Connecting energy monitors with Solar PV

[BrummyGit]

I'm not an electrician, but I'm hoping that you can help me work out how to solve a problem that's causing me headaches and sleepless nights. I recently had a Solar PV system installed in my home. For background information it's one of the free "rent-a-roof" schemes where the supplier gets the Feed In Tarrif and I get to use the electricity generated, with surplus exported to the grid. This also means that the supplier has satisfied their needs with a generation meter that they read via the mobile phone network.

In order to really benefit from the system I need to be able to monitor my generation and compare that to my consumption at any point in time, and ideally I'd like to keep history so I see how I'm doing. This means that I can then decide to switch on the dishwasher etc. Currently I'm using cost effective energy monitors from CurrentCost that clamp onto a single conductor and measure current flow, but not direction. I know these aren't the most accurate, but they are good enough for my needs and allow me to record the consumption on my laptop.

The Solar PV is connected to the consumer unit via a 16amp MCB on the non-RCD side, as are the house lights on a single bus bar from the 100a main switch. I have 1 30ma RCD feeding everything else via the RCD protected bus bar. My consumer unit is full and located in a full meter cupboard.

I have one energy monitor clamped on the Solar near the official generation meter which is fine for monitoring what I generate, but my problem is trying to monitor my consumption. Currently I have another monitor clamp on the meter tail, but the problem is that these monitors can only read current flow and not direction, therefore I can't tell if I'm exporting or importing power from the grid. Ideally I'd like to measure the total consumption of my home but I haven't got anywhere I can measure this as the input to the CU also exports.

My consumer unit is in my external meter cupboard and there is no spare space for installing extra kit (the generation meter had to be installed elsewhere). I don't want to speand a fortune, but I am willing to get a qualified electrician to make sensible changes where needed. I've so far had a couple of dubious ideas:

1. unscrew the consumer unit from the back-board and ease it forward in the hope of clamping all of the Live conductors (except solar) in a single clamp where they exit into the wall cavity. I have serious doubts over the possibility of this.
2. Use a double insulated 16mm or 25mm meter tie cable to make a loop outside of the consumer unit of the feed from the 100a isolator back to the non-RCD busbar and RCD for the remainder of the board. This would ideally mean moving the lights to the RCD or also splitting the lights from the solar
3. Install a small consumer unit between the grid meter and the current consumer unit - feed the solar in here and measure the consumption between this CU and the main household CU. This sounds to me like the obvious solution but the problem is that I don't think we could accomodate even an extra 2 way unit in the meter cupboard.

None of these sound like brilliant ideas, and I'm hoping that someone else can suggest a better solution, or even confirm any of mine are remotely plausible. Without being able to monitor my load I cannot start to maximise my benefits.

Thanks for reading.

 

[Ramjam]

Hi,

Good news and bad news...

Your system has been installed incorrectly. Under the terms of the MCS the systems has to be installed according to the DTI guide Photovoltaics in Buildings - Guide to the installation of PV systems (2nd edition) / Publications / Global Data / UK Home - Energy Saving Trust

Your inverter should be connected to a 30mA RCD as stated on page 16. This means that you have every right to call them back to put it right, any problems call REAL REAL Assurance Scheme

In the process of this get them to....

Break into the tails feeding your existing CU and add a second CU probably next to your generation meter by the sounds of it. Then you can clamp on meter onto the tails feeding your 'Consumption' consumer unit and another on the the new tails feeding your 'Generation' consumer unit. Then you can compare the two meters to optimise self consumption.

 

[BrummyGit]

Thanks for your reply.

They originally wired through the 30mA RCD however it constantly tripped.

The Fronius IG TL manual quotes the following relating to RCDs:

The design and function of the inverter provide a maximum level of safety
during both installation and operation.
The inverter provides operator and equipment protection through:
a) RCMU
b) monitoring the grid

RCMU = Residual Current Monitoring Unit
The inverter is equipped with an RCMU according to DIN VDE 0126-1-1.
It monitors residual currents from the solar module up to the inverter grid
connection and disconnects the inverter from the grid when an improper
residual current is detected.
Additional residual current protection may be needed depending on the
protection system of the installation or the requirements of the utility company.
In this case, use a residual current circuit breaker with a release
current of at least 100 mA.

I guess this means it will never work on a 30mA RCD?

