R.C.D's and Modulation Interference

My 2 Cents worth (as a former contracting electrician):
The line (input) side of the neutral terminal of an RCD connects to the main neutral bar. If that particular switchboard is the 'MEN' board, then the neutral bar will be connected to the earth bar via a removable link. As 'Admin' states, the load (output) side of the neutral terminal of the RCD connects to a small dedicated (independent) neutral bar - but only the neutral wires from the sub-circuits that are protected by that particular RCD get connected to that dedicated neutral bar. When that RCD either trips, or is turned off, both the phase & neutral wires of all the sub-circuits connected to that particular RCD are totally isolated from the rest of the switchboard wiring.

Now, Richard, you say that you have been able to measure 0.8Volts between neutral & earth of a power point. What you are probably measuring is the voltage drop across a current carrying neutral wire. What I mean is that there will be another appliance (possibly a fridge or a heat pump, but could be anything else) that is on the same sub circuit as the power point that you measured the voltage across. What I am trying to say is: The phase & neutral wires are the ones carrying the load current. They have resistance, therefore a voltage drop will be developed across each of those wires (very close to a 50/50 shared amount). What (I suspect) you were measuring was this voltage drop across the neutral wire, as the earth wire (normally) carries no current, there will be no voltage drop across the earth wire, unless a 'fault to earth' occurs. In 'bygone days' we used to install a MEN link at every switchboard in an installation, but this created a problem of the earth lead (between switchboards) always carrying a share of the load current. These days, only one switchboard per installation is to have a 'MEN' link fitted.

Back to Richard's original post regarding modulation troubles (this sounds like 'cross modulation'):
I can't help but wonder if the RCD's that have been installed have a half wave power supply (they have internal electronics, therefore need a power supply). I wonder if you have a freestanding range (stove) that has a power point fitted. If so, then plug a radio in to that power point, and turn off all of the newly installed RCD's, and note the effect on the radio. I suspect the problems might stop. This doesn't solve your problem, but might isolate what might be causing it. As RCD's can be wired 'either way around' (meaning the line in could be either at the top or the bottom of the RCD), therefore the RCD's power supply might be still 'alive' even when the RCD is switched off. However, it's worth a try to see what happens anyway. Some RCS's I've seen have one 'leg' of their electronics connected to one 'input' terminal, and the other 'leg' of their electronics connected to the diagonally opposite 'output' terminal. This means that the internal electronics are completely disconnected from drawing any current if the RCD is turned off.

Good morning Richard

That's great to hear! Possibly the next thing you could try doing is to see if there's one particular RCD that's causing the problem. What I would do next is to leave a radio plugged in to the stove, and turn the RCD's off, one at a time, and note if they all contribute to the problem, or if it's just one. If 'we' prove it's an RCD that's causing the interference, then substitution with a different brand might be a solution - your sparky just might be able to credit you for the ones he installed, rather than having you pay twice.

Thats a bummer - as if we didn't have enough modern interference issues to deal with when playing with valve radios... I was thinking of upgrading my c.1976 switchboard... now I'm thinking its just fine the way it is.

The isolating transformer in circuit works for now.

Thanks Peter, I'll give it a try

I found the (ir)relevant regs