Solar pv batteries distance to CU

[binky]

Might do better looking for a 300mA, there's a bit in fire protection for cattle barns that says we should install 300mA , so a little more common. It basically means the inverter you have is a leaky ****** that upsets standard 30mA RCDs. I've never understood how the hell DC could get to the AC side, plus most inverters have their own RCD detection built in. 

IMHO, it's akin to saying you don't really need an RCD on the cct, but if you have to put one in to meet BS7671 requiements for eg a TT earth system, it needs to be insensitive. 

https://www.proactivetechnicaltraining.co.uk/rcds-different-t

CEF do a 100mA unit Type A and a300mA for 3 phase

https://www.cef.co.uk/catalogue/products/4833584-100a-2-pole-100ma-type-a-rcd?gclid=Cj0KCQjw4ImEBhDFARIsAGOTMj810Q45_HJFvsOdu8o8p-i7nmtl83kZ708eLm-jllU0bVJmJc8mGu8aAp5WEALw_wcB&gclsrc=aw.ds

https://www.cef.co.uk/catalogue/products/4833618-100a-4-pole-300ma-type-a-rcd?gclid=Cj0KCQjw4ImEBhDFARIsAGOTMj9v0uEEKPUpjsaZPYULIM6av4HMa-EGcE-EE_qQ0GhVGrlF3QrVObQaAsETEALw_wcB&gclsrc=aw.ds

 

[revor]

Thanks Binky for coming back with reply so quick and much appreciate the links. From what have been told the inverter is well protected on the DC side but seems that there is a lot going on within the inverter on start up which could trip a 30 mA. So a 100mA type A with a 32A MCB would be OK? but what trip curve should the MCB be? . I wondered whether it could be done this way with not being able to find a suitable  RCBO.  From what you are saying and in the light of  rereading  the comments in the manual it looks like I may not need an RCD after all there is no BS 7671 requirement I can make out, we are on  TN-c-s . It seems common practice by installers around here to power inverters from spare way on a CU  and  I guess are on a 30 mA RCD. I do find the fact of using letters to signify differences in RCD types and also letters to distinguish MCB types rather confusing particularly when they  are combined in the same sentence /paragraph it just makes the communication a bit more ambiguous.

 

[binky]

you could use RCD as the 'main switch' and add suitable MCB for the cct. Can't remember if this is single or 3 phase? If it's single phase you should be abe to find an RCBO.

One thing not to do, is share any RCD in the house with the solar system, give the solar it's own RCD protection. If I remember rightly your inverter is in a shed next to the house, so you are coming off the board, through a wall and into shed? In which case , if you have run cable in a suitable way (ie not plastered into a wall at less than 50mm deep ) then as this is fixed equipment and you can't plug a lawn mower in, you don't need RCD at all. 

 

[revor]

Thanks again Binky hope you are enjoying the sunshine. To recap on the install the inverter (1P) was going to be in a purpose built shed  70 m from the house but when I got hands on the manual for the inverter had to put it in an external WC room as the voltage drop had to be 1% or less which just managed with15 mts 4 core SWA 6MM. The string isolators are in the shed that was originally intended for the inverter and battery.  The 2 strings go via 4 core SWA 6mm to the inverter in the ext WC.  The inverter is in a better location now as building is heated and of insulated block construction.  Also better for the battery as low temps will affect it's life. There is a separate power supply to the building supplying a small CU powering light and small heater.   The SWA to the inverter is underground in a duct it is all connected up apart from the grid connection to it. A local electrician was meant to call me on 4 occasions ( I live in manyana land) so now decided to finish off the job myself.  As I wired up the house during refurb and have had the pre inspection I am due for a final visit for part P certification in due course. (Does solar PV fall within Part P?) It means I will not get  MCS certified but worked out would need to sell 7000 Kwh just to get the registration fee back anyway.

A further thought I had is that I have spare ways in one of the CU and could shift the MCB's and RCD's along one way and put the non RCD 32 A MCB next to the main switch. But what trip type of MCB should be used ?.