The problem I have is that due to space restrictions, the generation meter is in an upstairs boiler cupboard probably 10 meters from the consumer unit and supply meter. I am thinking that I need to get a local electrician in to look at whether they can add another CU just for the solar - space is the big issue here.

 

[claypole]

Leaving aside your other issues what I'm proposing to do is this. Clamp monitor from generation meter tail + Clamp monitor from Import meter tail. For an instant indication of whether importing or exporting turn on a halogen light (or any high load) and if the import meter monitor reading increases you know you were (are importing) whilst if the import meter monitor decreases you know you were (are exporting). For recording purposes on a periodic basis, import meter monitor recorded total consumption minus import meter recorded consumption total = total exported for period. The only cleaner way that I'm aware of is to break into the tails and wire in series a monitor which will also record flow direction. An export meter would also of course give you a periodic record.

 

[Ramjam]

Do you have the requisite AC isolators? Lockable only in the off position, one by the consumer unit and one with the inverter?

 

[BrummyGit]

There are two AC isolators - a fully lockable unit installed with the generation meter in the upstairs cupboard (with warning stickers giving the location applied to the front of the consumer unit in the outside meter box) and an isolation switch next to the inverter in the loft. There is an accurate system schematic attached to the wall along with the AC isolator and generation meter.

There is also a DC isolator next to the inverter.

Emergency isolation instructions were also left with me.

I am not a qualified electrician, but have read the DTI guide and, assuming that the in-built residual current protection in the Fronius inverter satisfies the requirements for type B RCD protection, I cannot see anywhere that the system isn't fully compliant. The AC cables are surface mounted (external in plastic conduit) therefore I don't see the need for a type A RCD.

My original post was not intended to question the professionalism of the installers - in fact (whilst I accept that I am not qualified) I am very pleased with the installation quality. My problem is how to maximize the usefulness of the system for myself without disturbing the system which I do not own. This is my problem as the requirement was not part of the works agreed.

I've looked at 1-way CUs and non will fit in the cupboard. The simplest solution (but not the nicest) for me with such limited space in my meter cupboard would be to loop the switched live link from the 100a main switch in my consumer unit outside, clamp on the CT for the monitor and take it back in on it's way to the RCD. It wouldn't measure the lighting but as that's all low energy and not really used during daytime anyway I don't mind. If double insulated cable (eg meter tie cable) was used would it comply with regulations?

 

[Markc]

I think I need to question Ramjam's first reply here?

Why do you believe this should be fitted to a 30mA RCD from the original post? What am I missing?

I can not read anywhere where it states the inverter type (not until post #3 or has something been edited since the original post?)

Even taking the Fronius TL inverter into account it does not state a 30mA RCD in BS7671 or the DTI guide but it does state a Class B RCD.

Fronius TL inverters do nuisance trip 30mA RCD's so a way to comply with BS7671 (and if the AC cable is to be embedded at <50mm in wall would be to install a earthed metallic sheathed cable (SWA for example) If not embedded in wall then this reg isn't relevant.

The Class B RCD is required due to the lack of separation by the employment of an isolating transformer. Class B's I have seen are 300mA and not 30mA (please if you know of a 30mA Class B let me know)

Another requirement if a TL is used would be to earth bond the array. If TNCS system by the use of an earth electrode. If any other arrangement for example TNS then by bonding back to the MET terminal.

So reading back to the first post the OP states "one of the free "rent-a-roof" schemes" so I assume an On Roof System. I assume a inverter in the loft. I assume a AC cable run to a lower level of the property to connect into the consumer unit via a generation meter on the first floor. I can also assume a surface fixed cable, trunking or even clipped direct as it's a "free" system and knowing the installation company will want to maximise their investment by not offering chasing in their "free" price.

Obviously not knowing the full details of the installation it is very difficult to assume the installation is installed incorrectly nor correctly.

The manufactures literature (Fronius in this case) does state the unit contains a RCMU = Residual Current Monitoring Unit. This neither complies with BS7671 for RCD protection for AC cables embedded in walls at less than 50mm nor Class B RCD protection for prevention of direct injection DC voltage into the AC side. It is no more than a protection device for the internal components of the inverter as far as our regs relate. I questioned Fronius at the Eco Build event only last week and posted here about they claim Class B is not required.

I would suggest if there are any none compliances with the installation it is up to the client to contact the company and voice their concerns and if basic compliance with BS7671 has not been meet then there is a course of action open to them either direct with the company or their certification body (NICEIC, Nappit etc)

It's a shame we have not been able to help the OP by answering his original post.

