Earthing and grounding

Frequently asked questions

What is the difference between grounding and bonding?

They do different jobs, and the American code separates them because confusing the two produces installations that look safe and are not. BONDING connects metal parts together so that during a fault they are all at the same potential and a large current can flow back to the source, which is what makes a breaker trip. GROUNDING connects the system to the earth itself, which sets a reference potential and gives lightning and high-voltage surges somewhere to go. The crucial point is that the EARTH IS A POOR CONDUCTOR: the resistance of a ground rod is typically tens of ohms, which at mains voltage passes a few amps — nowhere near enough to trip anything. So the earth does not clear faults; the bonded metallic path back to the transformer does. Anyone who believes a ground rod protects them from a short circuit has the mechanism exactly backwards, and that belief is the reason the two words are defined separately.

Why does the British system have letter codes and the American one does not?

Because in Britain the source of the earth varies, and what you are allowed to install depends on which one you have. TN-S gives a separate earth conductor all the way from the supply transformer. TN-C-S, usually called PME, combines neutral and earth in the supply and separates them at the cut-out, which gives a very low earth resistance and, in exchange, a specific hazard if that combined conductor breaks. TT has no earth from the supply at all and relies on an electrode at the property, which means fault currents are far too small for a fuse and residual current protection becomes mandatory rather than optional. So the letters are a design input: they determine acceptable disconnection times, whether an RCD is required, and whether bonding an outbuilding or an electric vehicle charge point is safe. American practice standardises the supply arrangement instead, so the classification has nothing to vary over.

Why must an EV charge point be treated specially on a PME supply?

Because PME's specific failure mode meets an appliance held by a person standing on the ground. In a TN-C-S system the earth is derived from the supply's combined neutral-and-earth conductor, so if that conductor breaks somewhere upstream, every earthed metal part in the property can rise towards mains potential — and indoors that is tolerable because everything around is bonded to the same rising potential, so there is no difference for current to cross. Outdoors it is not: a person on real earth touching a car that is now at a raised potential bridges the difference. British wiring rules therefore require an EV installation on a PME supply either to have its own earth electrode or to use equipment that detects the voltage rise and disconnects. It is a good illustration of why the system classification is worth naming at all: the wiring in the house is unchanged, and the correct installation outside it is different.