AJJewsbury:
Just in case it adds anything, I've been looking back over the old regs. As far as I can tell the requirement for insulation testing started with the 3rd Ed in 1897 - then the limit was 10MΩ divided by the maximum demand of the installation in amps.
In the 4th Ed of 1903 it changed to 30MΩ divided by the number of 'points'.
By the 8th Ed of 1924 it became 25MΩ divided by the number of points, but with the added proviso that 1MΩ for the entire installation was acceptable even if the formula produced a higher number.
In the 10th Ed of 1934 the demand was for 100MΩ divided by the the number of points for the fixed wiring alone (before light fittings etc were connected) and 50MΩ divided by the number of points for the completed installation (without appliances), but again that entire installations need not exceed 1MΩ.
The 12th just added the proviso that readings below 0.5MΩ weren't acceptable because instruments of the time wouldn't be able to provide 500V into resistances that low.
By the 13th Ed (1955) it was 50MΩ divided by the number of points generally, but 12.5MΩ divided by the number of points if PVC-insulated cables used. Again 1MΩ for entire installation was acceptable.
The 14th (1966) made the requirement a flat 1MΩ but large installations could be sub-divided into groups of not less than 50 outlets.
The 16th introduced wording similar to what we have today (main switchboard and each distribution circuit tested separately) but the limit was reduced to 0.5MΩ
And the 17th introduced what we have today - main switchboard and each distribution circuit tested separately - and the limit restored to 1MΩ
- Andy.
AJJewsbury:
Just in case it adds anything, I've been looking back over the old regs. As far as I can tell the requirement for insulation testing started with the 3rd Ed in 1897 - then the limit was 10MΩ divided by the maximum demand of the installation in amps.
In the 4th Ed of 1903 it changed to 30MΩ divided by the number of 'points'.
By the 8th Ed of 1924 it became 25MΩ divided by the number of points, but with the added proviso that 1MΩ for the entire installation was acceptable even if the formula produced a higher number.
In the 10th Ed of 1934 the demand was for 100MΩ divided by the the number of points for the fixed wiring alone (before light fittings etc were connected) and 50MΩ divided by the number of points for the completed installation (without appliances), but again that entire installations need not exceed 1MΩ.
The 12th just added the proviso that readings below 0.5MΩ weren't acceptable because instruments of the time wouldn't be able to provide 500V into resistances that low.
By the 13th Ed (1955) it was 50MΩ divided by the number of points generally, but 12.5MΩ divided by the number of points if PVC-insulated cables used. Again 1MΩ for entire installation was acceptable.
The 14th (1966) made the requirement a flat 1MΩ but large installations could be sub-divided into groups of not less than 50 outlets.
The 16th introduced wording similar to what we have today (main switchboard and each distribution circuit tested separately) but the limit was reduced to 0.5MΩ
And the 17th introduced what we have today - main switchboard and each distribution circuit tested separately - and the limit restored to 1MΩ
- Andy.
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