 

[Markc]

There are two AC isolators - a fully lockable unit installed with the generation meter in the upstairs cupboard (with warning stickers giving the location applied to the front of the consumer unit in the outside meter box) and an isolation switch next to the inverter in the loft. There is an accurate system schematic attached to the wall along with the AC isolator and generation meter.

There is also a DC isolator next to the inverter.

Emergency isolation instructions were also left with me.

I am not a qualified electrician, but have read the DTI guide and, assuming that the in-built residual current protection in the Fronius inverter satisfies the requirements for type B RCD protection, I cannot see anywhere that the system isn't fully compliant. The AC cables are surface mounted (external in plastic conduit) therefore I don't see the need for a type A RCD.

My original post was not intended to question the professionalism of the installers - in fact (whilst I accept that I am not qualified) I am very pleased with the installation quality. My problem is how to maximize the usefulness of the system for myself without disturbing the system which I do not own. This is my problem as the requirement was not part of the works agreed.

I've looked at 1-way CUs and non will fit in the cupboard. The simplest solution (but not the nicest) for me with such limited space in my meter cupboard would be to loop the switched live link from the 100a main switch in my consumer unit outside, clamp on the CT for the monitor and take it back in on it's way to the RCD. It wouldn't measure the lighting but as that's all low energy and not really used during daytime anyway I don't mind. If double insulated cable (eg meter tie cable) was used would it comply with regulations?

I certainly would not recommend do this.

I gather from your posts you are trying to monitor your usage on your original circuits. This is going to prove problematic.

One option would be to fit single circuit metering (one to each circuit) and take regular meter readings of the usage on each circuit. This would not be a remote monitoring solution and regular visits to the consumer unit would be required. By metering each circuits use it would be irrelevant to know from where this is being generated from as it would be use.

Option two would be to fit X number remote monitors to each circuit (like you have excitingly done to your system) As the power usage will only be one way anything being used will register as usage and not generation.

Neither option is a cost effective solution but it is a saver one.

 

[claypole]

I thought OP original question was primarily this:

In order to really benefit from the system I need to be able to monitor my generation and compare that to my consumption at any point in time, and ideally I'd like to keep history so I see how I'm doing.

I can't see what's wrong with the simple idea of just using 2 remote monitors, 1 clamped to generated feed and 1 to grid feed. If the grid feed monitor reads close to zero you know you're on track. If the grid feed monitor gives a larger number you only need to very briefly adjust the load by switching on a light say, to establish whether you are currently exporting or importing energy. And for the historical record, grid feed monitor minus grid feed meter reading gives the total energy exported which you can then compare with your generation meter reading to give a % result. The lower this % figure the more successful you have been in benefitting from the system. To measure the total consumption of your home on a historical basis is then a simple sum. Generation meter reading minus export figure (as calculated above) plus grid meter (your original meter) reading = Home consumption total for the period.

 

[BrummyGit]

Thanks for the suggestions and also advice from Mark C.

In answer to Mark C's points - I do not see a fault with the installation, and as far as I can tell it complies with BS7676 as there is a single externally conduited cable run to up my outside wall, then surface mounted cables inside my loft, and surface mount plastic trunking inside the meter cupboard. I'm interested in the different opinions around class B RCD requirement between Fronius and yourself and might ask the installers for clarification.

The problem with the monitor on the grid feed is that I also want to record the data via my PC and an internet server and the data I collect cannot go negative and therefore gives invalid results - sadly Google Power Meter and similar software can't switch a light on and see which way the readings change.

It looks like getting an electrician in to lift the board off the wall and clamp the cables as they enter the cavity using as many (or few) as needed. My monitor will automatically add 3 together when operating in 3 phase mode so this may well solve my problem. There should be space inside the cavity for the clamps as there isn't in the CU.

Thanks

 

[claypole]

Sorry BrrummyGit, 1st part of your question appeared to be looking for instant reading at any point in time so you could turn on washer etc, so I figured you'd be there to do light test. 2nd part of question was for historical data which 2nd part of my answer would give you. If you're looking for historical data of each and every minute in time it's a bit tricky and your route is perhaps the way to go. Not sure how it helps you on a practcal level though since yesterday's weather will not necessarily be the same as tomorrow. Or to put it another way you might want to turn your washer on at 10am Monday, but on Wednesday 3pm might be the optimum time! Have fun.

