Connecting a PV systems the right way

[Barx]

Just to note there @binky an RCMU is not an RCD.

I emailed Afore (another cheap brand) to ask what type of RCD was built into their inverters as in their manual it just says RCD  -  Internal.

They replied saying their inverter does not have a built in RCD. LoL

 

[binky]

indeed it isn't, but the RCd is required against some form of DC leakage, so anything that monitors for said leakage basically performs the same function, probably more effectively....

 

[rugbyduffy]

ok found it.

Requirements of BS7671 connecting the PV a.c. mains.

At the connection to the a.c. mains, the PV supply cable must be connected to the supply side (i.e. upstream) of the OCPD's gor the final circuits in the electrical installation (regulation 712.411.3.2.11).

  therefore, the PV supply cable must be a dedicated circuit. It cannot be connected to an existing final circuit of the electrical installation.

When connected in this way, the operation of the protective devices of the other final circuits of the electrical installation is unaffected by the PV installation.

Fault protection

The PV supply cable on the a.c. side must be protected against fault current by an OCPD installed at the connection to the a.c. mains (regulation 712.434.1). A circuit breaker in a one way consumer unit fulfils this requirement.

  

When an RCD is not required at the PV end

If the purpose of the RCD at the PV inverter end is solely to prove protection against impact, the RCD would not be required if the supply cable was run in a floor or ceiling void, or on the surface or in SWA cable.

Regardless of whether or not the PV inverter has simple separation incorporated, an RCD would not be required in such circumstances.

Shared RCDs are not permitted

At the a.c. mains end the PV supply cable must not be connected to the outgoing way of a final circuit which shares an RCD with other final circuits (see IET code of practise for grid connected PV systems page 88). The reason this is prohibited is that the other final circuits maybe supplied by the inverter, during the shutting down period. This would mean the required maximum disconnection times for these final circuits could be compromised. In addition, sharing an RCD may cause unwanted nuisance tripping due to accumulation of earth currents. 

Good afternoon all,

New User here (although I have been an avid reader/spectator a number of years and found the forum a very good source of guidance and advice).

So firstly apologies if reviving this old thread was not the thing to do, but the quote above refers to a number of queries I had around my own PV installation so I thought it the most concise means of asking...

I have an SMA Sunnyboy Inverter installed 2012 into an independent circuit of a Dual RCD consumer unit, and whilst we'd always had infrequent nuisance tripping of the 80A RCD protecting that side of the board, the frequency had increased to the point where it became an inconvenience.  (I am aware that PV sharing RCDs is not a good practice from reading these forums, but can only presume that wasn't wide knowledge in 2012 as my New Build home subsequently passed Part P sign-off from my Local Authority).

I had an electrician colleague recently carry out some earth leakage tests on the other circuits which share that RCD in case a cumulative effect was causing it. He found no smoking gun, although he said the results were "not great, but okay". 

I carried out a series of investigations where I isolated the PV circuit for weeks at a time and experienced no nuisance tripping whatsoever; with it re-energised I'd typically get a couple a week (between 1-5 times, quite variable in frequency and not definitively linked to weather conditions which I also logged).

I recently requested my MCS installer return to my home to investigate as part of the Inverter 10yr warranty and their electricians stated during their visit that they have see a very small proportion of their installs suffer such nuisance tripping, and cited a recommendation that the PV needn't be RCD protected (I'd also expressed my concern after informing myself on this forum of the effect of a secondary supply sharing an RCD with other sockets).

Their course of action was to relocate the circuit to a non-RCD protected side of the board, which whilst having completely eliminated the nuisance tripping problem, has left me with a few queries.

1. I'd asked them to provide documentation of this modification for my own records, insurance purposes, etc. They seemed surprised that I'd requested this and stated that they could provide a Minor Works Certificate for me - is this the appropriate documentation for this work? 

