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OSG Ref B installation limits

WireWeHere

Table 7.1(ii) of the OSG includes Installation type B* which is "Enclosed in conduit or trunking on a wall, etc." This isn't included in any of the allowed installation methods of Table 7.1(i).

What "installation method" classification do you use when installing T&E on a blockwork wall in oval conduit, in a chase for subsequent plastering over? Equally, when the wires are just chased in and plastered over, or do you just consider that type C "clipped direct"?

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    But it doesn't explain why B isn't in table 7.1(i).

    I don't have the latest OSG to hand, but If it's for flat T&E cable and only covers methods 100, 101, 102, 103, C & A - I'd hazard a guess that it's a copy of BS 7671 table 4D5. That's a special for UK flat cables in certain conditions - for other installation methods you'd to refer back to  the general purpose table for any shaped multicore sheathed cables - 4D2A (which covers installation methods A, B, C & E). Hopefully that's reproduced in the OSG somewhere.

    4D5 is a bit of an oddity - we didn't have it until someone spotted that 2.5mm² T&E wasn't always compliant on a 32A ring circuit where there was thermal insulation involved - despite there being no actual problems being noticed in reality. Noticing that flat T&E were a little better at loosing heat than the internationally recognised round cables (by virtue having a thinner sheath and larger surface area) some experiments where done and new ratings for T&E cables in certain situations were drawn up. For all the other situations, we go back to the original table still.

       - Andy.

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  • 0

    But it doesn't explain why B isn't in table 7.1(i).

    I don't have the latest OSG to hand, but If it's for flat T&E cable and only covers methods 100, 101, 102, 103, C & A - I'd hazard a guess that it's a copy of BS 7671 table 4D5. That's a special for UK flat cables in certain conditions - for other installation methods you'd to refer back to  the general purpose table for any shaped multicore sheathed cables - 4D2A (which covers installation methods A, B, C & E). Hopefully that's reproduced in the OSG somewhere.

    4D5 is a bit of an oddity - we didn't have it until someone spotted that 2.5mm² T&E wasn't always compliant on a 32A ring circuit where there was thermal insulation involved - despite there being no actual problems being noticed in reality. Noticing that flat T&E were a little better at loosing heat than the internationally recognised round cables (by virtue having a thinner sheath and larger surface area) some experiments where done and new ratings for T&E cables in certain situations were drawn up. For all the other situations, we go back to the original table still.

       - Andy.

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