andyscotland

@Wolfman310 ah that is a shame!

I'd read it to mean that the 100A RCD and heater RCBOs were all in the same board, so swapping for a 100A MCB wouldn't affect cable protection on the incomer (there might of course need to be something at whatever upstream supplies the heating DB as you say, depending on the cable). The more likely complication would be if it's a split load board, or a board manufactured and supplied with an RCD main switch, as that can then get into issues with modifying a tested assembly. Like you say, lots of variables easiest assessed with the electrician's eye on site - hopefully not followed by the electrician's sucking breath through teeth 🤣

@Redbeard thanks, I'll try SIG. Scotland is probably achievable for me to collect myself (I have a Citroen Berlingo which is pretty good for volume) - certainly more achievable than Nottingham!

Good point, I'd misread that as an RCBO. Yes if it's an RCD it will need a 100A overcurrent protective device in series though I think that could be upstream or downstream. To be fair if the individual circuits are on RCBO and are the only thing in the CU then you could (and perhaps should for selectivity) just swap the 100A RCD for an MCB which would also solve that issue.

@Wolfman310 could I ask how you got the beads to you? Polypearl have been very helpful but they can't do delivery or palletising as they only deliver bulk in their own transport. So it has to be collection from Nottingham as individual 10ft³ bags. Unfortunately due to the size of the bags I'm getting fairly ludicrous quotes for couriers to collect and bring to Edinburgh : 10 bags is ~£1,200 from the parcel courier comparison sites. Man & van sites are coming in around ~£300-£400 - better, but still fairly prohibitive... 😬

"earth fault loop impedance" is the technical term for "wire impedance" - e.g. (in very simplified terms) the resistance of the loop of cable from the live on the generator at the power station to the end of the circuit and back through the earth to the generator. This resistance/impedance determines the amount of current that can flow through a dead short from live to earth. The Earth Fault Loop Impedance is therefore one of the factors governing the size of the protective device : as you say it has to be low enough that a dead short will allow enough current to flow that the breaker will trip within the required disconnection time for the type of circuit. Hence as you have correctly understood, the electrician is saying he needs to measure the earth fault loop impedance at the far end of each circuit before he can confirm that it's appropriate to change the overcurrent protective device (as the limit for a 20A breaker is lower than that for a 16A breaker, and indeed the limit for a type C is lower than for a type B). This does sound like it is due to the cold resistance (and therefore inrush current) of the circuit. If there is a single heater per circuit then there's not a lot you can do other than change the type/rating of the protective device or put an inrush limiter in front of the heater. The ratings of the protective devices do not of themselves necessarily equal the maximum demand of the installation. BS7671 does not provide any mandatory method for calculating maximum demand : it is up to the designer to use their expertise to make a reasonable assessment. If you have 6 x 3kW heaters on fixed wiring with no other appliances connected (and no sockets in the circuits) then your maximum demand is 18kW = 78 Amps at UK nominal voltage (between 71-83 amps on a supply within the voltage tolerance range). This is without applying any diversity, which probably wouldn't be appropriate in this situation. Changing the protective devices doesn't in any way change the maximum demand of the heaters. Therefore, it's entirely reasonable to size the protective device for the demand, current carrying capacity and loop impedance of each individual circuit. It is not automatically a problem to have breakers adding up to 120A downstream of an overall 100A breaker. Of course, you may find that if you make each individual breaker big enough to carry the inrush current, you move the problem upstream and end up with the combined inrush now tripping the 100A protective device. This however could be managed by adding control gear to switch one or two heaters slightly on after the others. 2.5mm² T&E is rated for 23A in conduit/trunking on a wall surface, 27A clipped direct to the surface. If the cable is buried / passes through or under insulation at any point, or is bunched with others / at a nonstandard ambient temperature this will reduce those ratings. So from that perspective a 20A RCBO would likely be fine for the current carrying capacity of the cable.

