.

That makes sense, and concurs with a lot of online chatter from 'influencers'.

Check the rules on using a dedicated hardwired connection to your balcony-solar micro-inverter. I believe you're allowed to output up to 2kW rather than just 800w. 😊 

Have a look at what Indevolt replied to James' question here. They are stating that a hard-wired installation using their storage battery (portable) micro-inverter is permitted to export 3kW. But I'm unsure if that figure remains true if the installation doesn't contain any element of storage. Whatever you find out, please post the answer here and provide a link from the other topic. There's considerable overlap, which would be useful to others.

Posted by: @old_scientist

↑

The outbuilding is fed by a SWA cable from the main CU in the house on a 32A MCB protected by an 80A (30mA) RCD. I very much doubt the RCD in the main CU is bidirectional.

If it's an RCD then it is bi-directional.

The issue over the uni-directional nature of earth-leakage trips applies only to RCBOs.

This is a message we need to convey as widely as possible. Please feel free to repeat whenever appropriate on this Forum!

Posted by: @old_scientist

↑

If connecting the micro-inverter into this secondary CU, what type of RCD or RCBO should I specify to best protect that 800W micro-inverter?

It would be permissible to install (only) an MCB. But that would leave you in a situation where a fault with the micro-inverter causing a path to earth would (should!) trip the RCD in the main house.

Personally I'd fit a bi-directional RCBO for the micro-inverter, and hope that it trips under fault conditions before the RCD in the house.

You need to consider the possibility of the freezer being left without power, whilst you are unaware. My electrical feeds to out-buildings don't have an RCD at the house-end. That allows me to have a dedicated socket for an appliance such as a freezer, which has its own earth-leakage RCBO trip (not shared with anything else).

Posted by: @old_scientist

↑

Would you recommend we also install a SPD in the outbuilding CU as there in no surge protection at the main house CU?

So we're now talking about a pair of SPDs on the 240v AC mains incomer to the house.

Those are certainly desirable, but not to be confused with my earlier comments about having DC-rated SPDs on the wires to the (micro-) inverter from the PV panels.

In both cases, SPDs require a nice fat copper wire to an earth stake. It must be able to divert 40kV to ground at two-thirds the speed of light!

@transparent thank you for your reply above.

As always, very clear and concise, I have learnt new stuff (that RCDs are bi-directional) and I now have a clear idea of my options.

Posted by: @old_scientist

↑

Posted by: @old_scientist

↑

If connecting the micro-inverter into this secondary CU, what type of RCD or RCBO should I specify to best protect that 800W micro-inverter?

 

It would be permissible to install (only) an MCB. But that would leave you in a situation where a fault with the micro-inverter causing a path to earth would (should!) trip the RCD in the main house.

Personally I'd fit a bi-directional RCBO for the micro-inverter, and hope that it trips under fault conditions before the RCD in the house.

You need to consider the possibility of the freezer being left without power, whilst you are unaware. My electrical feeds to out-buildings don't have an RCD at the house-end. That allows me to have a dedicated socket for an appliance such as a freezer, which has its own earth-leakage RCBO trip (not shared with anything else).

Great, that's exactly my thinking. Unfortunately I'm unlikely to change the distribution from the main CU, but certainly something to remember for the future when it's time for that CU to be upgraded or replaced.