I have searched for a suitable RCBO but to no avail and have emailed my local CEF branch to see if there is one.

As you suggest could put a 100mA RCD " main switch" and a MCB in an enclosure but having had the earlier thought from you and read the relevant paragraph in the manual can't now see a need for the inverter to be on an RCD.

 

[binky]

Ok so you have an armoured supply to inverter in WC, so in my opinion, you don't need a RCD for that cct. An RCD won't be doing anything very useful for you anyway. Just use a standard Type B MCB. If you want to install  a mini board with 100mA RCd main switch, that's fine also - works as an isolator for the system in case anyone needs to work on the house CU or meter. You should have an isolator for the solar next to main board. 

Part P doesn't cover solar, but getting MCS registered is a waste of time and money. There is no legal requirement yo be MCS registered to install solar, it was only the Feed in Tariff that drove that requirement, and that no longer exists. It will cost you MCS fees, plus fees for OFGEM and REAL, plus you will have to produce a small mountain of paperwork (if you have ever worked with ISO 9000 or Military Standard stuff, it's along those lines) the easy answer to which is to buy an off the shelf paperwork package at around £500, plus you need extra insurance. So you will be talking of around £2k, and you get nothing from it as there is no Feed in Tariff. I dropped MCS a few years ago, just couldn't justify the expense. You will have to register system with your local DNO for which you will need to produce a schematic diagram of the system (examples can be found online), and they will want to see the G99 / 100 cert for the inverter. 

 

[revor]

Thanks Binky will do it without an RCD. I had already put in an isolator for the inverter in the house plant room. I have also put in a changeover switch for the CU which is powered by the battery in the event of a grid outage because if the inverter goes down then no power goes to this privileged loads CU so the changeover means I can put the CU onto the grid if inverter goed down. The inverter powers this CU either via battery (if no grid) or via the grid but the load is limited to 20 A off the battery and 32 A on grid. I will put the lighter demands on this CU lights, heating ,MVHR, freezer and telly and 1 ring.

I have already had the OK from the DNO, a local installer has been extremely  helpful in doing the application for me and were going to do the MCS for me if their electrician got involved, who is not on their books, but contracts to them when he is needed for connecting up etc. As that has not happened in all probability now that won't happen and as previously mentioned the economics is not there.

I had not really wanted to do all the work myself as have a build to finish but no one was really interested in installing my choice of gear. Think it was out of their comfort zone not having heard of the inverter or battery or maybe just too many unknowns.

 

[binky]

I had not really wanted to do all the work myself as have a build to finish but no one was really interested in installing my choice of gear. Think it was out of their comfort zone not having heard of the inverter or battery or maybe just too many unknowns.

We are a little allergic to installing customer supplied gear . It's a mixture of lack of familiarity,  customers tend to buy cheap and nasty, and not having support form wholesalers in the event of a problem. In short we tend to get the blame from some people for rubbish they have bought!  :^O .

Sounds like a good set-up. I've never tried using the UPS facilities for the event of a grid fail - I like to keep things simple and it's not so easy to achieve on retrofit buildings. I would be very interested to know how you get on wth that facility. 

 

[revor]

I can quite understand a trade person not wanting to use customer sourced items but in my build which I managed myself with individual trades I was asked to source all the materials. It seems mainly to do with keeping them under the VAT registration level so all they had to account for was their labour. The only exception was a specialist rendering company.  I appreciate that electrical work has a different outcome if you get it wrong and the materials used, play a significant part in this.  Many electrical items do not have county of origin and that concerns me as well as others no doubt. Cable and Wago copies are examples of past issues. If it does not declare origin I assume it is from China. I had only sourced the panels (Q cells) and  identified the other components so that they could buy them themselves at whatever mark up / discount they could get. My objective was to have non China made items which was very difficult . The last installer told me to get the items and I could use the electrician they used and would then do me a MCS ( for a fee) if he did the work. Seems the company do the installs civils etc and electrician comes along and connects up. I think that is what seems to be the case with smaller solar PV organisation but I do not really know.  Certainly 2 I know of. In my quest for an installer I was quite appalled at the quality of work I was shown as examples of what they could do and the materials they used. My first approach was to a well known national company that got all the spec wrong because they had basically copied and pasted another job quote into mine and had even copied in the quote my provided details.  My job has worked out costing quite a bit more than had I gone for a local fully installed Sola X system. Basically cost of components. 

