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Electrical Engineering DISCUSSION
I measured about 2 V between neutral and earth at a socket while a heater was running, and close to 0 V with nothing switched on. I have also read that neutral and earth are connected together back at the supply transformer or service entrance.
If they are the same point electrically, why does every circuit carry both, and what would actually go wrong if the earth pin of a socket were simply linked to neutral?
They are joined at one point, but they do different jobs. The neutral is a working conductor that carries the full load current back to the source all the time. The protective earth carries nothing in normal operation and exists only to carry fault current.
Because the neutral carries current, it has a voltage drop along its length. Copper is about 0.0175 Ω·mm²/m, so 20 m of 2.5 mm² wire is 0.14 Ω, and a 16 A load produces 16 × 0.14 = 2.2 V. That is the 2 V you measured: the neutral at the socket sits a little above the earth wire, which has no current in it and so stays at the potential of the source earth.
Linking earth to neutral at the socket works until the neutral fails. If the neutral breaks or a terminal works loose upstream, the neutral on the load side of the break is pulled up to line voltage through any appliance that is switched on. With the earth pin tied to it, every metal case on that circuit becomes live at up to 230 V, with no fault current flowing and therefore nothing to trip.
With a separate protective conductor the cases stay at earth potential even with the neutral broken. The link also puts load current onto earth wires and bonded metalwork in normal use and stops residual current devices from working correctly.
The earth wire protects by making a fault trip the breaker quickly. If a line conductor touches an earthed metal case, current flows round the earth fault loop: line conductor, case, protective conductor, back to the source. The loop impedance Z_s decides the current. With Z_s = 0.8 Ω on 230 V the fault current is about 290 A. A B16 breaker trips instantaneously at no more than 5 × 16 = 80 A, so it disconnects within a fraction of a second.
If Z_s were 5 Ω, the current would be only 46 A. That is below the instantaneous threshold, the breaker would take many seconds on its thermal element, and the case would stay live meanwhile. Wiring rules therefore set a maximum Z_s for each breaker type and rating.
How the two conductors relate between the source and your building is what the earthing system codes describe. In TN-S they are separate all the way from the transformer. In TN-C-S they share one combined conductor in the supply network and are split at the service entrance, which is probably the join you read about. In TT the building has its own earth electrode and there is no metallic earth path back to the source.
A TT electrode may have a resistance of tens of ohms, so a fault cannot draw enough current to trip a breaker, and residual current devices are required for fault protection. Which system you have, and where a neutral-earth link is permitted, is fixed by the supply company and the national wiring rules.
Be careful with what a voltmeter or a plug-in socket tester can tell you. A small neutral-to-earth voltage under load is normal; exactly 0 V under heavy load at a socket far from the board can actually be a hint that someone has linked the two locally. A simple tester cannot distinguish a proper earth from such a link, and it cannot measure how good the earth is.
The meaningful tests are earth fault loop impedance and RCD trip time, done with an installation tester by someone qualified to work on the installation. If you suspect a missing or linked earth, have it inspected; do not work on it live.
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