Insulated Enclosures of Old and Earthing.

I am sure that I read about the need for all insulated distribution boards in the I.E.E. Regs. years ago on T.T. systems. What was a T.T. earthed system called before T.T? An installation with its own earth electrode and not supplied with an earth terminal from the distributor?

Depends how far back you go. At one point they didn't call it anything in particular - you could "Earth" things in almost any way you liked (from a supplier's terminal to just a metal plate buried on edge in the ground) - the only requirement then was that the resistance between any two exposed-conductive-parts anywhere within the installation didn't exceed 1Ω. In effect the entire installation was protected by supplementary bonding rather than ADS. From that point of view having an insulating CU isn't much of an advantage - as it'd be at pretty much the same potential as any other exposed-conductive-part anyway. Later on (14th say) they started talking about ADS and earth loop impedances, but still didn't really distinguish between TN and TT (even though PME does get a mention) - just noting that where the impedance was too high for 'excess current protection devices' to provide ADS you'd need some other device to provide 'earth leakage protection' (usually an ELCB). In those days ELCBs were normally all-insulated anyway, and installed in the tails before the CU, so there would be little need for the CU itself to be all/double/reinforced insulated. Come the 15th we had full TN-S/TN-C/TN-C-S/TT/IT descriptions.


  - Andy.

I am sure that I read about the need for all insulated distribution boards in the I.E.E. Regs. years ago on T.T. systems. What was a T.T. earthed system called before T.T? An installation with its own earth electrode and not supplied with an earth terminal from the distributor?

Depends how far back you go. At one point they didn't call it anything in particular - you could "Earth" things in almost any way you liked (from a supplier's terminal to just a metal plate buried on edge in the ground) - the only requirement then was that the resistance between any two exposed-conductive-parts anywhere within the installation didn't exceed 1Ω. In effect the entire installation was protected by supplementary bonding rather than ADS. From that point of view having an insulating CU isn't much of an advantage - as it'd be at pretty much the same potential as any other exposed-conductive-part anyway. Later on (14th say) they started talking about ADS and earth loop impedances, but still didn't really distinguish between TN and TT (even though PME does get a mention) - just noting that where the impedance was too high for 'excess current protection devices' to provide ADS you'd need some other device to provide 'earth leakage protection' (usually an ELCB). In those days ELCBs were normally all-insulated anyway, and installed in the tails before the CU, so there would be little need for the CU itself to be all/double/reinforced insulated. Come the 15th we had full TN-S/TN-C/TN-C-S/TT/IT descriptions.


  - Andy.