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High integrity earthing (!) on radial circuits

Discussion in 'Electrics UK' started by electronicsuk, 15 Aug 2009.

  1. electronicsuk

    electronicsuk

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    Thankfully my question is with regard to radial rather than ring circuits, so please let's not go down that route again!

    I have determined the required cable for the circuit to be 2.5mmsq thermoplastic SWA. As protective conductor current is likely to exceed 10mA, the circuit will also need to comply with 543.7, and I'm looking for a little clarification of exactly what is allowable regarding use of armour as a protective conductor.

    543.7.1.3 (i) states that we can create a high integrity earth by having a protective conductor with CSA > 10mm2, as long as it complies with 543.2 and 543.3. According to this table, the armour on 2.5 SWA has an equivalent CSA of 17mmsq. 543.2.3 says that a protective conductor must be of copper construction if less than 10mmsq, but as the equivalent CSA of the armour is greater than this, it should be permissible for use as a CPC.

    So, in short, I am correct in my assumption that I can achieve high integrity earthing solely through use of the armour of 2.5 SWA?

    If not, my other options seem to be 543.7.1.3 (ii) - "A single copper protective conductor having a CSA of not less than 4mm2... the protective conductor being enclosed to provide additional protection against mechanical damage, for example, within a flexible conduit". This will require me to move up to 4mm2 3 core SWA, using the third core as a CPC. However, I'd prefer to avoid the increased cost this would bring (there are a large number of circuits), and it's debatable whether the SWA armour alone is deemed a suitable enclosure to protect against mechanical damage. In my opinion it is, but I'd be interested to hear what others think.

    Finally, 543.7.1.3 allows two individual protective conductors incorporated within a single cable as long as "the total cross sectional area of all the conductors including the live conductors shall not be less than 10mm2". It also states that "one of the protective conductors may be formed by a metallic sheath, armour or wire braid screen incorporated in the construction of the cable...". The regs seem to make a particular point of including ALL conductors, including live conductors, so this presumably also includes the protective conductors. As the armour is an allowable protective conductor, then surely we can include this when calculating total CSA. So, a 3c 2.5mmsq SWA would have a total CSA of 3x 2.5mmsq + 17mmsq of armour, that's 24.5mmsq total cable CSA, well more than the required 10mm2.

    Personally, I'm inclined to go with method (i) of 543.7.1.3, although using 3c cable with one core as an additional earth to ensure CPC continuity even if a gland somehow becomes damaged. I guess this is sort of a hybrid between methods (i) and (ii) and perhaps more than required by the regs, but I'd consider it good practice. I'd like to hear your thoughts on my interpretation of the regs!
     
  2. Lectrician

    Lectrician

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    If the banjo'ed cable was also 10mm, yes, I guess you could. However, the best way with SWA?

    Use the third core of your cable as the CPC looping through the sockets using the two terminals of the socket. Use the SWA for the return loop at the final socket.

    Or, two radials, join the two ends together.
     
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  4. electronicsuk

    electronicsuk

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    Thankfully it's not a ring, I wouldn't even dare ask about such a circuit on here anyway!
     
  5. RF Lighting

    RF Lighting

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    As you say you've got lots of circuits to run it, then the easiest method is to link the CPC's of two circuits together at the end the run of each circuit.
     
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