saveasteading Posted April 3 Share Posted April 3 The kitchen lights went off. so I went to the CU and reset the fuse. Usually that means a cooker hood light has failed. The lights went back on but the fan and other bulb still didn't work. On the basis that a chain doesn't break in 2 places I assumed the worst, that the fan had a major fault and the local spur fuse had failed when it was turned back on. (We hear mice crawling thereabouts.) New fan assembly needed? As a matter of course I checked the local spur fuse, which I found was was blown, and changed it, and expected it to immediately fail. But it didn't and it's all working with a replacement bulb. Hoorah. How can a light bulb fail and cause the spur fuse and the CU to ping at the same time? One must fail first and open the circuit. Fundamentally I don't know why a bulb failing (a fuse wire in itself) causes a trip at the fuse box anyway, let alone a ceramic fuse as well. Link to comment Share on other sites More sharing options...
Adsibob Posted April 3 Share Posted April 3 If the fuse spur fails, at the time of failure is it possible it allows an excess amount of power through to the bulb which then fails? I suppose the CU is the final failsafe point, so it must have detected something wasn’t right and did its job. Just my guesswork. 1 Link to comment Share on other sites More sharing options...
dpmiller Posted April 4 Share Posted April 4 exactly what kind of breaker tripped in the CU?, was it an MCB? RCBO? Link to comment Share on other sites More sharing options...
Alan Ambrose Posted April 4 Share Posted April 4 An old-fashioned filament bulb? There’s often a big fast spike of current when they fail and a corresponding flash of light (think thermal runaway) which will take out fuses / trips etc. Not so much of.a problem these days since we don’t use many filiament bulbs. But … a big spike of current can still be a thing when electronic components: capacitors, transformers, resistors etc in modern LED assemblies, dimmers etc go pop. 1 Link to comment Share on other sites More sharing options...
MikeSharp01 Posted April 4 Share Posted April 4 Was it an incandescent lamp or a LED. If the former then it is entirely possible that the blub going will cause a current spike. Triping both the CU and the fuse is unlucky but not impossible given the right circumstances. 1 Link to comment Share on other sites More sharing options...
SteamyTea Posted April 4 Share Posted April 4 8 hours ago, saveasteading said: We hear mice crawling thereabouts. Get that sorted, never a good thing. Link to comment Share on other sites More sharing options...
saveasteading Posted April 4 Author Share Posted April 4 Yes it's a filament bulb. Cooker bulbs all still seem to be, presumably because of heat that would melt an led. Link to comment Share on other sites More sharing options...
ProDave Posted April 4 Share Posted April 4 8 minutes ago, SteamyTea said: Get that sorted, never a good thing. Very hard in an "ordinary" house. A normal build with all the ventilation requirements leaves plenty of mouse sized holes everywhere. Mice in the cold loft was a big problem at the last house. They could climb straight up the rendered walls, and in through the soffit vents that must be there to meet building regs. Remember if a hole is big enough for a biro pen to fit through, so can a mouse. This is the first mouse free house we have ever had. Achieved by making it air tight, and with a warm roof, so the air tight layer follows the roof and the loft space is warm, dry, inside the air tight envelope, and mouse free. We only once had a mouse in, entered through an open velux overnight. Link to comment Share on other sites More sharing options...
SteamyTea Posted April 4 Share Posted April 4 3 minutes ago, ProDave said: Very hard in an "ordinary" house What poisons are for, has the advantage that it discourages friends with dogs visiting. Link to comment Share on other sites More sharing options...
ProDave Posted April 4 Share Posted April 4 13 minutes ago, SteamyTea said: What poisons are for, has the advantage that it discourages friends with dogs visiting. Yes but it did sometimes result in "that smell" of rotting dead mouse somewhere. Link to comment Share on other sites More sharing options...
