Diagnosing cause of RCD trip

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u33446z

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Scenario: lighting circuit serving several rooms including shower room. Shower room light switch also controls extractor fan. After a use of shower room in a morning (no other room lights in use), shower room lights and fan ceased to operate, since RCD had tripped.

I disconnected the fan at the fan, and disconnected the light switch, but still the RCD would not return to its 'on' position.

What could be the problem?
 
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What circuits ,other than lighting circuit,does the RCD protect ? Or is it an MCB or RCBO for the lighting circuit that tripped and you are incorrectly calling it an RCD ?
 
IMG_1044.jpg

Perhaps I used the wrong term?
 
Its an RCBO , (functions like an RCD and MCB combined ,giving overload protection as well as earth leakage protection ). Has any new work been recently carried out on this circuit ,which required this device to be fitted ?
 
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No, the device was fitted about 14 years ago. Last year I changed all the lights in the shower room from halogen to LED and installed a different extractor fan.
 
Probably failed power supply for the led stuff- if it has failed N-E then disconnecting the switch won't clear the fault. Tho saying that, most of those things don't have an earth....still probably N-E fault somewhere, did you disconnect all poles of the fan or just the live?
 
The fault that caused the trip can be anywhere on the circuit ,not necessarily In the shower room ,and can be a short circuit or earth leakage fault.If you switch off the main switch ( the red one that isolates all circuits ) then switch the rcbo to on ,then put the main switch back on ,does the rcbo trip instantly ?
 
I disconnected all the poles of the fan.

If I switch off main switch, move rcbo to on, then main switch to on, rcbo trips immediately.

One more thing: when I removed the cover from the fan terminal block I found a small pool of water in the cover which must have come from the fan itself, since the loft environment is dry. The fan supplier has accepted that is a fault and I'm preparing to return the fan for a replacement.
 
Water present at fan connections can certainly cause the rcbo to trip. You need to find out where the water is coming from. Is it a ceiling fan or wall ? Does it have a timer ,continues to run after light is switched off ?
 
The water must have come through the gland in the casing that the wires use. The fan is a Manrose Mixed Flow in-line fan mounted on a joist in the loft. It has a timer so runs on after switch-off. But the fan is no longer connected electrically and yet the circuit fault remains.
 
Is there a triple pole isolator for the fan ? If you have disconnected the cables at the fan ,and the conductors / cable have been exposed to water you will need to isolate or disconnect the conductors at the feed . And you still need to establish where the water has come from.
 
The fan wires are disconnected at the fan. The wires at the light/fan switch are also disconnected (is this the 'feed' you asked about?)
 
No. The cable going to the fan will have four conductors. Permanent line ,switched line ,neutral ,and earth. Disconnecting the light switch will only take the switched line out of the equation . The other three conductors are still connected back to the consumer unit .Although the fan is not now connected ,and no longer part of the circuit ,its the moisture in the cables conductors that's suspect.
If indeed this is the fault ,it may be something else completely.
 
Last edited:
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