Specific requirements for earthing and bonding on metal sheds fitted with solar microinverters.

10-10-20

We have a discussion ongoing in relation to fitting solar panels and microinverters onto popular shed roofs.

https://www.boards.ie/discussion/2058292335/add-panels-on-steel-tech-shed

Just so we're clear on what a microinverter is, it's a small inverter device which directly converts the solar DC power, from either a single solar panel or a small group of panels, into AC which is fed back into the residential circuit. The devices typically operate in the range of 300 to 400w (ie, a solar panel's worth of power), but may be daisy-chained up to much larger installations. The inversion is carried out on the device by frequency matching to the incoming domestic AC and they are mounted directly underneath the panel, hence 240v AC is present at the rear of the panel on the roof in this configuration.

The system is often deployed using SWA off an RCD at the CU, a dual-pole isolator is typically fitted near to the solar panels and the panels are bonded off the CPC and SWA armor. The micro-inverters are self isolating in the event of an AC disconnect (EN50549), removing the requirement for a fireman's switch.

I brought up the topic of bonding and earthing in the context of providing earthing of the panels and the extraneous-conductive-parts within the shed. The manufacturer's guidelines typically are that "...the following equipment must be bonded electrically and grounded:"

The PV panel frames

The panel support rails (such as ground bonding across splices in the rail)

The micro-inverter case grounds (if a grounding lug is supplied)

Metal PV array junction box (if fitted)

Metal disconnect switch of the house (if fitted)

The question arises though:

If the panels are mounted onto a laminated steel-skinned shed which is constructed using metallic structures (such as the roof supports), and the AC supply cable routes around or within this structure; how is the bonding of the shed supposed to be handled in this situation? Do these components of the shed then require bonding as being extraneous-conductive-parts, and is this sufficient to carry-out using 10mm2 earthing back to the SWA CPC using suitable (and suitably installed) bonding points?

kirk.

Outbuildings my understanding was you brought out a main bond from the MET in the house

That could be the swa itself or a separate 10sq

I think the reasoning was the high fault currents possible through through the main bond and buried steel in a cattle shed for example if a ESBN neutral broke etc

Appliances at the shed would just take a CPC to earth the exposed metal in the appliance itself

Bonding in the outbuilding would be to extraneous metalwork

So I'd probably test from the earth terminal at shed board to metalwork (with CPCs disconnected) to see is it extraneous.( I think it's less than 25kohm resistance from shed board to metalwork)

What's the system of connecting microinverters to the supply ? not familiar with solar

10-10-20

https://www.boards.ie/discussion/comment/120494880#Comment_120494880

Thanks Kirk. I'll take time to digest that info and read-up on it more.

In relation to the supply side of microinverters, they are actually connected directly onto the AC, effectively at the rear of the solar panel, via a junction box. PDF below shows the typical setup with the inverters in red. In fairness, that diagram is missing an isolator switch.

https://www4.enphase.com/sites/default/files/downloads/support/IQ-7-7PLUS-micro-wiring.pdf?_ga=2.260708805.975668950.1681905770-41175939.1681905769

In the situation where these are self-installed then there would be no NC6, so no return on power exported to the grid possible in the absence of this cert.

kirk.

The size of the main bond coming out to the shed would be important for cattle sheds etc where you've steel girders

Not so much for a wooden shed

Afaik a load of steelwork including metal roof up on top of an insulated wooden structure wouldn't need a bond unless it's earthy some way ( not including appliance CPCs connected to structure)

Same sort of thing as copper pipework and cylinder. It has an immersion heater CPC connected to it but I think the main bond is done away with now unless the copper itself is extraneous

Could be wrong on some of that , could be something specific for solar I dunno

10-10-20

When you say "earthy", I presume you mean that the metal is physically installed into the earth as part of the foundations?

What about a steel garden shed which is on a damp concrete plinth and or has soil in or around the metal structures of the shed - would that be 'earthy' enough to cause concern?

kirk.

https://www.boards.ie/discussion/comment/120495792#Comment_120495792

I edited some of my first post there

Ya earthy means physically earthed , they use 25k as a figure in UK anyhow to determine extraneous ( between shed earth terminal and steel with megger afaik)

But that could vary depending on weather

Dunno does anyone bond those steel sheds dropped on concrete, no idea

kirk.

If the supply neutral gets broken just before the link

Load current flows out through the main bonding and structural steel . Those currents can be high depending on .

Probably not that relevant here

kirk.

Rules refer to main bonding of buildings. Buildings probably doesn't include metal sheds, I dunno ?

If you're connecting metallic equipment to the structure then it makes no difference ,probably as well to stick on a decent bond I would say

Other than than that if a shed metal structure is non-extraneous then it's preferable not to bond as you're only running the risk of a live shed with an open PEN

In the UK they have TT wiring for outbuildings which prevents that scenario

Scoobydoobydoo101

For what it's worth my sparky was here 2 days ago to connect up my 6kw hybrid on my steel shed.....10sq earth from the inverter back to the consumer box and 4sq bonded to each panel back to consumer box.