Earth. The conductive mass of the Earth, whose electric potential at any point is conventionally taken as zero.
Functional earth. Earthing of a point or points in a system or in an installation or in equipment, for purposes other
than electrical safety, such as for proper functioning of electrical equipment.
In real terms, not so easy to separate, a functional earth can have non fault current travailing down it. Be this a radio transmitter, or a filter, pre-RCD days it was common to have radio interference filters which 'leaked' some current to earth, these stopped being used when the RCD came out as they could trip the RCD.
Today we have a problem, we want the functional earth to be the same voltage as the earth, so somewhere somehow they need bonding together, I have seen with a large gas installation a massive resistor about the size of a transit van connecting the DNO earth to the site earth. But not seen this with smaller installations, I have also seen a 2.5 mm² earth wire which connected a radio shack to the main house melt, caused by a broken PEN, but most people do not have an earth rod at each corner of the garden, and flat copper braid joining them. The earth was for his radio transmitter. He had some problems, which turned out to be a rusty bolt on the aerial acting as a diode, and the earth had been laid while hunting for the problem.
So in today's domestic premises, we may well have an earth rod, but not large enough to cause massive currents to flow, and as far as a clean or functional earth goes, somewhere somehow it needs to connect to the main earth. I have given up Armature radio for some years now, at least where it needs an earth, last time we used copper pipes below the concrete slab that the mast was anchored to again looking at a surface area large enough that to connect a TN-C-S earth to it directly would cause problems under fault conditions, so the signal box was on a TT supply.
I am sure when I did my RAE this was covered, but this was some 40 years ago. If I needed to fit a functional earth today, I would need to do some heavy reading. There are some odd systems, I seem to remember diodes in the earth as galvanic protection,
it seems wrong to rely on the current through a diode to earth bond, but it is common with canal boats. The instructions
I will guess more than one diode in the unit, as difference between copper and iron is more than 0.5 volt. Also in salt water the hull would be a good earth anyway, although not so much in fresh water.