The big challenge would likely be demonstrating that the VAT reclaim on the materials was genuinely for the purpose of trade. That would imply that you would need an arms length transaction on commercial terms for the installation. The claim that it was a show home / marketing expenditure would be very hard to evidence if you did not subsequently end up making any sales to anyone else. You might be able to argue that a discounted rate was justified, but that would almost certainly need to include at least some labour costs as well as cost of raw materials. You then have the challenge that the labour fees are in the company and can't get out of there without being taxed on some level. Is probably the wrong question, because the boot is largely on HMRC's foot. With most tax things, it's usually on you to prove you've done things right rather than on them to prove you haven't. The right question is therefore likely "If HMRC query my VAT reclaim, can I produce strong enough evidence to convince them I am genuinely intending to trade, and how much time / professional advice might I require to win that argument?". If HMRC think it "smells a bit off" then the likely outcome is they raise an assessment and challenge you to appeal that at Tribunal if you think they're wrong. It's very unlikely to be cost effective to go down that route.

@joe90 is correct that the special self-build VAT reclaim is only for standalone homes - it wouldn't cover a garden office or any property that can't be sold as a separate home (e.g. it wouldn't cover a "granny flat" in the garden either). If it was for a VAT registered business it might be possible to claim the VAT on the VAT return if the building was built entirely for business purposes, and will be used entirely for business purposes for a long period (10 years, I think). However this would require clear evidence and might also mean there is Capital Gains Tax to pay if/when the house is eventually sold. There are many pros, cons and complexities so this should all be discussed with an accountant. Technically, there is no top rate if you can demonstrate a reasonable calculation from shares of the utility bills and similar costs according to the portion of time used for business. However if you claim more than the HMRC flat rates (which they offer as a concession to simplify life for taxpayers and themselves) you can expect them to be interested in how you got your figures. It's not often worth going above their rates as by the time you have the hassle of calculating a fair figure you probably won't make much. However for a garden office it might be if t was used almost exclusively for business (but make sure you show some private/domestic use to avoid Capital Gains Tax) and e.g. has standalone electric heating that you can easily get a usage figure for.

I guess maybe @SteamyTea was thinking regs might be different, but this is just BS7671 which applies UK wide (not France though 🤣). I'm not sure why the electrician didn't like Wagos if they were in a proper containment (other than that some people don't like new things). When installed per manufacturer's instructions in a wagobox they are certified for use even in a totally inaccessible location (unlike anything with screw terminals). And are very hard to get wrong (again unlike anything with screw terminals). They do need to be in a proper box to provide protection for the unsheathed cables and strain relief, among other things, and the cable tie to hold the box closed is also important. I would definitely use a wagobox to repair that damaged section, ultimately even if it's not the cause - and it could well be - it needs to be repaired anyway. Hard to see from the photos how far it is to the junction box, if it's not too far best/cheapest bet is probably just one wagobox and then replace the whole section to the junction box. Alternatively get 2 boxes and just splice in a short replacement section. Make sure the cable you use matches or exceeds the size of the existing (I'd assume 1.5mm² will be fine, but check). Unless you have a spare bit of suitable old cable lying around for the repair you are going to have different core colours in the replacement section. Old red = Brown, Old black = Blue. You should therefore also have a label like this at the origin (you can make your own) - not that there's really any electricians out there who aren't well aware of the harmonised colours by now! Technically speaking as this is a repair to the fixed wiring it is Minor Electrical Works and should be done by a competent person, properly tested (with calibrated test equipment) & documented.

Ah, I'd misunderstood the original pic. So I assume the circuit that feeds the suspect light is also on the left hand half of the board? That would make sense. Top one would be fairly unlikely to be causing this type of fault (but should obviously still be fixed, for safety). Bottom one would be a candidate as there is a copper pipe nearby and if the exposed neutral contacted that (or was bridged to it by moisture) that would be exactly the type of trip you're getting. Worth checking, but equally if we have now established that's on the other half of the board to the RCD that tripped then it is probably not the culprit in this case.

