Why is there a potential difference between neutral and ground (earth)?

Question

I was surprised to find that bridging neutral and ground in a socket at home tripped the RCD. Measuring with a multimeter, there is indeed 0.1 V between them. If the RCD trips at 30mA that would mean that the wiring in the house must have resistance lower than 3 ohms. (Sounds likely if household wiring is around 0.01 ohm/meter.) So the questions are:

1) why is there a p.d. at all?

2) isn't it a problem for doing electrical work, since switching off the circuit breaker for the ring only disconnects the live wire, meaning the RCD can still trip the whole house if the neutral and ground wires touch?

EDIT: since there are different earthing standard apparently, this question relates to the UK and to a normal urban setup (2-wire 1-phase mains). Also, the live wire is disconnected on the ring. I think the RCD trips because the (small) potential difference between the neutral and earth creates a small but big enough current through the neutral wire, which is not balanced by current in the live wire.

Answer 1

1) Why would you expect a potential difference not to appear? N and earth are tied together only in distribution/transformer boxes, that's a long way till your house, the N wire is usually carrying some current so its potential might slightly differ from ground.

2) If you call tripping the whole house an issue, well that's an issue then. I'd say that safety is not compromised at all, it is instead enforced by this behaviour. Disconnecting also the N through the circuit breaker would work.

Answer 2

With earth and neutral being tied at a distribution panel but the neutral being used to close the power circuit with live, there will be a potential difference but that will not be why the RCD tripped.

An RCD checks for a balanced current in the live and neutral. If you have paralleled live and earth there will be an imbalance in the current and thus the RCD trips.

Answer 3

1) Because the currents in the neutral and ground are different.

2) Switching off the live wire means that there will be no current in the neutrals served by that breaker, effectively floating them, so - other than a momentary spike caused by charge accumulated on the neutrals - touching them to ground shouldn't affect the RCD's balance.