Hi All, looking fo some support/ guidance with my solar PV causing my main CU RCD to trip randomly.

[Nathan_95]

First post on any kind of forum so please be gentle with me!!
I moved into this house a couple of months ago and the vendor had a solar PV installed 5 years ago now. Recently I have had random tripping of my main RCD for what I believe to be a result of the Solar PV. I’ve attached a couple of images one of my CU (its pretty old!) and one of the installation schematics.

Currently the MCB labelled up “immersion Heater” (16A ) feeds DB2 which supplies the solar panels.
Now it is my understanding that since the cables for the solar PV are not buried in a well shallower than 50mm that the supply for the solar PV does now need to be on an RCD. Also from looking at the schematics it looks as if the supply to the solar PV should be on the incoming side of the CU.
Any guidance or help on this would be greatly appreciated!

 

[buzzlightyear]

Rcd protection is for cables buried in a wall and additional protection.

 

[Nathan_95]

Rcd protection is for cables buried in a wall and additional protection.

Yeah that was my understanding also. I’ve looked through a lot of forums and done a fair bit a research and from what I’ve seen a lot of solar PVs are not installed on a shared CUs RCD as they cause a lot of random tripping. I’m currently trying to find the manual of the inverter to see what protective device they recommend for use.

 

[HappyHippyDad]

Rcd protection is for cables buried in a wall and additional protection.

This could be a TT though and therefore need 30mA RCD on everything.

To Nathan.. Why do you think it is the solar array causing the RCD to trip? It sounds as though you are trying to negate the need for an RCD on the solar part of the install. I do not install solar so am unsure of the RCD requirements but I would be EXTREMELY wary of removing RCD protection from something especially given that it may be showing you there is a fault.
Apologies if you know this, but a 30mA RCD is a life saver, allowing only <30mA of fault current to flow before tripping thus (probably) saving your life.

 

[davesparks]

Solar PV should not feed in to the load side of an RCD which feeds any other circuits. If a person were to receive an electric shock the RCD will detect it and operate within 200mS but the solar PV could continue to provide power to the load side of the RCD for a couple of seconds.
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I do not install solar so am unsure of the RCD requirements but I would be EXTREMELY wary of removing RCD protection from something especially given that it may be showing you there is a fault.

The circuit which connects the inverter to the normal mains supply is unusual as it has a source of power at both ends of the cable, the grid at one end and the inverter at the other.
When the grid supply goes off the inverter will shut down, but this can take a few seconds sometimes.
It's very hard to provide effective RCD protection for this circuit as you would need to somehow ensure simultaneous disconnection at both ends of the circuit.

 

[Nathan_95]

Solar PV should not feed in to the load side of an RCD which feeds any other circuits. If a person were to receive an electric shock the RCD will detect it and operate within 200mS but the solar PV could continue to provide power to the load side of the RCD for a couple of seconds.

Yeah I read that elsewhere, I’ve thought about contacting the installer of the system but by the looks of it they are no longer trading! With regard to getting this corrected do you know if a normal domestic electrician would look at this or would I be better contacting a certified solar PV installer?
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This could be a TT though and therefore need 30mA RCD on everything.

To Nathan.. Why do you think it is the solar array causing the RCD to trip? It sounds as though you are trying to negate the need for an RCD on the solar part of the install. I do not install solar so am unsure of the RCD requirements but I would be EXTREMELY wary of removing RCD protection from something especially given that it may be showing you there is a fault.
Apologies if you know this, but a 30mA RCD is a life saver, allowing only <30mA of fault current to flow before tripping thus (probably) saving your life.

Hi Happy,
I’ve disconnected all circuits in the CU apart from the solar array an which point the RCD will still trip. With the solar array disconnected the RCD doesn’t trip.
I’m wary of doing any work personally with regards to this just because I’m based in control engineering not power! It’s just from some research on other sites/ threads I’ve seen a lot of people coming across a similar issues with shared RCD protection and Solar PVs

 

[pc1966]

Currently the MCB labelled up “immersion Heater” (16A ) feeds DB2 which supplies the solar panels.

That is not what the installation diagram says!

As Davesparks has already pointed out, you should not have a 2nd source of power on the "protected" side of a circuit breaker.

• Firstly your set up compromises the main RCD protection for personnel on all circuits
• Secondly if that immersion heater is still in use it also compromises the over current protection as a fault at the heater can draw both the 16A breaker's limit and the PV inverter's output.

Without changing the CU to have separate RCD protection on all other circuit, the safest and least-expensive option would be to split the meter tails, add a fuse/switch for over current protection, and a separate RCD to cover the feed cable to the solar inverter (basically as the installation diagram shows). Due to the possibility of a DC fault it is essential to have at least a 'type A' but preferably a 'type B' that will function even on a true DC fault (definitely not 'type AC') .

That is definitely not DIY work, but I expect any professional electrician should be able to do it.
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Just to add, you already have a switch and MCB for over current in the existing DB2 unit, you ought to have it fed from the main supply and have an RCD on the output.

However, it might be cheaper/easier for the spark simply to replace the MCB with a 2-pole 16A RCBO (MCB & RCD combined) and upgrade/re-route the incoming cables for safe connection to the incoming supply.

Also for dumb reasons folk decided to call the RCD trip characteristics AC,A,B,F... with respect to the sort of fault waveform they will trip on, and the trip characteristics for MCB as B,C,D depending on how fast the "instantaneous" magnetic trip fires. This makes checking RCBO specifications difficult!
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Sorry? Not sure what you’re asking/ saying

The point is you ought to blank out any personally identifying information before posting (client and/or contractor).