Protection by RCD

[HandySparks]

Hi Tel.
I quite agree that the duration of the fault is set by the resistance of the fault path and the characteristics of the OCPD. Hence why we measure Zs.

Now, "a line/earth fault will cause the potential of the earthed component to rise to line value, at the point of the fault".

Well, yes, except that a line/earth fault also drags down the line voltage at the point of the fault; so the local line voltage is reduced. So, say for example, the line and earth parts of the fault path are of equal resistance, then the voltage at the fault will be half the supply voltage. In effect, the resistance of the two parts of the fault path form a potential divider across the supply.

Or am I misunderstanding?

 

[Deleted member 26818]

In a healthy circuit, the line and neutral have similar if not equal resistance.
Would this combined resistance halve the supply voltage?
In many instances, the CPC has a smaller CSA than the live conductors, as such some volt drop would be expected, but not by a significant amount.
In many cases where an earth fault occurs, the neutral is still connected.
In such cases, the CPC would become a parallel conductor with the neutral, as such reducing resistance.