30 milliAmps I assume. The thing is the RCD is across both the sockets and the lights so that would not necessarily matter. But if still tripping then sounds like that's not a factor. The red switch is just a switch so it won't have been that. If it was caused by the light it should have been the RCD (the black one with a test button) on the right of the board e.g. furthest from the red switch as that is the one that protects the half of the board with your lighting circuit breaker (the one that is switched off). It is possible for 2 RCDs to trip simultaneously - the tripping current and time are both so low that they trip easily and fast. Opening the circuit is a mechanical action so it's rapid but not instant. So the downstream can detect a fault and start opening but the power remains connected for enough milliseconds that the upstream also sees the fault. Ideally an installation would be designed to reduce the risk of this happening but it's not always possible (and even with the best designs can't be guaranteed in all circumstances). Depending how your lighting is wired, I would also check any cable from the suspect lamp to the next fitting. And also look out for any junction boxes nearby and similarly check them for dead things/damp/visible damage. Make sure you de-energise the circuit at the distribution board before you start poking around, especially as there is a chance of damaged/wet connections. And if there is anyone else home who might be tempted to turn circuit breakers on, tell them what you're doing.

@saveasteading just to be sure I've understood: the box marked DB3 that @G and J has marked up is at the origin of your supply, and the power is fed from there to the multi-way split load board in your final photo? And the trip that is going is the one G&J has circled red, knocking out the whole system? As @G and J says, that one is an RCD and will trip on a very small imbalance between the current going out on the phase (live) conductor and coming back on the neutral. It's not possible to see in the photo what rating that is, it will likely either be 30mA or 100mA. The one you have marked/switched off in the downstream board is an MCB and will only fail on overload (likely 6A if it's a standard lighting circuit). Therefore you have a fault that is causing a very small amount of current leakage. This means either a high-resistance short between phase and earth (e.g. a mouse body, or degraded insulation) or possibly a dead short between neutral and earth : the neutral is not quite at zero volts so connecting to earth will cause a very small current to flow. Assuming you haven't recently made any changes to the system, and have tried removing/replacing the lamp itself then the most likely explanation is either recent cable damage, moisture ingress to a connection point, or failure of a connected electronic circuit (which sounds unlikely if you have isolated it to only happening when a specific light is switched on even with the lamp removed). There could be a couple of reasons why it is tripping at the origin rather than at the RCD protecting the right hand half of your downstream board: * Possibly, both RCDs are 30mA and the same type. You would generally try to avoid wiring RCDs in series like that because there is no guarantee which will trip first on a fault. Usually I would either try to run cable between boards in a way that means it doesn't need an upstream RCD (eg using steel wire armoured cable) or use a 100mA time delay RCD upstream to increase the chances that the downstream RCD trips first. However it is not always possible/allowable to do this and so you get the nuisance that sometimes a fault will trip the whole installation. * It may also be that the lighting circuit itself is only producing a very low leakage but there is leakage elsewhere in the installation which means when it all adds up at the origin it is just enough to trip the RCD. It's common for electronics (computers, LED drivers, smart stuff) to produce a small level of background leakage due to the way electronic transformers work. In fact it is even possible that the lighting circuit has always had some cable damage causing leakage but only recently have you connected more electronics elsewhere and so it has now become a problem. Ultimately the only thing you can really do is get a proper insulation resistance tester and try first to isolate whether there's any part of the system with a lower-than-expected live / neutral resistance to earth. You will need to be very careful to disconnect absolutely everything electronic before testing to avoid damage.

