Protective Multiple Earth

In many places across the UK, the electricity supply companies use an earthing system for the mains supply known as 'PME' - protective multiple earthing.

This is a slightly odd system in that the earth connection is bonded to the neutral at the non-consumer side of the incoming electricity supply.  To protect the integrity of the neutral line, the system is connected to earth rods beneath several supply poles along the route.

Not what it seems at the fuse box - the PME system connects earth to neutral at the incoming cable.

In extremely rare - amounting to almost unheard of situations - the neutral line can be broken somewhere along the route.  Very unlikely, but possible.

The RSGB has a somewhat technical document on PME here.

Now, if you have a separate RF earth - and typically, you will - then if a neutral continuity break occurs, you could find that RF earth reaching mains voltage, and a fairly high risk of fire and electrical shock.

The use of an RCD unit on the circuit to which the ham equipment is connected is certainly an improvement over a conventional wire fuse, which could take maybe 20 seconds to blow, and which are still quite common across the UK, especially in older properties.  An RCD will trip in just a fraction of a second.

But to manage this potential neutral fault, the RF earth should be connected to the neutral as close as possible to the incoming line.  The recommended wire cross sectional diameter is often given as 30mm2, which makes this an expensive and stiff wiring job.  A consumer can only wire it in to the fuse box, but the earth from there to the incoming supply line is often very thin - mine is only about 4mm2, and seems to have kept the electricity company happy because they changed my meter and incoming wire just a couple of years ago.

Whilst I never really like to invite the 'authorities' to interfere beyond my front door, this is a situation where some advice would be useful.  I've just had an e-mail from Scottish Power Network, starting the process of making an appointment to examine the situation.  I'm a bit surprised they haven't got any standard advice from the outset.  I'm also a bit concerned they might try to push their weight around, especially as hams are often regarded with unfair suspicion.

Once I've run the gauntlet, I'll update you with what they say...