- Reaction score
- 157
PME already has an earth rod installed along the distributing main every 40m or less, and at the end of the main. How would an earth rod connected to an installation's MET create a problem that all those other earth rods don't?
Not to mention the bonding of extraneous parts which could be connecting a much bigger earth electrode to the installation.
PME is a major cause of elevated 50 - 100 - 150 Hz magnetic fields in the UK. The multiple Earthing should be good - but why was it done. It was primarily done due to failures in the 1970s and 1980s (and later!) of early three-phase el-cheapo 3 core XPLE cable that used the armouring for Neutral/Earth. Unfortuntely when water gets into those it eats through the poorly plated armouring and bothe Neutral and Earth are lost. I have found places (commercial installations) where more than 350 amps are flowing though earth, old gas pipes, old water pipes because of this problem. When it happens, without PME, the 3-phases become unblanced and one may get 150 volts and another 350 volts rms. So customers equipment dies - and there are quite a few case studies of this happening - hundreds of homes with damaged equipment. PME tends to hold Neutral fairly well when this happens; some people get a little too high a voltage and some get dim bulbs, etc., that flags a fault without doing much damage.
The problem if you put and Earth rod in and bond it to metal water and gas pipes (ok, older houses with metal supply pipes) then stray current (mostly due to PME!) that has coupled onto those pipes can flow through the bonding and into the ground at your house. It does not matter much if all 3 services enter the house at the same place and are bonded there - but I have visited houses (especially council and ex-ones) where the water and sewage go along the back gardens but the electricity and gas come in at the front. I have measured 37 Amps flowing throug the bonding conductor from back to front of a terraced house (with a child who developed leukaemia) due to this problem - creating enormous magnetic fields (20 microteslas when UK average is 0.04 microtesla) inside the house as the go and return currents are taking completely different routes.
Easy way to tell if there is a problem is (always!) to put a clamp meter around the bonding conductors (in every installation!) and measure the current. It should be zero. If it more than an amp, I believe it should be rectified. Usual way is to insert a piece of insulating pastic pipe in the gas or water pipe wehere it enters the building to break the stray current path.