2. I have been reading the available extracts of BS7671, in particular "522.6.202 Cables in wall less the 50mm from the surface" which I realise stipulates that I require 30mA protection for this circuit - my solar PV cables are within safe zones in plastic capping under render.   Now that this circuit has been modified am I required to comply with this (presumably more recent) regulation? I say 'presumably more recent reg' as I believe this was a 2015 update, which our home pre-dates (built 2012).

3. Assuming so, is my only option an independent 30mA RCBO for the the single PV circuit, and hope that without any cumulative leakage effects from the "okay, not great" circuits that this will survive my Sunnyboy inverter switching?
4. I have read a lot of manufacturer advice relating to PV nuisance tripping RCD which focuses on uprating to 100mA or 300mA ratings! Am I right to presume that these are applicable only in cases where cables are routed differently to mine, effectively snookering me (as ours were installed in build, not retro-fitted)?

5. Assuming I do need a 30mA RCBO and this suffers similar nuisance tripping, are my only other options to redesign the wiring routes to not require the RCD protection (Regulation 522.6.204 I think stipulates such examples?), or to upgrade our Inverter which is presumably deteriorating somewhat due to the increased frequency of RCD tripping and its 8 year age (I think we were suggested these had a potential life of 10yrs when we purchased the system, although I got the impression the industry was so new that nobody actually knew this as the time we were quoted it).

Appreciate a number of queries here, but I would really appreciate the guidance / understanding / experiences of anyone whose come across / overcome similar issues so that I can go back to my MCS Installer with an informed response!

Thanks in advance for any support offered.

Cheers

 

[Murdoch]

Good afternoon all,

New User here (although I have been an avid reader/spectator a number of years and found the forum a very good source of guidance and advice).

So firstly apologies if reviving this old thread was not the thing to do, but the quote above refers to a number of queries I had around my own PV installation so I thought it the most concise means of asking...

I have an SMA Sunnyboy Inverter installed 2012 into an independent circuit of a Dual RCD consumer unit, and whilst we'd always had infrequent nuisance tripping of the 80A RCD protecting that side of the board, the frequency had increased to the point where it became an inconvenience.  (I am aware that PV sharing RCDs is not a good practice from reading these forums, but can only presume that wasn't wide knowledge in 2012 as my New Build home subsequently passed Part P sign-off from my Local Authority).

I had an electrician colleague recently carry out some earth leakage tests on the other circuits which share that RCD in case a cumulative effect was causing it. He found no smoking gun, although he said the results were "not great, but okay". 

I carried out a series of investigations where I isolated the PV circuit for weeks at a time and experienced no nuisance tripping whatsoever; with it re-energised I'd typically get a couple a week (between 1-5 times, quite variable in frequency and not definitively linked to weather conditions which I also logged).

I recently requested my MCS installer return to my home to investigate as part of the Inverter 10yr warranty and their electricians stated during their visit that they have see a very small proportion of their installs suffer such nuisance tripping, and cited a recommendation that the PV needn't be RCD protected (I'd also expressed my concern after informing myself on this forum of the effect of a secondary supply sharing an RCD with other sockets).

Their course of action was to relocate the circuit to a non-RCD protected side of the board, which whilst having completely eliminated the nuisance tripping problem, has left me with a few queries.

1. I'd asked them to provide documentation of this modification for my own records, insurance purposes, etc. They seemed surprised that I'd requested this and stated that they could provide a Minor Works Certificate for me - is this the appropriate documentation for this work? 

2. I have been reading the available extracts of BS7671, in particular "522.6.202 Cables in wall less the 50mm from the surface" which I realise stipulates that I require 30mA protection for this circuit - my solar PV cables are within safe zones in plastic capping under render.   Now that this circuit has been modified am I required to comply with this (presumably more recent) regulation? I say 'presumably more recent reg' as I believe this was a 2015 update, which our home pre-dates (built 2012).

3. Assuming so, is my only option an independent 30mA RCBO for the the single PV circuit, and hope that without any cumulative leakage effects from the "okay, not great" circuits that this will survive my Sunnyboy inverter switching?
4. I have read a lot of manufacturer advice relating to PV nuisance tripping RCD which focuses on uprating to 100mA or 300mA ratings! Am I right to presume that these are applicable only in cases where cables are routed differently to mine, effectively snookering me (as ours were installed in build, not retro-fitted)?

