Are solar panel inverters tied to earth?

[oscar21]

I'm in a questions asking mood tonight. I briefely looked at a job today and I need to recommend a board change but need to justify it. Firstly there isn't any main switch, just two RCD's. Looks like an old 16th ed board and someone has replaced the old main switch with an RCD and then looped the tails straight across to the 2nd RCD so its kind of a dual RCD board but no means of isolation.

But secondly its had a solar install done at some point and the MCB for it is after the second RCD so if the RCD was to trip the solar would still be supplying the various circuits on the same bus bar, including the faulty circuit that had caused the trip. Is this an actual dangerous fault, could you still get a shock of the exposed metalwork if it has a live to earth fault via the solar panels, ie is the neutral side of the inverter grounded, I suppose it would be if its connected to the neutral bar in the consumer unit.

I don't do solar installs so how are they actually tied into the consumer unit, the cable is twin and earth so it would need RCD protection, do you have to use a 1 way unit or can it just be fed of an RCBO before any other RCD's/RCBO's

 

[timhoward]

A shared RCD shouldn’t be used for an inverter supply (and the feed backwards) for the reason you give.
The inverter will shut down if mains fails but not quickly enough to ensure disconnection times are met.

 

[oscar21]

A shared RCD shouldn’t be used for an inverter supply (and the feed backwards) for the reason you give.
The inverter will shut down if mains fails but not quickly enough to ensure disconnection times are met.

Ah so the inverter shuts down anyway, so lets say you are filling an electric kettle and are touching the kettle and a grounded tap, the kettle has an earth fault so the RCD trips, you get a slight shock but the majority of the fault current goes via the earth wire, what then happens with the inverter power that is still there, is there still a path for it through you and the tap? lets say its a TNS system with separate earth and neutral supplies so they aren't connected together at the cut out point.

 

[pc1966]

Ah so the inverter shuts down anyway,

Yes, but it could be a sizable fraction of a second to maybe more than one second. I.e. not acceptable for shock protection!

 

[pc1966]

I would add you should check the manufacturer's instructions for suitable RCD protection.

A friend of mine had one fitted but lost out a lot of money as the 30mA RCD would trip and they would not spot it. I checked and found it wired in SWA so no additional protection needed, and MI suggested 100mA or 300mA AC type S as suitable so fitted cheapest 100mA S-type one I could find and it was fine since then.

 

[pc1966]

can it just be fed of an RCBO before any other RCD's/RCBO's

I would say it has to be a dedicated circuit. Check to see if N-switching is needed though, personally I would treat it as for EV and have N-switching in all cases, but if no RCBO to fit the CU that has that feature, maybe the MI says not needed.

 

[pc1966]

Finally I would add that some PV inverters / battery chargers are designed to work off-grid should the supply fail, rather like a large UPS, so the installation has it own power island. They probably also link N-E and isolate incoming L & N under such conditions and they ought to be in-line between the supply and sustained load(s) so they can do that switch over, etc.

That is not such a common setup to see I'm guessing.

 

[oscar21]

I've done the board change on this now, what a lash up it was before, old 16th board hacked into a dual RCD one, think they have taken the main switch out and put it in an enclosure outside the CU, then bodged another RCD into the unit and bridged the two.

Solar wired in T&E but it needed to go through a meter so just strip it back until you get singles, great idea. L&N to meter, cpc to consumer unit.

I did what I could with bits in the van.

However is it wired right? the way they had done it was the feed from the inverter went to the meter input first, then out of the meter into the isolator and then to the MCB. I thought that was wrong and the inverter feed should go to the AC isolator first. That way if you want to work on the meter you can switch the isolator and MCB off and its totally isolated but I might be wrong.

Also the T&E to the inverter is in trunking so I don't think it needs any sort of RCD so for now I've just put it on the unprotected side, I've not even found the inverter yet let alone know what make it is so have no idea of any specs, I suspect its in the roof void somewhere but its a biddy bungalow, the roof is extremely low and I'm extremely big and the loft hatch is in the lowest bit of the roof.