RCD's with Photovoltaic's?

[Markc]

However, if the inverter was fed by an RCBO instead of an MCB then I would expect this to trip as well as the main CU RCD thus cutting the inverter off from the rest of the circuits

I understand your logic here although not fully agree. If a fault (customer, nail hammer scenario) is on the PV, AC side there is still not supplementary protection. Nor can I think of a way to provide it.

 

[dougalthedog]

Fair point Mark - I was suggesting the RCBO to protect the other circuits from the PV circuit. If the PV cable itself needs RCD protection the only solution I can think of would be some sort of RCD at the inverter itself.

 

[VMan]

you only need an RCD if there isn't electrical seperation, although even that is debateable as some inverters have inbuilt earth fault protection which satisfies that requirement (so I believe!)
Unfortunately for you the reason you need an RCD is because your on a TT, and if you need an RCD for PV it has to be a type B.
main switch RCDs are a pain because if they trip everything goes off. RCBOs is the gold standard option but it can be tight getting them in a board on every way. twin RCDs is the more common solution. A high integrity board gives you the best compromise IMHO as it gives you twin RCDs plus a couple of spare ways for circuits you may want on ther own RCBOs (freezers for example)

I agree with a separate dedicated CU, but i have a full 4 way unit (Rcd as main swith and 4 circuits inc PV), and no room for an upgrade in size, it physically wont fit, so after this discussion i think the best option is for a normal main switch and 4 x RCBO's.

The inverter has a transformer (i.e. electrically separated) so RCBO's do not need to be Type b.

 

[Markc]

Fair point Mark - I was suggesting the RCBO to protect the other circuits from the PV circuit. If the PV cable itself needs RCD protection the only solution I can think of would be some sort of RCD at the inverter itself.

I understand the logic. Having a RCBO feed off a RCD protected way the thought is any fault occurring on a the RCD protected circuits will not only "trip" the RCD but also the RCBO in <40mS? Designed cascade affect.

The problem is, as I have witnessed on numerous occations, the cascade effect isn't fool proof. Several times I have seen an up stream RCD not trip where an equal sized down stream has. My example of this is caravan parks. We have a number of sites we look after where the number of static hook ups is over 100 and all have individual RCD or RCBO protected hook up and RCD overall CU's in the actual caravan. Several times the van will trip the hook up and not the caravan and the caravan and not the hook up. It will also not trip any other RCD in the circuit or feeder pillars?

I think this subject will run and run. Its all to good for the registered bodies to bring this to our attention but notice how silent they are with a reasonable solution?

 

[SolarCity]

Re. Biggs' point that the RCBO would only trip if the fault was on the inverter side of the RCBO, I'm not entirely sure. I can't see any reason why an RCD/BO should be directional and once the main RCD trips, there will still be an imbalance between the line / neutral currents so it should trip. I probably wouldn't want to bet my life on it so will stick to installing a separate CU.

Surely if this were the case then under circumstances where a non-RCD protected circuit developed an inbalance, it should trip an RCD protected circuit which is part of the same CU. This doesn't happen. Or am I missing something?

 

[SolarCity]

Fair point Mark - I was suggesting the RCBO to protect the other circuits from the PV circuit. If the PV cable itself needs RCD protection the only solution I can think of would be some sort of RCD at the inverter itself.

This is an interesting point - perhaps this is the only way we could possibly achieve <5s disconnection times. I'm still not convinced it would work though.

 

[moggy1968]

You have it a bit the wrong way around Moggy.
712.411.3.2.1.1 is the relevant reg

That reg does not require an rcd.
But it says if there is an rcd for ADS (eg TT)
and if no simple separation (ie transformerless)
then use a type b rcd
unless the inverter meets the exception at the bottom.

There is a longer debate around this subject that you will find on here, but that is what the reg says.

err,
I think actually thats exactly what I said but in rather more words
I'm well aware of the regs on this as I entered the other discussion on here you refer to but didn't fancy going out to my van in the rain to get the big green book!!
If you read my post in the relevance of the original post which refer to a TT system then you will see it is correct.
I think you have taken my first paragraph out of context of the original question

 

[dougalthedog]

Once the main RCD has tripped, the PV inverter becomes the only "supply" so if there is still a fault to earth on one of the other circuits, current will flow from the inverter, through the line terminal on the RCBO and some will then go to earth and the rest back through the neutral. Because the line and neutral currents through the RCBO will be different it will trip (this assumes an RCBO is not directional and I can think of no reason why it should be but may test this!).

In the case of a non RCD protected circuit developing an imbalance, this doesn't trip the RCBO because the current flowing through the line terminal of the RCD remains the same as that returning through the neutral. In the case of a particularly severe short circuit, the current flowing through live may be reduced but there will be a corresponding reduction in neutral current so the RCD remains balanced and does not trip.