 

[BrummyGit]

There's no apology needed - I appreciate all input. Indeed I currently do the light test myself, but the wife just sees this as a step too far and won't play.

I'm a bit sad/---- I guess, and would love to see the impact Solar has made on my power usage in graphical form. It also allows us to plan when we are most likely to have the most surplus power to set appliances such as dishwasher and washing machine on timer. Whilst I accept that today's weather isn't the same as yesterday, by using averages I should be able to get reasonable benefits while I'm out at work - previously these appliances ran exclusively on grid supplied power so any savings are good. When I'm home I will use instant meters and the windows in my house.

Your last sentence is key - I'm having fun, saving a little money (to spend it on energy monitors) and maybe gaining a "green credentials" badge.

 

[claypole]

I think we're very much on the same wavelength, and I can match your sadness and raise it - I haven't even got the darned panels on the roof yet and I'm already burning all this midnight oil on these thought processes. Lord knows what I'll be like next week when we're actually generating! Keep us posted if you get the consumption monitoring set up and working - I do like your thinking.

 

[Ramjam]

Firstly I'd like to apologise, I posted some misleading info, sorry!

MarkC you are completely correct. I always put inverters on RCD protected circuits a a matter of course, additional protection is becoming standard and that's how I work. I glanced through the DTI guide looking for confirmation that my methods were regulations rather than just my own practice and saw RCD and thought bingo. The Inverter doesn't need to be on an RCD protected circuit if the cables are surface mounted etc.

My preferred practice is to install a second DB cut into the main tails where I have an RCD main switch and a 16amp breaker. By doing this I don't need to go into an existing DB, many of which are old and ugly. Additionally it procides a clear and simple solution for customers like Brummygit who can then clamp an energy monitor onto the tails to the 'Consumer Unit' and if they want to another to the tails of the 'Generation Unit'.

Again it's a matter of personal preference but I think it's less than ideal to send electricity in two directions in one consumer unit. I know we have the stickers and it's common practice but I didn't yet meet an electrician who wasn't surprised when he first heard that you simply feed into an MCB in the main CU.

Given the cost cost of a PV system I think that if space permits that a separate generation DB is best practice and that's how I try to do my installs.

 

[claypole]

Gets my vote! Us sad "need a life" customers can then do anything we want with our clamp ons! I can't really comment on the actual problem of 2 way flow in the CU but my initial gut reaction when I realised that was how it was done was also "really?" When I was first looking at this with knowledge base zero I wondered if generated power and grid power could be T'd before the CU so that I could monitor consumption and was promptly put right when advised the PV must feed via a CB on the CU. Your method clearly satisfies from every angle.

 

[SolarCity]

Again it's a matter of personal preference but I think it's less than ideal to send electricity in two directions in one consumer unit. I know we have the stickers and it's common practice but I didn't yet meet an electrician who wasn't surprised when he first heard that you simply feed into an MCB in the main CU.

I'm afraid this is unavoidable. The CB will always have current flowing in both directions.

I think installing on a seperate consumer unit is a good idea but it won't avoid the problems that you describe.

 

[Markc]

Firstly I'd like to apologise, I posted some misleading info, sorry!

MarkC you are completely correct. I always put inverters on RCD protected circuits a a matter of course, additional protection is becoming standard and that's how I work. I glanced through the DTI guide looking for confirmation that my methods were regulations rather than just my own practice and saw RCD and thought bingo. The Inverter doesn't need to be on an RCD protected circuit if the cables are surface mounted etc.

My preferred practice is to install a second DB cut into the main tails where I have an RCD main switch and a 16amp breaker. By doing this I don't need to go into an existing DB, many of which are old and ugly. Additionally it procides a clear and simple solution for customers like Brummygit who can then clamp an energy monitor onto the tails to the 'Consumer Unit' and if they want to another to the tails of the 'Generation Unit'.

Again it's a matter of personal preference but I think it's less than ideal to send electricity in two directions in one consumer unit. I know we have the stickers and it's common practice but I didn't yet meet an electrician who wasn't surprised when he first heard that you simply feed into an MCB in the main CU.

Given the cost cost of a PV system I think that if space permits that a separate generation DB is best practice and that's how I try to do my installs.

