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Bad electrical design

Discussion in 'Electrics UK' started by Jon c, 10 May 2018.

  1. Jon c

    Jon c

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    Hi

    I’ve got a new build house (2006), which I’ve moved into recently. It has a 16th edition Wylex split load board, and currently the 2.5mm SWA supplying the garage is on a B16 MCB, on the RCD side of the board. At the garage end, there is a garage consumer unit, with an RCD, and B6 MCB for the lights, and B16 MCB for the sockets. Both RCDs are the same rating:30mA. The house has a PME earth, which has been exported to the garage. The distance from the house to the garage is about 30m.

    Clearly this is a poor setup, and worryingly it seems to be original from when the house was built.

    I’d like to do something about this, as there is no discrimination between the house and garage at all. I’m thinking:
    1) Supply the garage from the non-RCD side of the board, however there are no spare ways so I’m thinking I can put the burglar alarm and doorbell onto the same B6 MCB (currently they are separate).
    2) Upgrade the MCB at the house end to a B20, so there will be some discrimination.

    So the new setup would be 2.5mm SWA supplied from non RCD side, on a B20 MCB. Garage end would stay the same.
    I would like to upgrade the size of the SWA as well, but it runs under a communal parking space, and there is no route a new cable could run without digging up some of the communal parking space, so unfortunately it’s not viable in the foreseeable future.

    How does my plan sound?
    Thanks for any replies
     
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  3. JohnW2

    JohnW2

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    That sounds fair enough - and would remove the redundancy (and potential inconvenience) of having two RCDs in series for the garage supply (and the risk that a fault in the garage would take out all RCD-protected circuits in the house.
    That will give you little, if any, discrimination. The B16 in the garage is really redundant (i.e. not necessary) whether the main CU's MCB is a B16 or a B20. You really need a ratio of MCB ratings of at least 2:1 (e.g. B16 and B32) to stand much chance of getting the expected discrimination, and even at 2:1 it's far from guaranteed.

    Kind Regards, John
     
  4. John D v2.0

    John D v2.0

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    not sure about poor, it's not ideal, and not necessary to RCD protect properly terminated SWA, but for a garage maybe that's not the end of the world.
    Regarding exporting the PME, if you don't need to bond anything in the garage, it wouldn't be a problem. If you did, you'd need a 10mm bonding cable which would be prohibitive over that distance. The same issues about using earthed equipment outside would apply as to the main house.
    Regarding double RCDs that's just pointless as they'd both trip. But you need to vary the delta I rating and the time delay if you want discrimination. Moving it to the other side would make sense.
    You would need to measure loop impedence and volt drop if you want to upgrade to 20a at the house, and it's not necessary. Nothing bad will happen if you have two MCBs with the same rating in line. Why do you need a bigger cable? Are you wanting to run bigger loads at the garage?
     
  5. Jon c

    Jon c

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    Ok I will go ahead and move the supply to the non RCD side, but just leave it with the B16.
    I would prefer a larger cable because I would like to run a welder, and have better heating in the winter. Also I have noticed that the lights temporarily dim when I use my mitre saw or other high power tool, so assuming this is due to volt drop, a larger cable should fix this.
     
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  6. flameport

    flameport

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    The existing 2.5mm is already too small for voltage drop on a 16A circuit when supplying lighting. It complies for non-lighting loads.
    Changing to 20A with the same cable is still non-compliant, and 20A is marginal for volt drop for non-lighting as well.
     
  7. Jon c

    Jon c

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    It’s probably too late to talk to the developers about that non-compliance:rolleyes:. If I ever get a chance I will replace to cable, but there’s not much I can do about the cable at the moment.
     
  8. john4703

    john4703

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    How long is the length of SWA under the communal parking space? If you have good access at both ends and it is short you might by able to pull out the existing SWA and pull in a new bigger cable in one go. The existing cable would be at a suitable depth and have marker tape left from the existing cable as the new one would be in the same place.
     
  9. Jon c

    Jon c

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    Unfortunately most of the SWA is under the communal parking lot, about 15m, so not much chance of pulling a new cable through. There is also an alarm cable running to the garage with the SWA, so I checked if there was a conduit running to the garage, but there isn’t. Presumably there should have been a conduit, as the alarm cable is just buried directly in the ground.
     
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  11. Risteard

    Risteard

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    That's not correct.