5. Assuming I do need a 30mA RCBO and this suffers similar nuisance tripping, are my only other options to redesign the wiring routes to not require the RCD protection (Regulation 522.6.204 I think stipulates such examples?), or to upgrade our Inverter which is presumably deteriorating somewhat due to the increased frequency of RCD tripping and its 8 year age (I think we were suggested these had a potential life of 10yrs when we purchased the system, although I got the impression the industry was so new that nobody actually knew this as the time we were quoted it).

Appreciate a number of queries here, but I would really appreciate the guidance / understanding / experiences of anyone whose come across / overcome similar issues so that I can go back to my MCS Installer with an informed response!

Thanks in advance for any support offered.

Cheers

@binky

 

[NozSpark]

The one point that I'll pick up on is that they removed RCD protection for the circuit.... the regs while the regulations are not retrospective, any work that we do has to comply with the regulations that are in place at the time that the work is carried out,,, so if this cable is buried <50mm in a wall, goes through a metal frame stud work or passes through a bathroom then it needs RCD protection... the best way to achieve this of PV would be to protect it with a RCBO or through it's own consumer unit with appropriate fault and overload protection... as for the type of RCD that it should have,,, I'd say that an "AC" type RCD is no longer appropriate for a new installation or alteration of a PV circuit and that an "A" or possibly even "B" type RCD would be better suited

 

[binky]

1. I'd asked them to provide documentation of this modification for my own records, insurance purposes, etc. They seemed surprised that I'd requested this and stated that they could provide a Minor Works Certificate for me - is this the appropriate documentation for this work? 

seems reasonable to me, it's not a new circuit as such, and not a new MCB, so the electrical characteristics haven't really been changed. 

2. I have been reading the available extracts of BS7671, in particular "522.6.202 Cables in wall less the 50mm from the surface" which I realise stipulates that I require 30mA protection for this circuit - my solar PV cables are within safe zones in plastic capping under render.   Now that this circuit has been modified am I required to comply with this (presumably more recent) regulation? I say 'presumably more recent reg' as I believe this was a 2015 update, which our home pre-dates (built 2012).

rediculous as it may seem plastic capping is regarded as 'mechanical protection' same as plastic trunking, ergo any need for RCD is removed in my opinion. I would not be looking to reroute cables to suite 2018 regs or citing current regs as a reason to do so, it is not a new circuit.   Would have to rewire half the houses in the UK if we did. As it stands it is compliant with 2012 and only thing required is the OCPD (cable protection) , so as long as that complies with requirements for overload protection, ie right size MCB then that is fine. Moving cct off RCD has just enhanced your safety. Inverter shut down times can be as long as 3 seconds (in reality most work far faster) so RCD could be tripped out whilst you are cutting through your lawn mower cable and you still get belted by your PV system for up to 3 seconds, rather than 40mili-seconds RCD normally takes to trip. This could kill you!  

3. Assuming so, is my only option an independent 30mA RCBO for the the single PV circuit, and hope that without any cumulative leakage effects from the "okay, not great" circuits that this will survive my Sunnyboy inverter switching?

Sunny boys aren't normally as issue on their own RCD - I still prefer no RCD.  

4. I have read a lot of manufacturer advice relating to PV nuisance tripping RCD which focuses on uprating to 100mA or 300mA ratings! Am I right to presume that these are applicable only in cases where cables are routed differently to mine, effectively snookering me (as ours were installed in build, not retro-fitted)?

See comments in previous discussions.  Power One are effing terrible for earth leakage after being bought out by ABB, my own earlier model Power One has no issues with earth leakage. Later models after a couple of years leak like a sieve hence need 300mA RCD to cope. I've had several long arguments with their technical support and got precisely no where with any complaints, and stopped fitting these invereters - they used to be my favourite unit. Not known earth leakage to be an issue with any other make of inverter. When installing cables I've always routed to avoid RCD requirement under current regs, but any requirement is not retrospective. 