I agree with the cascade effect - in fact it may be worse with PV. Take the example we have been discussing where a split load box with 30mA RCD feeds a PV circuit via a 30mA RCBO. If the other circuits on the same side of the split load are drawing 8A and the PV system is supplying 4A there will be an equal 4A being supplied from the external supply.

If we were to introduce a hypothetical earth leakage of 50mA presumably there would be 25mA leakage through the main RCD and 25mA leakage through the RCBO (currents are split equally) so neither would trip in spite of an earth leakage of well over 30mA!

 

[VMan]

Once the main RCD has tripped, the PV inverter becomes the only "supply" so if there is still a fault to earth on one of the other circuits, current will flow from the inverter, through the line terminal on the RCBO and some will then go to earth and the rest back through the neutral. Because the line and neutral currents through the RCBO will be different it will trip (this assumes an RCBO is not directional and I can think of no reason why it should be but may test this!).

In the case of a non RCD protected circuit developing an imbalance, this doesn't trip the RCBO because the current flowing through the line terminal of the RCD remains the same as that returning through the neutral. In the case of a particularly severe short circuit, the current flowing through live may be reduced but there will be a corresponding reduction in neutral current so the RCD remains balanced and does not trip.

I agree with the cascade effect - in fact it may be worse with PV. Take the example we have been discussing where a split load box with 30mA RCD feeds a PV circuit via a 30mA RCBO. If the other circuits on the same side of the split load are drawing 8A and the PV system is supplying 4A there will be an equal 4A being supplied from the external supply.

If we were to introduce a hypothetical earth leakage of 50mA presumably there would be 25mA leakage through the main RCD and 25mA leakage through the RCBO (currents are split equally) so neither would trip in spite of an earth leakage of well over 30mA!

Good point, another reason not to have PV on RCD with other circuits! - this is actually a major concern and the more reasons im angry at my assessor!!

 

[moggy1968]

my head hurts!!

 

[SolarCity]

Good point, another reason not to have PV on RCD with other circuits! - this is actually a major concern and the more reasons im angry at my assessor!!

To be fair to your assessor, a few issues have come up on this thread that I hadn't considered before. As others have said, it is a relatively new industry to the UK and issues are bound to pop up, some of which are unique to the British market, that are going pass your assessor by.

Great thread.

 

[Markc]

Once the main RCD has tripped, the PV inverter becomes the only "supply" so if there is still a fault to earth on one of the other circuits, current will flow from the inverter, through the line terminal on the RCBO and some will then go to earth and the rest back through the neutral. Because the line and neutral currents through the RCBO will be different it will trip (this assumes an RCBO is not directional and I can think of no reason why it should be but may test this!).

But (Sorry...........I Know, I Know!) But, once the main RCD has tripped you have lost your "LINE" conductors, Live and neutral. The fault being on the now none protected side of the RCBO as the main RCD has now tripped. Will the RCBO activate? I don't see where the imbalance is coming from to trip the RCBO as the "Line" conductors are now isolated. i.e Fault in front of RCBO not behind. Yes cascade is still a factor. I'm not sure but worth a test I think.

 

[martin fuller]

Re Rcd's in CU's and inverter feed!
My installer and I have rung the Niceic helpline on this matter, question; is it ok for an inverter output to reed into a circuit breaker on a common rail Rcd controlled supply???,
Answer Yes, as long as theres no other circuits on the inverter to circuit breaker line, ( no problem)
Mr Niceic did ask what the install method was; 6,7 B, sheathed cables in clip trunking, surface mount.
I posed the scenario that if my rcd tripped for any reason, then if the inverter didnt shut down in <40ms, and potentially took up to 5 seconds to shut, then the common rail would be live for that time!
The answer was not convincing and I was unsure if Mr Niceic understood what i was saying, so I left it there.
My preference is a seperate rcd in the pv line feeding the non Rcd side of the board,
any comments ! thanks.

 

[moggy1968]

my preference is no RCD at all if I can manage it!!
I think until the situation becomes clearer the best bit if an RCD has to be used is to use a dedicated one for the PV that has no other circuits on it.

 

[screwdriver]

Just been following this thread and wanted to throw another metaphorical spanner in the works as it is all interesting conjecture and probably worth a play with some rcds and inverters over the next few months (whilst its quiet!)
How many inverters are referenced to earth? ( I mean a solid earth neutral connection) I have been thinking that if you had a system which I know couldn't be referenced to earth, eg. 4 soladin 600s which only have a 2 core cable feeding them then how would there ever be an imbalance in the feed from/to the inverter as the earth path is going to be via the cutout or transformer dependant on tns or pme supplies. So once the main rcd trips in the house there will only be voltage between live and neutral whilst the inverter shuts down and not live to earth as its return path through the neutral has been cut off. I understand that the main rcd would be slow to trip but I just cant see an rcd at the inverter would ever be able to trip unless the inverter was referenced to earth!
I am almost wondering if we should consider inverter supplies like the fire alarm supplies of old where they were totally separate and usually in an earthed conduit or micc..................