TurboGen said:

The cable route from the attic

Just a couple of further thoughts:

• If the inverter is going in the loft, BS 5839-6 recommends fire (smoke or combined smoke and heat) detector in there, connected to the fire detection and alarm in the rest of the property (this would imply a minimum of interlinked smoke alarms, and the minimum requirement may be subject to devolved national requirements outside England).
• are you having DC-coupled storage batteries installed? If so, is the attic the appropriate place for those, as they would usually be installed close to the inverter? In addition to being heavy and having some inherent fire risks, they are usually subject to a tight band of operating temperatures for efficiency, battery life, and safety.

I would agree the attic is a poor place to locate both an inverter and certainly a poor place for temperature sensitive and high-energy equipment like batteries, initially when I started looking I wanted these in the garage. But if protecting AC cables with a bit of earth leakage in a wall is challenging, running DC cables around the building is an order of magnitude worse, such that it almost drives you toward keeping the DC in the attic, unless you run the cables on the surface externally but even then you have a long cable drop with no form of electrical protection from the energy from the solar PV side.

From a bit of initial investigation I can see why many of the solar installers opt for the simple solution of having inverter and batteries in the attic to minimise the DC cable run, keep it on the surface within the attic, with the AC cable down the exterior wall. It isn't particularly neat or elegant and from an equipment safety, maintenance and longevity perspective having inverters and batteries in a dusty loft subject to extreme ambient temperatures is far from ideal, but other approaches come with a lot of difficulty of how to install it in a compliant way.

Even the recommendation of how to protect against fire is a challenge because a lot of fire safety guidance advises against smoke detectors in attics because of their sensitivity to dust / dirt / insects causing nuisance operation, so drives towards just a heat detector but that should be reasonably achievable. That said, I imagine if I want that it would be me needing to extend my existing interlinked system, I have yet to see any solar installation companies even mentioning smoke or heat detection as part of their standard packages.

TurboGen said:

with no form of electrical protection from the energy from the solar PV side

Unless you go for a system with optimizers in ... certain types of optimizers have all sorts of safety features built in. In addition, to help mitigate the risk of short-circuits between L+ and L- ought to have wiring equivalent to double or reinforced insulation from each other (if installed to BS 7671:2018+A2:2022). 

There are always innovations, and in cabling for Solar PV, although I've not seen it yet myself, I believe there's a cable type available either now or the near future, that contains the 2 or 4 insulated-and-sheathed solar DC cables in a common UV stabilized outer sheath - so apparently a 2-core or 4-core multicore Solar DC cable that the manufacturers say meets BS 7671 latest requirements: https://www.youtube.com/watch?v=OmHtcfWUI4w

And further to innovation, there are a few other things that might be coming along. In some countries, fire service emergency isolators (I guess what we might call a firefighter's switch system) are starting to be required for certain Solar PV installations.

Some studies into solar PV fires at inverters have identified high temperatures in lofts being a cause.

Some insurers are providing guidelines for batteries and Solar PV, and this may bring in some other requirements.

Research into lithium storage battery and EV battery fires is ongoing.

gkenyon said:

Some studies into solar PV fires at inverters have identified high temperatures in lofts being a cause.

Is that inverters alone, or ones with batteries? If the latter, is that with lithium ion or lead-acid please?

gkenyon said:

Some studies into solar PV fires at inverters have identified high temperatures in lofts being a cause.

Is that inverters alone, or ones with batteries? If the latter, is that with lithium ion or lead-acid please?

Chris Pearson said:

Is that inverters alone,

At the moment, inverters alone.

What I don't know is whether the conductor operating temperature of cables has been taken into account in these studies, and potentially the compatibility of the cable operating temperature with the terminal operating temperatures of equipment?

Graham, thank you.

A substantial part of the roof of my house is flat and the 2nd floor rooms can get quite hot in summer - say 30 deg C, maybe a little more. The difference in the loft space is that there is just bare roofing felt (and tiles) between the rafters. I have been contemplating insulating, but for energy conservation in winter.

That notwithstanding, if the southern part of the roof were covered by PV arrays with a gap beneath them, wouldn't the roof be in shade?

I am now thinking that before going further, it might be useful to install a temperature recorder (if there is any summer left).

Chris Pearson said:

I am now thinking that before going further, it might be useful to install a temperature recorder (if there is any summer left).

That would be wise. Battery warranty is often temperature dependent, and both ambient and internal temperatures may be logged in the monitoring built into the battery itself.