It actually did pretty well at that until it got to the certificates question, where it is completely wrong (but to be fair a high number of humans including accountants also get this wrong). For new builds and conversions to "dwellings" no certificate is required nor should one be issued (VAT 708 Section 17 and the definitions in 14.2-14.5) In those cases HMRC expect the builder to know what they are building/supplying and how that type of work should be rated for VAT - in much the same way they expect a shop to know whether they are selling biscuits or cakes. The certificates are for cases where the building would not normally be zero / reduced rated, but the customer is entitled to a lower rate because they are a charity or are building a care home / hospice / etc. In that case it's for the customer to prove to the supplier that they are eligible and the certificate is part of that.

You've probably solved this now but thought I'd add a couple of details for anyone that finds it in future. First, I'd echo @Mr Punter's recommendation that you should discuss this with your spark - once you start departing from the standard circuits, cable sizing & de-rating can be complex and depends on a lot of factors (e.g. supply earth arrangements, circuit lengths, protective device selection) so the right answer will depend on the specifics of your installation. The starting point in BS7671 is 523.9 which says that cables "shall, wherever practicable, be fixed in a position such that it will not be covered by the thermal insulation". So it will usually be better - for any loading of cable - to use a service void, or notch studs rather than drilling the centres. If that's not possible for structural / mechanical protection reasons then your suggestion of clipping the cable out to the edge of the stud, across the face of the insulation, and then back in to the hole in the next stud is the next best. That does assume you can safely extend the length of the circuits without causing knock-on issues for things like voltage drop or loop impedance. If you can achieve that, that gets you into the territory of installation method 102 for T&E cable in a stud wall where the cable is touching the wall surface (though there are some caveats depending on the exact materials). Table 4D5 gives the current carrying capacity for those methods. That allows 13A for 1mm², so a 6A lighting circuit should be fine. For 2.5mm², the rated current in the table is 21A - assuming the ring you mention is a standard 32A socket ring final, regulation 433.1.204 states that the cables must have a current carrying capacity of not less than 20A so again that's just OK. So long as the loops into the stud centre and back out are touching the stud, and are short, I would not count them as being "totally surrounded by thermal insulation" and therefore they don't require additional de-rating. I don't think 7671 actually differentiates between an interior or exterior wall surface, but working from first principles I'd say if you can take the cable along the outside wall (especially if you can clip them to the OSB) that would give you more of a safety margin as both the stud and the surface will obviously be colder than the inside wall, giving the cables more chance to dissipate any heat that builds up on the short loops into the stud centres. If you can't go in and out, and have to run through the centre of the insulation, then you would be looking at installation method 103 - T&E in a stud wall not touching the surface. Table 4D5 gives 8A for 1mm² so the lighting would be OK, but 2.5mm² can only handle 13.5A in that setup and you would have to go to 6mm² to meet the 20A requirement for a 32A standard ring final. At that point I would probably look at alternate circuit designs / protective device sizing as although theoretically you can get 2 x 6mm² into most socket terminals it is a real pain. If at any point the cable is totally surrounded by insulation then you would need to de-rate for that - by 50% if it's surrounded for more than 0.5m, or on a sliding scale for shorter sections. Again, this is really intended for background information to assist the discussion with your spark.

When you isolate individual circuits, you are only isolating the phase (live) conductor. The neutral remains connected. If other circuits in the installation are energised, there will be current flowing in the neutral. This, combined with the resistance of the neutral, means that the neutral will have a small voltage difference to earth/the neutral at the origin. Normally that doesn't matter because the current can still only flow along the neutral and through the RCD. However if you touch neutral and earth together (e.g. when cutting a cable) this creates a parallel path which will allow some current to flow through the circuit you have created. Since this bypasses the RCD, it causes an imbalance and causes it to trip. Time delayed RCDs - like all RCDs - still display shorter tripping times as the current increases above the rated value. For example a 100mA S type RCD at 500mA is allowed to trip in 40ms - a lot less than the 130ms minimum with a 100mA current. If you had a phase to earth dead short then the current imbalance was likely into the hundreds of amps, and therefore well outside the parameters the time delay was specified/designed/tested for, so all bets are off. The time delay is really for situations where e.g. a human is in the loop, rather than cable shorts.