I will keep you posted  how it all goes. It is not a "proper" UPS system as there is a 30 sec delay before switch over to battery and as we have more v short outages ( a few seconds) w.r.t long outages perhaps that is not a bad thing.

 

[binky]

 The last installer told me to get the items and I could use the electrician they used and would then do me a MCS ( for a fee) if he did the work. Seems the company do the installs civils etc and electrician comes along and connects up.

Most companies use subbies these days. It's not often you find a company with more than office staff employed on proper contracts. It's part of the reason there's very few apprenticeships available for youngsters, most of us have gone self employed working as 1 / 2 man bands, and taking on an apprentice is a pain n the neck due to lack of incentives, employment legislation and all the other hoops you have to jump through to have an apprentice. It's easier not to bother and crack the work out / earn money as an apprentice skows you down for about the first year. I'm a one man band electrician /solar installer ( I used to have 6 staff), I drag in a friend when I need help and get him to do the electrics whilst I work on the roof, then he helps me lug the panels up and into position and 'job's a good un' 

 

[revor]

I guess the greatest issue with employing an apprentice is H& S you would need to be confident they are not going to do something unsafe and electrocute themselves or someone else which means standing over them so slows you down. The company who came to do my  inspection have about 40 staff many of course will be office based and would probably be the size of company that could afford to have  apprentices. I reckon they will have been hit hard by the pandemic as they did lots of electrical provisions for outdoor events .

I mentioned earlier that the privilege loads  CU will be provided with an alternative feed via a changeover switch if the inverter goes down so there is a grid supply to those elected loads on that CU. I was going to take the supply from the Henley block via a 2 pole 32A MCB in an enclosure except can't get one locally.   I do need a breaker in the supply as it is coming via 6 mm tails so need to protect them. I have a spare way on another CU which I could take supply from but that mean that the privilege loads CU will have its own RCD and that of the supplying CU.  Have often seen debates about having such an arrangement but wondered what your take is on this. I am expecting not to have to use this arrangement as it is only a emergency backup in event of inverter failure or maintenance.

 

[binky]

sounds reasonable to me. I'm just a little surprised you managed to get 6mm meter tails - not seen those for years, and they confuse the hell out of other electricians. I would be inclined to fit 16mm to save arguments  :^O .

First year of any apprenticeship is teaching basic use of tools. Simple stuff like how to drill holes and fit rawl plugs - so many oarents are hands-off these days, and schools no longer teach handskills, so youngsters are basically clueless. It's this that slows you down. Teaching safe isolation proedures is basic from day 1, but you have to also show the training college a paper trial of non sense like 'equal opportunites policies', 'complaints procedures' etc etc, which as a busy 1 man band, is time consuming. 

 

[revor]

Thanks Binky. I'll take the supply from the other CU.  Yes students  are poorly equipped on practical work not how it was when I was at school. For a time I taught wood skills to young adults and was very surprised that some could not use a ruler or do simple sums e.g. divide a piece of wood into 2 equal lengths. Too much emphasis on academic subjects but a lot of kids are not able or willing to learn but have other skills to offer on the practical side of things but no longer get the opportunity to develop them, as where are the woodwork and metalwork classes now? They get bored and are disruptive to others.

https://www.edwardes.co.uk/categories/6-0mm-6181y-double-insulated-cable

Edwardes have a very good range of cut to length cables. Only recently used them. Got quite a bit of stuff from them for the install. Have found our local CEF very poorly stocked and what says on their store stock check is inaccurate but I think their web site is the best of the lot so easy to drill down to what you want, so get most from CEF  but I usually buy on line, click and collect or get delivered.

 

[Murdoch]

I don't know you could still get 6mm tails. I use 16 or 25 as those are the ones I always have.

