The £1300 AFDD consumer unit

I am sure that you are right Mike. However, the point you are making is important. The only voltage we can "control" (hopefully rather better than the Government attempts to control Covid) with PME (which is, in reality, all we have) is the L-N voltage. We do not have a low impedance Earth which really is the bulk of the planet, so we cannot divert the surge elsewhere whatever we do. The voltage between a telephone cable and the mains Neutral, or any other service cannot sensibly be controlled. The items which can be damaged (in my case over the years Modems and ADSL filters on telephone lines in the exposed country) are not protected by mains surge suppressors because the failure is in the line interface. I have never had a failure on the same site on the mains part of any equipment, despite a local pole TX, TT supply, and a very exposed 11kV line of miles length. The site got quite a lot of lightning and several local trees were hit over 15 years. All the equipment I have seen for sale assumes a TN-S supply and a good local Earth, probably not to a cable armour with the sheath insulating it from real ground. This does not match the reality of the UK, the earth connection is often not local, bonded service pipes are often plastic and not available, and so the Earth connection has significant resistance and series inductance. Perhaps this is why there was pressure for foundation Earthing, to provide a local Earth for the surge suppressors. Unfortunately, I was not party to the discussions!


Following on, the lightning flash can only induce a similar voltage, as a series addition to both or all live conductors, raising the end voltage to real Earth. However real cables have conductors both twisted and very close together so these voltages must be pretty much identical. Overhead HV lines are slightly further apart, but still very close in inductive terms, similar to an open-wire transmission line. Other switching transients may well mainly affect one line, giving a L-N or P-P voltage which could be clipped by a surge suppressor, but it is not clear why this has recently become a serious problem to consumers because that is not my experience.

I am sure that you are right Mike. However, the point you are making is important. The only voltage we can "control" (hopefully rather better than the Government attempts to control Covid) with PME (which is, in reality, all we have) is the L-N voltage. We do not have a low impedance Earth which really is the bulk of the planet, so we cannot divert the surge elsewhere whatever we do. The voltage between a telephone cable and the mains Neutral, or any other service cannot sensibly be controlled. The items which can be damaged (in my case over the years Modems and ADSL filters on telephone lines in the exposed country) are not protected by mains surge suppressors because the failure is in the line interface. I have never had a failure on the same site on the mains part of any equipment, despite a local pole TX, TT supply, and a very exposed 11kV line of miles length. The site got quite a lot of lightning and several local trees were hit over 15 years. All the equipment I have seen for sale assumes a TN-S supply and a good local Earth, probably not to a cable armour with the sheath insulating it from real ground. This does not match the reality of the UK, the earth connection is often not local, bonded service pipes are often plastic and not available, and so the Earth connection has significant resistance and series inductance. Perhaps this is why there was pressure for foundation Earthing, to provide a local Earth for the surge suppressors. Unfortunately, I was not party to the discussions!


Following on, the lightning flash can only induce a similar voltage, as a series addition to both or all live conductors, raising the end voltage to real Earth. However real cables have conductors both twisted and very close together so these voltages must be pretty much identical. Overhead HV lines are slightly further apart, but still very close in inductive terms, similar to an open-wire transmission line. Other switching transients may well mainly affect one line, giving a L-N or P-P voltage which could be clipped by a surge suppressor, but it is not clear why this has recently become a serious problem to consumers because that is not my experience.