Distribution Circuit

Discussion in 'Electricians' Talk' started by Bobby green socks, Sep 28, 2021.

  1. Bobby green socks

    Bobby green socks New Member

    Hi, bit green on distribution circuits and need advise please!

    I have been asked to convert an outside socket to a Caravan hook-up that meets 18th Edition (interlocked switches and 2 x 16amp RCBOs 30ma) feeding 2 x 16amp commandos where I will convert to a TT system.

    The installation as stands is as follows:

    - TNCS main Feed to
    - 32 amp breaker protected by 30ma RCD (Garage UNIT) to
    - 70 metre approx buried 16mm swa cable run
    - Caravan Hook-up (as above)

    - ZS @ distribution point 0.48 Ohms (calculated)

    Any thoughts ...is this how you would do it?
    My main question is this now considered a 'Distribution Circuit' in which case my permitted disconnection times would change? I am reading definitions and still confused as whether the caravan hook-up is considered another CU as before the ZS values would be well within permitted if just outdoor sockets. Normally I wouldn't use 30ma RCD's in sequence but as I'm converting the caravan hook-up to TT then I'm assuming this would prevent any tripping that would take out the whole circuit (?) I know an old school approach would be to front it with a 100ma S type.

    Thanks ...

    I'm also getting conflicting advise on the old 'exporting a PME' issue !
    Last edited: Sep 28, 2021
  2. Bob Rathbone

    Bob Rathbone Screwfix Select

    Their is no conflict on exporting the earth to a caravan outlet, the regulations forbid it, must be TT. Other types of outdoor supplies have the option to export the earth, but many, including myself, will go for TT.
    Bobby green socks likes this.
  3. Bobby green socks

    Bobby green socks New Member

    Thankyou for responding ...Sorry I should qualify that. It is a caravan outlet but being used for a Glamping Pod...
  4. Comlec

    Comlec Screwfix Select

    Much of what you need to know is fully explained in Guidance Note 7.
    Bobby green socks likes this.
  5. The Happy Builder

    The Happy Builder Screwfix Select

    I cannot see anything about the proposal that complies with the wiring regulations.
  6. Comlec

    Comlec Screwfix Select

  7. The Happy Builder

    The Happy Builder Screwfix Select

    Note the RCDs are double pole and disconnect both live and neutral.
  8. Bobby green socks

    Bobby green socks New Member

    Yep, the Rolec units all comply with these standards including 'Interlocked switches'. Sorry what aspect of it does not comply? from the diagrams ...best practise seems to be front end with a 100ma
    Thanks for this so the first image posted by Comlec is 'exporting a pme' please correct me if I'm wrong! I'm grabbing my bluebook now!
  9. Bobby green socks

    Bobby green socks New Member

    So juts to clarify... the image posted by Comlec below is not correct? as it depicts exporting PME, at least to the pitch pillar . Thanks
  10. Lectrician

    Lectrician Screwfix Select

    Having the 30mA upfront is very undesirable. It's TN-C-S so does it need it? If you Zs at the hookup meets the disconnection time for the 32amp MCB at the start, then the 16mm SWA doesn't need the RCD protection, and certainly not 30mA. Divorce the earth at the hookup, rodding the hookup down and segregating the earths from the socket pins from the SWA. The 30mA RCDs in the hookup provide the TT arrangement for you.

    As you have TN-C-S for the 16mm, its disconnection time is 5 seconds. The hookups will be 0.2 seconds TT.
  11. bright_Spark

    bright_Spark Screwfix Select

    Exactly this.
    Bobby green socks likes this.
  12. Bobby green socks

    Bobby green socks New Member

    Thanks this was my first take on the design change so Im happy you have confirmed !
  13. The Happy Builder

    The Happy Builder Screwfix Select

    Let’s start at the beginning, you said you have been asked to “convert an existing outside socket”.

    So is this a 32 amp socket and what is the type and specification of the circuit that supplies it, also what other sockets and loads does the circuit supply?
  14. Comlec

    Comlec Screwfix Select

    The diagram shows the the armour connected to earth at the supply end but it is not shown at the distribution end. So if you TT at the ‘post’ then you don’t connect the armour!
    Bobby green socks likes this.
  15. Bobby green socks

    Bobby green socks New Member

    Sorry I've confused things by mentioning that . What exists now is a 16mm SWA (with nothing connected to it) fed by a 32amp breaker and and 30ma RCD Main switch (A BG garage unit basically)...On the end of the SWA I'm going to connect the CARAVAN hook up unit which is a standard 2 x 16amp outlet interlocked switched unit) . Also convert to TT at the socket outlets. I now think best to convert the RCD to a main switch in the garage unit and uprate the breaker to a 40 amp . Can't remember max ZS but will check . Thanks all for comments

    No other loads, no other sockets fed ...one cable / one Caravan hook up.
  16. The Happy Builder

    The Happy Builder Screwfix Select

    That makes more sense, you don’t want a 30 mA RCD at the source and the sockets, also you want diversity between the MCB and MCBs/RCBOs.
    Bobby green socks likes this.

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