Sub main works.

Discussion in 'Electricians' Talk' started by Davidkent89, Oct 16, 2016.

  1. Davidkent89

    Davidkent89 New Member

    Good evening all.
    I've got myself confused and starting to panic.

    By the way, I'm new here, signed up after seeing a few interesting conversations via google!

    Anyway. To the point.

    I've gone to a customers house and rewired 2 circuits (so far) in his garage.

    I'm being told lots of different things about what next.

    It started life with an old consumer unit which I've binned and out an RBCO board in, and now gone to start doing some testing and this is where I hit an issue.

    There's the cutout and meter at the house.
    Tails go into blocks and get split.

    One set to the house consumer unit, and main earth (TT so on a rod) - No problem.

    Second set goes to a smaller consumer unit where NO EARTH is connected (bear with me).
    From here (63A) MCB, an SWA runs to the garage - where my new consumer unit (and also 2 new circuits) are.
    This end, has an earth rod of its own into the consumer unit, where the earth of the SWA and the sheath of the SWA are earthed.

    This is where the issue is...
    I'm being told this isn't correct..

    Now I didn't install anything at the house end, Nor did I install the SWA.

    I'm assuming this is going to cause issues come sign off, and am looking for some advice on what is actually correct and what / if I need to make any changes.

    Obviously currently the house and the garage earths are totally seperate.

    Anyone got any advice?
    Thanks in advance
     
  2. leesparkykent

    leesparkykent Well-Known Member

    The garage is TT and so is the installation its fed from. Link the Rods together using the CPC of the distribution circuit. It will only help to reduce the overall Ra.
     
  3. Davidkent89

    Davidkent89 New Member

    Aren't they supposed to be seperate?

    It would be MUCH easier to link them because both ends have SWA glands into metal consumer units, I just didn't think that was 'corrrect'
     
  4. leesparkykent

    leesparkykent Well-Known Member

    Why wouldn't it be correct? You are not mixing earthing arrangements as its all TT so no worry about potential differences.
     
  5. Davidkent89

    Davidkent89 New Member

    That does make sense, perhaps that's where I thought the issue was.

    Does the supply still need RCD protection?

    The garage circuits are all RCBO but the last thing I want is a fault to trip both an RCBO and the RCD at the house.

    Is it a necessary addition, and if so, I assume 100mA time delay?
     
  6. leesparkykent

    leesparkykent Well-Known Member

    More than likely because I doubt the Ra is low/stable enough to achieve TN values and consequently disconnection times for the rating/type of over current protective device. Using a 100mA time delayed RCD upfront will give you a maximum EFLI value for the distribution circuit of 500Ohms as the maximum safe touch voltage is considered to be 50V. 50V/0.1Ohms=500Ohms.

    Now the problem you have is the RCBO's used will more than likely be single pole solid neutral as the majority of them are. I only know of 2 makes of single pole switched neutral RCBO's and they are SBS and Garo. If you have a neutral-earth fault on one of the garage circuits it will trip the corresponding RCBO but it will also trip the 100mA time delayed RCD because the neutral doesn't disconnect on the RCBO as its of a solid neutral construction.

    The easiest option would be to replace the main switch in the garage board for a 30mA RCD and change the RCBO's to circuit breakers. The best option would be to use a SBS board in the garage and use their compact RCBOS which are switched neutral and this will give you division of circuits to minimise disruption in the event of a fault.
     
  7. Davidkent89

    Davidkent89 New Member

    That makes sense.

    I've had to change it around for my assessment with Napit anyway and he seemed happy - hopefully I don't get any trip issues at the house end though can't guarantee this and won't know until I know..

    Annoying really though could cause alsorts of issues...

    It's now got an RCD at the house and the earth at the garage has been separated adequately.

    Of course this doesn't mean that if there's a fault on a garage circuit that it'll separate and it could still cause the house to trip, as you rightly said - the RCBOs are only cheap so probably aren't double pole in honesty.

    Time will tell!
     
  8. Lectrician

    Lectrician Screwfix Select

    You don't explain at all what is in place? What earthing system at origin and at garage? What RCD at origin and at garage? What length of run, size of cable, Ze and Zdb?
     
  9. Davidkent89

    Davidkent89 New Member

    Origin now has a 30mA RCD (NAPIT advice)
    TT system
    Now earthed from origin end using earth conductor and the sheath too.
    16mm SWA running approx 30 metres.

    Garage is also a TT, has its own rod. Earths are seperate by using plastic gland into the metal consumer unit and the earth conductor is in a connector block in the consumer unit so not connected.

    This was existing with seperate rods, I installed the RCD at the origin and sorted the earths (the cable was earthed from the garage end and seperate at origin but defeats the object of the RCD at origin so swapped this all over.

    Ze at origin using that rod was 3.19ohm from memory, ze at the garage using its own rod I believe to be around 24.9 ohm.

    I'm sure it'll be fine now I just don't really want the house RCD tripping as it'll be a nightmare to deal with, especially when the boss is out and the house is locked.
    Tripped RCD = no office. I need to do some RCD trip testing and see if the house trips I think...
     
  10. Lectrician

    Lectrician Screwfix Select

    As they're both TT, there is no need to segregate. An RCD at origin doesn't need to be 30mA. It could be 100mA, 300mA or 500mA, and it should be time delayed. You then have 30mA local protection as required. What you had before, and now, both sound poorly designed.
     
  11. Davidkent89

    Davidkent89 New Member

    I was also advised that the garage doesn't need its own rod, but because it has it and it's in use, that it needs to be separate.

    Napit advice too, which is annoying - it was a pain to separate.

    I've tried to make the best of a bad situation really...
     
  12. leesparkykent

    leesparkykent Well-Known Member

    have you still got RCBO's at the far end?
     
  13. Davidkent89

    Davidkent89 New Member

    Yes RCBOs far end on every circuit,
    Doesn't mean they're double pole though does it?
     
  14. Lectrician

    Lectrician Screwfix Select

    Depends which you get!
     
  15. leesparkykent

    leesparkykent Well-Known Member

    No and even more unlikely to be double pole or switched neutral if single module
     
  16. Davidkent89

    Davidkent89 New Member

    That's what I thought, I'll see what happens I think!
     

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