    No two BS EN 60898 circuit breakers of any rating in series will discriminate when it comes to fault protection.
     
  12. Risteard

    Risteard

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    In fairness the 3% and 5% isn't actually a Regulation - it's merely guidance. The Regulation is simply that the volt drop won't affect the proper functioning of the equipment.
     
  13. JohnW2

    JohnW2

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    If it's any consolation, strictly speaking there is probably no "non-compliance". There are no regulations which specific explicit limits for voltage drop. Guidelines (in an Appendix of the regulations) suggest a limit of 3% for lighting circuits and 5% for all other circuits, but all the requlations actually require is that the voltage drop should not be such as to "impair the safe functioning of the equipment" - and I know of no lighting that would become 'unsafe' because of reduced voltage.

    To put this in context, these considerations of VD only really become relevant if you are one of the very few people who normally has a supply voltage close to the minimum permitted (216.2V). For those with a more typical supply voltage of around 240V, they could have a VD of over 10% and still have their lights/loads receiving a higher voltage that in the installation with a supply voltage of 216.2 (even with zero VD within that installation). If the supply voltage was close to the maximum permitted (253V), there could be about a 16% VD without equipment receiving less that the permitted supply voltage.

    Kind Regards, John
     
    Last edited: 10 May 2018
  14. JohnW2

    JohnW2

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    I thought it would have been obvious that I was talking about overload protection.

    As you say, as far as fault protection is concerned, there's never any certainty about what will happen if one has MCBs in series. If a high-rated one happens to clear the fault extremely quickly, a low-rated one in series with it may not operate.

    Kind Regards, John
     
  15. Jon c

    Jon c

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    Thanks for the explanation, my house has a voltage of 240v, so the amount of voltage drop should be acceptable.
     
  16. JohnW2

    JohnW2

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    Indeed, that would be my feeling - given that an installation starting at (permitted) 216.2V could go down to below 210V without exceeding even the 3% guideline for lighting VD.

    Kind Regards, John
     
  17. ericmark

    ericmark

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    Since we are told we must fit two 30 mA at 40 mS RCD's in series with boats and caravans, can't really say it is wrong to fit two in series, even if a little pointless, you could always change garage one to 10 mA.

    As to MCB where it is likely you can overload then having some discrimination may be prudent, my caravan has a 10A and 6A MCB which is fed from a 16A outlet which means if I put the kettle on with heater running in error I can reset without going outside. But before you go up in a rating you would need to inspect and test to see if you can without changing some thing else. I have seen many lighting circuits where the 6A has been swapped for at 10A however although the cable can take 10A, and your permitted up to 16A for a lighting circuit, the ceiling rose which acts as a junction box is only rated 6A (although sure it can handle a lot more) so technically you should not fit a 10A MCB in lighting circuits which have 6A ceiling roses.

    So you would need to check on the ELI before you can change to a larger MCB.

    As to volt drop and if the appendix is considered as a regulation well when one takes the C&G2382 exam they do ask questions about items in the appendix so it would seem C&G consider the appendix as part of the regulations. However in real terms looking at fluorescent lights. With the old wire wound ballast the fluorescent lamp was very sensitive to voltage, a 230 volt fitting can draw well over rated current if fed with 254 volt, and can fail to strike at 215 volt. I had a 240 volt 65W fitting in my kitchen, supply to house was around 245 volt, with a 58W tube as 65W discontinued it would strike most days, but then it stopped working all together. On testing voltage now house is at 230 volt, seems some solar panels have been fitted in the street and so they don't lock out on over voltage the voltage needs to drop.

    This has been talked about a few times, we can test incoming supply and it may be 240 volt with an ELI of 0.20Ω but without telling you, the supplier can change that to 230 volt with an ELI of 0.35Ω as for ELI with all RCD protection not really a problem, but volt drop is another matter. I have been caught out a few times with volt drop, but fluorescent fittings and transceivers the latter receives OK with volt drop, but on transmit it can have mains hum which one is unaware of. OK licensed ham should know better and CB limited to 4W and pre switch mode power supply radios, or record players you can hear the main hum as voltage drops so well aware of problem.

    As said other than pre-HF ballast fluorescent fittings there is very little likely to fail due to volt drop. In the main the modern SMPS has removed the problem with volt drop. However one tends to use old equipment in garages, you buy a nice new radio alarm for bedroom, and old one it taken to garage.
     
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