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Discuss Was this the right decision? in the Solar PV Forum area at ElectrciansForums.co.uk.

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  1. sambotc
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    sambotc Electrician's Arms

    Location:
    Devon
    Wired a P.V job today, TT system, old wylex rewirable board and a 100ma RCD on incoming tails. Ze of 50ohms but earth in old 2.5mm or something, black sheath which disappeared behind the spar dash which was obviously done at a later date and earth spike seems to have been tarmaced over!

    I put a new sub board in off some henleys with a 16a mcb and a 30ma RCD, the idea being this will trip before the main 100ma so the P.V isn't sharing the 100ma RCD? I also ran a 10mm earth to a new rod that i put in, but left the old 2.5mm and rod (where ever it was) in. The new rod on it's own gave a Ze of 75ohms, but leaving them both connected Ze was down to 23ohms which I thought was pretty good and also main earth size now complied with correct size for meter tails (16mm tails)

    Was the RCD the right idea, as obviously with such a high Ze the disconnection times would not have been satisfactory, but more importantly any faault on the P.V would have caused the main 100ma RCDto trip, and also potentially created a back feed situation and causing the 100ma RCD to fall outside of 0.40 disconnection from what I hear on here?


    Just wondered what you would have done and if my thoughts were justified?

    Thanks in advance
     
    Last edited: Feb 23, 2012
  2. moggy1968
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    moggy1968 Electrician's Arms

    In a word, no
    You should have used a type B RCD as opposed to the more generally available type A which is probably the type you have used (note a type B MCB is not the same as a type B RCD).

    The use of the PV may interfere with the operation of the main RCD or even stop it tripping at all.
    PV should never be on a shared RCD of any type.

    Also, unless you used a time delay 100ma RCD in the event of a fault on any other circuit it is likely that both the 30ma RCDs on those circuits and the main 100maRCD will trip. To get discrimination on your circuits (and therefore comply with the regs) a time delay main RCD should be used.

    A 2.5mm earth is, bizzarely, fine as long as it is protected against corrosion and damage.
    The Ze is about right for a TT, but did you do it without the bonding attached. I have seen good Ze on TT when there is no spike because the lad didn't disconnect the bonding, the only earth was the pipework!!
     
    Last edited: Feb 23, 2012
  3. yellowvanman
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    yellowvanman Electrician's Arms

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    Dorset
    For TT the earth size is not related to the tails size.
     
  4. sambotc
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    sambotc Electrician's Arms

    Location:
    Devon
    Ok thanks for the replies, I see what you are saying now and it makes sense regarding a type B RCD. As for earth size, OSG (green) table 4.4ii, beneath this table it states

    Notes:
    1 assuming protected against mechanical corrosion by a sheath

    2 The main protective bonding conductors shall have a cross sectional area at least of not less than half that required for the earthing conductor and not less than 6mm

    Am I interpratating that wrong then?

    Thanks in advnce
     
  5. BruceB
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    BruceB Electrician's Arms

    Location:
    Bath/Bristol
    You do not mention it is a transformerless inverter so type B rcd might not come into it. And even if it is transformerless then most of them claim to be exempt the type B requirement.

    You would have been alright if you had connected your solar pv box upstream of the existing rcd. But as Moggy says, downstream is a no-no because you are disobeying 712.411.3.2.1.1 and you would not have rcd discrimination unless you put in a time-delayed one upstream.

    A 30mA rcd can be low for a pv inverter - check the manufacturer's instructions.

    Regards
    Bruce
     
  6. VMan
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    VMan Guest

    even with an inverter with electrical separation i.e transformer inverter??
     
  7. moggy1968
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    moggy1968 Electrician's Arms

    It's a TT system, you need an RCD

    How would you meet your disconnection times with a Ze of 50 otherwise!! (even with a Ze of less you need an RCD because the earthing is variable depending on environmental conditions)
     
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    Last edited: Feb 23, 2012
  8. sambotc
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    sambotc Electrician's Arms

    Location:
    Devon
    Sorry, should have stated that it was impossible to cut in upstream of 100ma RCD as it was fed from overhead lines which look like they are plastered into the walls. Its one of those 'house that jack built' type places, asbestos ceilings and all sorts of hidden surprises etc

    What would be the best way around this situation as I will flag it up? and someone will have to go back, possibly not me depending on my employment situation.

    I hate getting things wrong but its the first TT job i've done, and being one of these done in a day type set ups, had to think on my feet which is where i've come unstuck! Hopefully won't have to do this much longer, again depending on employment status over the next week or 2
     
  9. sambotc
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    sambotc Electrician's Arms

    Location:
    Devon
    Just to add its a fronius IG30 which as far as i'm aware has a transformer
     
  10. moggy1968
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    moggy1968 Electrician's Arms

    I would recommend talking to Sibberts. I would say fitting a 100ma time delayed type B RCD might do the trick but your going to have to cut the meter tails and put them into a henley block in order to seperate it from the household electrics.
    Good choice for your first one!! Welcome to the vertical learning curve society:banghead:
     
  11. moggy1968
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    moggy1968 Electrician's Arms

    reg 411.5
     
  12. VMan
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    VMan Guest

    I dont believe it needs to be type b as its an isolating transformer not transform-less inverter
     
  13. sambotc
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    sambotc Electrician's Arms

    Location:
    Devon
    Vertical learning curve sounds about right. Thrown in at the deep in is another way of putting it. In my defence I do at least try and get things right and safe even if it doesn't always work out that way!

    So I take it the issue is the fact that a fault current of 100ma or more would effectively knock out both RCDs simultaneously or that any fault current from the PV could stop the 100ma RCD from tripping under fault conditions? Just trying to picture the chain of events but if i'm honest I cant figure it out in this loaf of mine.
     
  14. moggy1968
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    moggy1968 Electrician's Arms

    Thats why I suggested talking to Sibberts as this is a bit beyond my level. I would think a type B but am quite happy to be proved wrong!
    It would certainly keep the cost down!
     
  15. moggy1968
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    moggy1968 Electrician's Arms

    basically yes, and well done for asking, most people would probably just stick their head in the sand and not give a ****!!
     
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