Earth Electrode.

I had to connect up a consumer unit's earth terminal to a new D.N.O. provided P.M.E. earth terminal today. The previous earthing arrangement was TT. As an experiment I did a loop test on the earth electrode. I was interested in its resistance after many weeks of no rain here and the well draining sandy soil. The reading was about 100 Ohms.

Then as I was connecting the new 16.00mm2 green and yellow earthing conductor I saw arcing across from the cable to the new earthing terminal before I tightened the terminal screws. At first I was concerned. I then measured the Voltage across the earthing conductor from the consumer unit to the new P.M.E. earthing terminal with a D.M.M. It read about 3 Volts A.C. So I had a Voltage rise of about 3 Volts from the C.N.E. and true Earth. Despite that the sparks were impressive.

The property has solar panels and the local D.N.O. had written 249 Volts 0.36 Ohms on the main cut out.



  • This is a practical lesson in why main equipotential bonding, and occasionally, supplementary local equipotential bonding, is necessary.

    It's also not restricted to TN-C, TN-C-S and TT earthing arrangements.

    Even with TN-S, voltage differences in a system without bonding exist, because of 'protective conductor currents' or 'earth leakage currents'. This was also the case before electronics - because of 'leakage' through insulation. The larger the installation (physical area, and supply), the greater the currents. They also may be affected by the influence of other electrical installations (e.g. transients or ground currents from HV or LV installations) - not to mention lightning on the odd occasion.

    It's also an abject lesson, learned the hard way by some, for example working in telecomms infrastructure without adequate training, that the green-and-yellow wires:

    • are not always safe to touch, especially when disconnected
    • may well carry intended current in normal conditions
  • Despite that the sparks were impressive.

    Ah, should we consider the need for an AFDD on protective conductors then? Thinking

       - Andy.

  • Under some conditions very large fuses or at least a known weak link might be better - just before the point where the whole path of the CPC becomes a fire hazard it becomes much better if it lets go in a known and controlled way at an inspectable location. Such thoughts are anathema to the traditional wiring regs of course, but such  things are contemplated and done in certain high energy circles.