Beware the inspector who thinks that tails must be 25mm or it's a C2

 

[revor]

25 mm tails are needed because  the main incoming fuse is 80 or 100 amps so 25 mm has rating of 90 to about 110 amps depending on fix method.  The onsite guide states minimum of 25 mm.  Anything less of a cable could not take the current and would overheat  and possibly blow before the company fuse went. So in this situation you would not use 16 mm tails on a 100  A fuse. So in what situation do you use 16 mm tails.?

All my main CU's  of which there are 3 are 25 mm tails (reference c fixed) because they are all connected via a Henley block  and through the meter to the incoming company fuse.

The inverter max output is 32 amps so using 6 mm cable is consistent with this (46 A method c.) The inverter feeds the privileged loads consumer unit (number 4) via grid / battery the consumer unit tails is protected by the inverter feed 32 AMP MCB and the  changeover inverter feed in event of inverter being down is via a different 32 A MCB. 

 

[Murdoch]

So in what situation do you use 16 mm tails.?

60a or 80a DNO fuse

OR 

a 2nd CU with a small load - say a hot tub or similar when 10mm or even 6mm would be more than adequate.

As for the OSG ......... oh dear.

 

[revor]

 So no real difference to my 6mm  tails on a 32 A MCB providing power to the small load CU inverter privileged ?  

 

[binky]

 So no real difference to my 6mm  tails on a 32 A MCB providing power to the small load CU inverter privileged ?  

Within the regs somewhere is a statement that suitable overload protection can be provided at either end of a cable, so the fact you have a single fixed load cct on a 32A MCB is adeqaute to protect the tails. 

 

[revor]

You asked earlier to let you know how the project is working out. Overall it is doing very well. I am achieving anything between 96% and 100% self sufficiency that is less than 4% coming from grid. ( Month ave 98%) I loose out every 20 days or so as battery goes into maintenance mode and is not available so reliant then on PV and grid. I have not tested out yet the changeover to battery only in event of power cut, been on other things. I am waiting a firmware upgrade which is promised to give some better efficiency and iron out some glitches but I am not noticing any issues. What does not get used either via battery or PV goes to grid for 5.5p Kwh which for the moment is more than covering the daily charge except on the dullest of rainy days and gives me some money back instead of making payment. Winter will no doubt change that. We are not a big user no more than 10Kwh per day. We aim to get battery 100% full by evening, overnight and evening battery goes down to about 70% to power the freezer and fridge ,which this hot weather  is more than usual.  A cooked breakfast will take it down further to say 60% given a good day will be 100 % by lunchtime. late am will use dishwasher washing machines etc to make use instead of going to grid. What I am very impressed with is how it handles a 9Kw shower. It will take up to about 5Kw from the battery and rest from grid and PV. This PM the shower was in use and did not take anything from grid. We would not use 2 big loads at same time obviously as battery max's out at 5Kw. Have noticed if you put a big load on it takes from the grid for a split second or so whilst system decide where the mix is coming from. In using the hob or shower say, we will put on low and ramp up that minimises spikes which all add up at end of day to use grid, One might think we get free juice from battery but there is an investment cost involved in it and provided no performance issues over its life it is calculated that each unit is about 12p but off course that is fixed and with prices going up at about 10% pa is cost effective.

 

[binky]

@revor thanks for the feedback, it's always interesting to know how people are getting on with their installations. Inflation busting energy rises has always beem a selling point for PV, but never officially allowed to be quoted in any figures. 

 

[revor]

Back again with further query. Trying to understand what I am seeing on the inverter display. Have 2 strings in parallel string 2 in some circumstances shows 0.1 amp lower reading than string one e.g  MPPT 1 10A MPPT 2 9.9 never the other way around. MPPT 2 occasionally showing on the inverter as 0 A and is at this for 10 secs at a time. Not sure if inverter is doing some form of control or the problem is with the string output. Next sunny opportunity will investigate. The other observation it that seems to do it when there is a lot of activity around charging of the battery as it gets 80% full it discharges and charges I think to even out the cells.

Any testing I can do to determine what might be going on.