And no one can knock you for committing to better practice over minimum requirements (BS7671). On the RCD front if you are fitting a 30mA RCD on a TL transformer you will still need a Class B RCD to comply with BS7671.

 

[Markc]

Thanks for the suggestions and also advice from Mark C.

In answer to Mark C's points - I do not see a fault with the installation, and as far as I can tell it complies with BS7676 as there is a single externally conduited cable run to up my outside wall, then surface mounted cables inside my loft, and surface mount plastic trunking inside the meter cupboard. I'm interested in the different opinions around class B RCD requirement between Fronius and yourself and might ask the installers for clarification.

The problem with the monitor on the grid feed is that I also want to record the data via my PC and an internet server and the data I collect cannot go negative and therefore gives invalid results - sadly Google Power Meter and similar software can't switch a light on and see which way the readings change.

It looks like getting an electrician in to lift the board off the wall and clamp the cables as they enter the cavity using as many (or few) as needed. My monitor will automatically add 3 together when operating in 3 phase mode so this may well solve my problem. There should be space inside the cavity for the clamps as there isn't in the CU.

Thanks

I can defiantly confirm the internal RCMU on the Fronius TL inverters do not in any way form protection on the issues being raised here. Disregard the need to have a 30mA RCD on this system and no AC cable is embedded in the wall. This type of RCD will be the one as fitted to your existing consumer unit.

The Class B RCD will be a standalone unit located adjacent to the consumer unit in a majority of cases. This deceive prevents DC voltage being injected into the AC system i.e. your sockets and lights and grid network and so protects you and your family. This unit is a regulation and must be fitted to comply with BS7671. If it is not there it is a none compliance. Further to this it is also a regulation to earth bond the array. This is done in two ways and is dependent on the earthing arrangement of your property. If you have as TNCS also called PME system a earth electrode (Stake) must be driven into the ground and a 10mm green and yellow earth cable taken form the stake to the array on the roof. The second option is if TNCS or PME is NOT at your property and you have an alternative arrangement TNS or TT then the same size and colour earth cable is again taken from the array on the roof to the main earth terminal within your consumer unit. One way to check if this is done would be to look in your loft and a majority of the time this earth cable would take the same route as the AC cable.

All the above is only a requirement if using a transformerless inverter (TL). A TL is more efficient to standard transformered inverters due to less losses. As your system is a free-rent a roof design the installation company will obviously want to maximise their return to investment. By employing a TL inverter this is achieved HOWEVER the costs of such sundrie items such as the Class B RCD can be considerable considering their size. Nether the less it is a requirement and is for protection.

Our aims here is to provide constructive and honest advice and as an accredited installer feel it important to to post honest and correct advice. I am not claiming your system is none compliant but it may be worth a look and check. I am sure the installation company will be more than willing to give advice and if need be corrective action to comply. After all they have already moved the installation to a none RCD side.

Apologies for hijacking your thread and good luck with trying to achieve what you originally asked.

 

[BrummyGit]

Hi Ramjam - thanks for your contributions which are gratefully received. I would also have preferred to have the meter tails split and a second CU installed exactly as you describe and would have been willing to pay for it, but without fitting a larger meter box I can't see how it could be done. To be honest my existing 200mm tall CU could do with splitting into dual RCDs but I couldn't even fit one of the newer tall CUs in the box.

Hi Mark C - thank you. I am sure the installers would take seriously any installation issue from my experience so far. I know an earth spike was fitted and cabled up as far as my inverter, however I am not sure that they earthed the array itself unless the DC cables also carry earth. Additionally I need to check if a type B RCD has been fitted as I'm pretty sure it hasn't.

Both - I agree that a type A RCD would have been desirable, and was initially happy when my inverter was wired through the RCD side of my CU. But the Fronius IG TL simply won't work on a 30ma type A RCD as it needs 100ma instead.

 

[Worcester]

I had a long chat today with the devlopment team at CurrentCost (Current Cost - Reducing your energy bills so you can live a greener life) and explain the challenge with Solar PV - they are working on a solution ... so watch this space.

Soon you'll be able to upload the output from the three monitors to THEIR website, (it'll need the EnviR) and they don't believe that any changes need to be done. If they do it'll be the bridge and they said it would just need reprogamming (probably got linux on a chip) and they would offering free reprograming on a swap in / swap out basis.

I am working with them to produce a specific package for Solar Installations. We'll see how it goes.