Why does my RCDs Trip?

[rickie]

I have a 17th edition Consumer unit fitted with Upstairs circuits on one RCD and the Downstairs circuits on the other RCD. Works fine until I switch on the landing light and then both RCDs trip taking out all the power in the house. If there is no bulb fitted no problems except that im not getting any light.

Anyway, I have spoken to 2 electricians. 1 is adamant that it is either the feed or load cable at that light. the other says it is the switch wires. Are either of them right? I just want someone who knows what the problem is and how to fix it.

[therron57]

An RCD detects any difference in current between the hot wire and the common wire. A difference would mean that some current is not flowing in the proper path, indicating that some current is flowing to ground. If a difference is detected, the RCD trips, disconnecting the circuit. An MCD trips only if excess current is detected.

I suspect that there is a wiring error which has caused the common side of the landing light to be connected to the common of the RCD other that is not the one that supplies the hot connection. That means that the current for that light flows through the hot side of one RCD and the common side of the other, causing an unbalance in both.

If the problem started when the RCDs were installed, the wiring error has probably been there all along. If the problem started after some other work was done, a common wire may have been moved during that work.

I can't think of another possibility that would cause both RCDs to trip, but that doesn't mean that there isn't another possibility.

[otis]

get the opinion of a third. That way you can select the opinion of two out of three !

[marmion]

they are both right (or rather 1/2 right)

As Vshade says - it looks like the live feed to the light is connected to one of the RCD's and the neutral to the other RCD, causing an imbalance in both whan the light is switched on
(no bulb = no change in current in live or neutral, therefore no imbalance)

(Get an electrician to check it, but dont get electrician 2 - all he did was state the bl//ding obvious.
- And you MUST by law get a licensed electrician - cos it will take expertise to trace the wiring.
We had a similar problem - took a day to trace and check all the cable routing to find the crossover in the circuits.)

[delton]

Is the landing light by any chance controlled by two switches, one upstairs and one downstairs?

If so, is the downstairs switch one of several switches on a common plate?