5. Assuming I do need a 30mA RCBO and this suffers similar nuisance tripping, are my only other options to redesign the wiring routes to not require the RCD protection (Regulation 522.6.204 I think stipulates such examples?), or to upgrade our Inverter which is presumably deteriorating somewhat due to the increased frequency of RCD tripping and its 8 year age (I think we were suggested these had a potential life of 10yrs when we purchased the system, although I got the impression the industry was so new that nobody actually knew this as the time we were quoted it).

SMA I would expect to last 15 - 20 years unless subjected to a large voltage spike form something like a lightning strike in your local area, we are still in the dark about actual inverter life, but SMA design for 20/25 year life. 10 years is sales BS for flogging new inverter upgrades or getting foot through your door on pretence of 'safety inspections' or some other BS to sell you other stuff you don't need! The earth leakage issue will not be the inverter on its own, all modern appliances have some earth leakage to protect the electronics, so a build up of earth leakage from multiple sources eventually gets to the magic 30mA (in reality probably more like 25mA) that causes RCD to trip.

Hope that helps. 

 

[binky]

The one point that I'll pick up on is that they removed RCD protection for the circuit.... the regs while the regulations are not retrospective, any work that we do has to comply with the regulations that are in place at the time that the work is carried out,,

Interesting point, would correcting a mistake still be subject to todays regs? In this case, inverter should never have been put on a shared RCD (not really known at the time), so moving it off RCD has improved safety, corrected mistake, but not involved any alteration of the cct or OCPD. In my opinion, any requirement to work to current regs is not applicable or a bit over -zealous? If the cct was being alterred to say, add a socket, a modification to the cct, then I would agree with your point.

 

[NozSpark]

Interesting point, would correcting a mistake still be subject to todays regs? In this case, inverter should never have been put on a shared RCD (not really known at the time), so moving it off RCD has improved safety, corrected mistake, but not involved any alteration of the cct or OCPD. In my opinion, any requirement to work to current regs is not applicable or a bit over -zealous? If the cct was being alterred to say, add a socket, a modification to the cct, then I would agree with your point.

I'm sorry, but it's a buried cable.... it needs RCD protection...... whilst moving the circuit off a shared RCD has made the installation as a whole safer, removing RCD protection from that cable has made that circuit less safe

 

[Murdoch]

I'm sorry, but it's a buried cable.... it needs RCD protection...... whilst moving the circuit off a shared RCD has made the installation as a whole safer, removing RCD protection from that cable has made that circuit less safe

Isn't the simple answer replace  the MCB with a RCBO?

I find that some nuisance tripping (on dual boards) is due to accumulated leakage across the circuits - seen it more on "16th" boards where all the sockets are RCD protected

22mA isn't hard to achieve in the average house

 

[binky]

I'm sorry, but it's a buried cable.... it needs RCD protection...... whilst moving the circuit off a shared RCD has made the installation as a whole safer, removing RCD protection from that cable has made that circuit less safe

but capping counts as mechanical protection.

 

[Sharpend]

but capping counts as mechanical protection.

not sure that it does. 

 

[Murdoch]

not sure that it does. 

Doesn't that depend on how thick it is and what it's made of?

 

[binky]

Doesn't that depend on how thick it is and what it's made of?

never seen any 'guidleines' refering to mateial or thickness. Personally I think plastic capping is about as much use as a rich tea biscuit against a drill bit or screw. 

 

[SPECIAL LOCATION]

Most capping is designed so that it can be nailed to a wall...

Therefore it cannot be considered as mechanical protection for buried cables...

As bullet point (iv) of 522.6.204 specifically states mechanical protection will prevent penetration by nails / screws etc..

You can use an 'earthed metallic covering' to protect buried cables..

But capping's main function was to protect a cable from the plasterers trowels etc.. 

From the days of wet-plastering, when putting on the first bonding coat.

hence why plastic capping was an acceptable cheaper alternative to metal..

