RCD tripping when a bulb blows

[tony sayer]

Got into a silly argument with an electrician the other day re RCD nuisance tripping. In his humble opinion RCD's were a PITA 'cos anything and everything will cause them to trip and "every bl**dy bulb that fails will cause them to trip".

A bulb failing?. Never heard of this so has anyone else and assuming quite reasonably that as a bulb could only short out the live and neutral with no apparent earth fault current, is this a possibility or was he being a prat?....

As he seems to have been with some of the waffle he's come up with....

[Andrew Gabriel]

It would be amusing to probe a little and see if he really understands the difference between an RCD and an MCB...

It's not a completely impossibility. If there was some high impedance cross-connect between neutral and earth which could pass enough current to trip the RCD only when the the neutral-earth voltage rose dramatically during a fault current, that might do it. So if he's referring to one installation, it's not impossible, but clearly _most_ installations don't have this issue.

[TheScullster]

Not an electrician, but sounds like he's confusing RCDs and MCBs to me! Yes, we too lose all the lights on the ciruit when one bulb goes. My father in law, who is an electrician (retired), has actually rewired his bungalow such that alternate rooms are on a different circuit. This way he only has to open one door to get to a light source.

Phil

[chris French]

In message , TheScullster writes

Try replacing the relevant MCBs with Type C ones

[Peter Parry]

It is not uncommon that a bulb failing causes a 30mA RCD to trip. Mains halogen bulbs seem to be particularly prone to causing it. Somewhere I have a technical report from Siemens explaining the mechanism by which high current pulses can cause a sensitive (30mA) RCD to trip.

[tony sayer]

In article , Peter Parry writes

That would be very interesting to see, can you mail or post it anywhere please Peter?....

[Dave Plowman (News)]

Hmm. Against the perceived opinion here and elsewhere, I put all the lighting circuits on the RCD side of my split load unit. Just that fiddling with a blown bulb on a multi way switched circuit doesn't appeal when you're not certain it's off. Plus the same applies to push on push off dimmers, etc.

I specified Type C MCBs, and although I've had several bulbs blow since, not one has taken the MCB.

[Peter Parry]

If I can find it I will certainly do so.

[kdband]

What exactly are type C MCB's. I too have problems with the MCB tripping everytime a bulb blows but I also get an earload of abuse from SWMBO whenever it happens.; "It never happened with the old fuse-box...."

[fred]

In article , tony sayer writes

Don't know if you're one of those bods here who has some electronics knowledge in their background but it sounds valid and a Common Mode Rejection (Ratio) CMRR problem. The breaker will subtract the live current from the neutral current to get the leakage. In steady state this works fine but when there is a big current spike, like say a (possible) short when a bulb blows, the measurement (subtraction) of live & neutral may be in error by an amount greater than the trip level. Immunity from this kind of unwanted effect is measured as a ratio, the Common Mode Rejection Ratio. I'm sure they work very hard at immunising their breakers from this sort of nuisance trip but if you imagine a spike of say 200A when a bulb blows short circuit then it takes only an error of 0.015% (==76dB CMRR) in the measurement of a very dirty current waveform to cause a trip.

Hope that makes some sense :-)

[PhilÃÂÂÂ]

Yep, I would like to also know this. When my bulbs blow they always trip the MCB for that ring. Right pain in the arse as the switches are in the corner of the basement and it is as scary as hell going down in the night in the dark to flip the switch.

[Chris Bacon]

Buy a torch or torches. Get batteries for them, checking the "best before" dates. Don't use rechargeables. Put the torches where you know you can get them easily, and don't use them for anything but emergencies.

[Jim Scott]

Chris Bacon wrote in news:42bab087$1 [e-mail]:

OR buy a big rechargable torch with beam and fluorescent. You can leave it on charge in the basement so that when the power fails the fluorescent comes on. S'easy.

[Adrian Berry]

Or better still, put basement light on a fused spur from a ring cicuit....

[Tony Bryer]

Get the basement light(s) put on their own circuit

[Adrian Berry]

But only if the power goes - not if the lighting circuit trips.... :-)

[John Rumm]

The "correct" way would be:

if you want to wire and use it as an ordinary light as well:

to the ligting circuit that powers the lights in the vicinity of the CU. That way when you loose that circuit the light will come one.

(A type C MCB will be cheaper though ;-)

[Dave Plowman (News)]

You've no lights in the basement?

[John Rumm]

A type C requires a bigger overload to activate its "instantaneous trip", and so is more tolerant of short duration surges.

Tell SWMBO that when the reason that the circuit is tripping is because it is her connected to it and not a failing light bulb she may be thankful!

[John Rumm]

How does that help? Since the RCD ought not trip on a bulb blowing anyway?

(or do you just mean that the circuit may still be live but the consequences of sticking your finger in the lamp fitting, or the bulb collapsing in your hand are now less serious?)