SteamyTea Posted April 4 Share Posted April 4 (edited) 5 minutes ago, ProDave said: Yes but it did sometimes result in "that smell" of rotting dead mouse somewhere. The smell of victory. Edited April 4 by SteamyTea Link to comment Share on other sites More sharing options...
saveasteading Posted April 4 Author Share Posted April 4 3 minutes ago, ProDave said: that smell" of rotting dead mouse somewhere. Mice one day. Rats a week. I justify the killing on the basis of the following generations avoided that I don't have to poison. They all go outdoors again about now. They are eating bulbs and seedlings now! Link to comment Share on other sites More sharing options...
saveasteading Posted April 4 Author Share Posted April 4 9 hours ago, dpmiller said: exactly what kind of breaker tripped in the CU?, was it an MCB? RCBO? It's an mcb. Link to comment Share on other sites More sharing options...
saveasteading Posted April 4 Author Share Posted April 4 So am I to surmise that when a light bulb filament fails, it is less like a fuse melting and more like a dramatic spark? In this case the surge of that has also caused the 3A spur fuse to fail, but the filament then fell apart, and that has made the mcb trip becsuse it senses a problem.....somehow. 3 links in a chain breaking at the same time. I'm trying for an analogy in chain terms but can't think of one. This because I don't understand electricity, though do believe in it. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted April 4 Share Posted April 4 Well we're in the wrong forum for the detailed behaviour of filament bulbs, fuses and MCBs (try eevblog or some of the more inquisitive physics/electrical youtube channels). Here's the short answer: The bulb is normally ~40W i.e. 0.2A i.e. not much. This is a total guess on my part, but I am going to suggest that the current spiked to, say, 10-30A for, say, 100ms. If you had current clamp and an oscilloscope, and could repeat the experiment at will, you would be able to see the fast spike and estimate the maximum current amplitude. That spike was enough to blow the fuse and trip the trip. The fuse blew in say 800ms, but while that was going on the MCB tripped - yes, the MCB shut off the current, but the fuse was already red hot by that time. There's endless detail (as there is in any subject if you dig deep enough) but here's a bit more: https://physics.stackexchange.com/questions/341134/how-to-calculate-melting-time-of-a-fuse https://library.e.abb.com/public/114371fcc8e0456096db42d614bead67/2CDC400002D0201_view.pdf >>> I'm trying for an analogy in chain terms but can't think of one. OK this: suppose you have a chain with a breaking load of 10T. There are two weak links of various sorts, one 1T, one 0.5T. You suddenly drop a 500T load on the end of the chain (remembering to stand well back). Everything flys apart, both weak links shatter with the shock load and two bits of the rest of the chain shoot past your ears dangerously (the final part is still secured to the ceiling). Theoretically the main chain would break too but the very short time before the weak links break means that there isn't any time to do the work (force x time) to make the main chain break before the weak links give way and remove the force. In this case, the analogy for the main chain is the mains cable. Now, be happy that this worked as it should ... because you didn't melt any mains cable and you didn't have a fire. I think that mice don't nibble cable so much but rats and squirrels like to sharpen their teeth on everything. So best to remove the problem promptly. 1 Link to comment Share on other sites More sharing options...
saveasteading Posted April 4 Author Share Posted April 4 1 hour ago, Alan Ambrose said: mice don't nibble cable so much Something small did this. Link to comment Share on other sites More sharing options...
saveasteading Posted April 4 Author Share Posted April 4 I asked here on purpose as I might understand the reply... And I do, thanks. Re mice and rats. There is a regular visitor to the greenhouse which snipped the head off my one courgette seedling, and dug up ( but didn't eat) the beans planted 2 days ago. This has never happened before so it is a new skill or piece of knowledge, and so poss a small rat. Special food there now as a reward. Link to comment Share on other sites More sharing options...
kandgmitchell Posted April 22 Share Posted April 22 As a self builder haven't you been told to get the professionals in when your skill set isn't enough..... 2 1 Link to comment Share on other sites More sharing options...
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