With so much dry-line boarding done nowadays..

many don't bother with any form of capping at all over the cables to be hidden behind the boards. 

bottom line is.. if you can hammer a nail through it..

It is NOT mechanical protection!

Guinness  

 

[Sharpend]

Concur with Spec as my understanding. 

 

[rugbyduffy]

Thanks everyone for your expertise and guidance! 

Thanks to @binky for taking the time to respond to each of my queries, and to @NozSpark, @Murdoch, @Sharpend and Spec @SPECIAL LOCATION for continuing the debate and providing your own thoughts and interpretations.

So what I take from your responses:

• Plastic Capping is unlikely to be classed as suitable ‘mechanical protection’ per the definitions (I must say as homeowner I wouldn’t feel comfortable in relying on it for this purpose);
  
• Assuming this, I would need to reinstate the RCD protection of this cable which I have lost since the modification (recognising I have actually made all of the circuits previously shared on the common RCD safer by eliminating the issue of the inverter shutdown lag). The reason for this is that the 17th edition wiring regulations BS7671 cover my system as they apply to all installations after 30th June 2008.
  (An alternative to this would be to re-route the cabling so as not to require RCD protection, which isn’t really practical as its wet-plastered into the walls since the house was built.)
  
• My SMA inverter on its own 30mA RCD is not likely to be an issue with nuisance tripping by removing the cumulative earth leakage from the previously shared circuits.
  
• The easiest means of achieving this would be to replace the MCB with an RCBO which will add earth fault protection and maintain Overload protection for the PV circuit.
  
• An ‘AC’ type RCD is not appropriate for this application and I should seek an RCBO with ‘A’ or ‘B’-type characteristics; I presume my installer will be able to advise on the exact type from OEM recommendations, but any thoughts on this from the experts?
  
• The previous MCB move didn’t require a Minor Works Certificate as this wasn’t a new circuit or MCB. On that basis if this were to be replaced with an RCBO should I expect to receive a Minor Work Certificate (or other paperwork for my records)?
  

Thanks again for all of your times and wisdom.

Cheers,

 

[rugbyduffy]

Thanks everyone for your expertise and guidance! 

Thanks to @binky for taking the time to respond to each of my queries, and to @NozSpark, @Murdoch, @Sharpend and Spec @SPECIAL LOCATION for continuing the debate and providing your own thoughts and interpretations.

So what I take from your responses:

• Plastic Capping is unlikely to be classed as suitable ‘mechanical protection’ per the definitions (I must say as homeowner I wouldn’t feel comfortable in relying on it for this purpose);
  
• Assuming this, I would need to reinstate the RCD protection of this cable which I have lost since the modification (recognising I have actually made all of the circuits previously shared on the common RCD safer by eliminating the issue of the inverter shutdown lag). The reason for this is that the 17th edition wiring regulations BS7671 cover my system as they apply to all installations after 30th June 2008.
  (An alternative to this would be to re-route the cabling so as not to require RCD protection, which isn’t really practical as its wet-plastered into the walls since the house was built.)
  
• My SMA inverter on its own 30mA RCD is not likely to be an issue with nuisance tripping by removing the cumulative earth leakage from the previously shared circuits.
  
• The easiest means of achieving this would be to replace the MCB with an RCBO which will add earth fault protection and maintain Overload protection for the PV circuit.
  
• An ‘AC’ type RCD is not appropriate for this application and I should seek an RCBO with ‘A’ or ‘B’-type characteristics; I presume my installer will be able to advise on the exact type from OEM recommendations, but any thoughts on this from the experts?
  
• The previous MCB move didn’t require a Minor Works Certificate as this wasn’t a new circuit or MCB. On that basis if this were to be replaced with an RCBO should I expect to receive a Minor Work Certificate (or other paperwork for my records)?
  

Thanks again for all of your times and wisdom.

Cheers,

Apologies for the bump, just one final call for help in case anyone more informed than me can advise on my assumptions/queries above, so that I know what to ask before I go back to my installer with a query...

Thanks in advance for any assistance!