The £1300 AFDD consumer unit

That would be great, but we ground stuff in a three-phase system.


I think what you mean is for the purposes of ADS we provide a low impedance at 50Hz back to the substation, and on its way the neutral or earth is connected to a number of metllic objects that are in the earth (terra-firma).


This is generally  irrelevant for short pulses / high frequencies,  as the impedances and propagation times are all too high. Actually what we do as far as they are concerned is connect the system to a number of low loss resonators of varying length and indeterminate termination impedance. Somewhere between a few tens of KHz and a fraction of a MHz the mains wiring is resonant.


Also if you put an ideal coax cable in an RF field the same voltage is induced along the outer and the inner - which is why the difference voltage  between outer and inner remains  the same at both ends, and is exactly what we want. Shielding efficiency / transfer impedance is a measure of how much this perfect state does not happen.

Of course if you allow un-equal current to flow in inner and outer, then then there is an imbalance term that will appear as a variation between the outer to  inner voltage at the two ends- that is where things like ferrite beads help. making sure the common mode current sees a high impedance, and the voltage gradient on the outer and the inner is the same.

We need to be careful how we use words like 'ground' and 'earth', as to what frequency we mean it for, a good DC or 50Hz ground is not generally the same as a good RF one - which may indeed not be earthed at all in any DC sense.


regards Mike.

That would be great, but we ground stuff in a three-phase system.


I think what you mean is for the purposes of ADS we provide a low impedance at 50Hz back to the substation, and on its way the neutral or earth is connected to a number of metllic objects that are in the earth (terra-firma).


This is generally  irrelevant for short pulses / high frequencies,  as the impedances and propagation times are all too high. Actually what we do as far as they are concerned is connect the system to a number of low loss resonators of varying length and indeterminate termination impedance. Somewhere between a few tens of KHz and a fraction of a MHz the mains wiring is resonant.


Also if you put an ideal coax cable in an RF field the same voltage is induced along the outer and the inner - which is why the difference voltage  between outer and inner remains  the same at both ends, and is exactly what we want. Shielding efficiency / transfer impedance is a measure of how much this perfect state does not happen.

Of course if you allow un-equal current to flow in inner and outer, then then there is an imbalance term that will appear as a variation between the outer to  inner voltage at the two ends- that is where things like ferrite beads help. making sure the common mode current sees a high impedance, and the voltage gradient on the outer and the inner is the same.

We need to be careful how we use words like 'ground' and 'earth', as to what frequency we mean it for, a good DC or 50Hz ground is not generally the same as a good RF one - which may indeed not be earthed at all in any DC sense.


regards Mike.