In one bedroom we have a ceiling fan supplied from the lighting circuit. It's been there for over 10 years no problems. In the past few nights a couple of times when switching it off the RCD protecting the ring mains has tripped. But the lighting circuits don't go through it and the lights are still on including the one which is part of the fan and the fan itself can be switched back on. Puzzled as to how this can happen - and where to start looking.
It would suggest that it's not supplied by the lighting circuit...
Tim
Possibly a suppressor in the motor has failed and the resulting inductive spike is coupling into an adjacent circuit. Makes sense as you say it doesn't always happen, probably only when the voltage happens to be on a high part of the AC cycle.
Cheers
Are you sure that your RCD *only* protects the ring mains and not *all* circuits such as cooker, lighting in addition to ring mains?
My understanding is that an RCD in the "fuse box" is usually upstream of the ring main MCBs (over-current sensors, equivalent to wire fuses) and protects the whole house. Our previous house had two RCDs: one was a separate unit that was between the meter and the fuse box, and the other was within the fuse box. Even if the RCD within the fuse box was tripped, all power went off, including to non-ring-main circuits such as lighting and cooker.
What happens if you manually turn off the RCD? Does the lighting still continue working?
Earthed via a the ring-main circuit?
Probably not, RCDs don't just detect earth leakage but any current imbalance between live and neutral.
It wouldn't actually matter where it was earthed if there was leakage to earth and it wouldn't affect other circuit unless they also had an earth leakage problem as RCDs aren't connect to earth.
Quite agree. I think Clive is probably right as to the cause. Within the consumer unit, all the lives and all the neutrals are connected together, so there's scope for some coupling between circuits. The only thing that's really surprising (to me) is that the RCD protecting the lighting circuit does not trip.
(I may be making a fool of myself here. Hope not!)
I wouldn't assume there there is an RCD protecting the lighting circuit. Very many homes don't have them.
Neutral-Earth faults can trip the RCD.
Doesn't explain why it can be switched on again when the RCD trips.
Of course the 18th edition has altered that for future installs (I think you are correct about the OPs lights)
Just for info and not related to the OPs question. Domestic properties are now required to have RCD protection on the lighting circuits even if the cables are run in a manner or of the type that would not require RCD protection (eg in trunking or SWA)
Is that on the basis that it's okay to get electrocuted by the lighting circuit? Perhaps it's a more refined way to die.
Where is the switch for the fan, in the fan/light itself? I'd start there. Brian
I was thinking of back EMF. That could make it back through the lights to the other circuits as a spike of some kind. Brian
Yes those were my thought s as well. Brian
Lighting circuits have generally been classed as safe without RCD protection. For some reason domestic lighting circuits they are now unsafe without RCD protection.
The RCD, when it first came into general use was for outside sockets and using the lawnmower etc. Remember the RCD 13A plug in breakers before RCDs were common place in CUs?
It used to be on the basis of cost-effectiveness: the number of death and serious injuries from lighting circuits was much less than from sockets, showers, cookers etc.
I have no idea if the cost-effectiveness has changed with cheaper RCDs (bearing in mind there are also now safer light fittings and less need to change bulbs) as those who decided such things don't see fit to publish their assessments of costs and benefits of changes - if they even bother to make them rather than going with what they just know makes sense.
I still have a plug-in RCD which I use when working outside. It started off as 30mA but now reliably trips with less than 10mA so usually saves me a walk to the CU. I only wish we could such in the UK (as I think someone raised here before).
Our CU was replaced in 1998, and no RCDs for us. We have a couple of those RCD adapters. I have no idea whether they work, though, as they've never tripped.
The CU looks like one of these:
If the Starbreaker has the plug in MCBs it may be possible to fit a couple of RCBOs.
I was fitting RCD CUs well before 1998.
A photo of your actual CU will help me decide if you can fit an RCBO (height restrictions apply).
As for your plug in RCD breakers. They should have a test button. This is nothing more than a switch to imbalance the load via a resistor. It will not tell you if the RCD is tripping within the required times but it will give you a guide as to it's ability to work.
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