So just now I was adding an extra light over my day bed it's a vintage 60 watt bulb looks like an old radio valve it even has the little glass pip on top. Anyhow as I was wiring it up two whiskas of wire touched there was barely any sound but it popped the 5 amp fuse in the plug adaptor and tripped 16 amp MCB how is this even possible even under partial short circuit conditions surely a 5 amp fuse should clear before a 16 amp MCB?
I just did a quick selectivity study and the fuse should have beaten the MCB as in theory full selectivity is achieved. But that is only theoretical and in realatity, as you have proved, theory is based on variables not all of which can be quantified. I have seen recently a sparks drilling a hole in trunking hit a cable inside and blow the MCB on the board and the MCCB back at the main panel. He also needed a new drill bit.
In our study/home office there are two extremely well ventilated ceiling fittings, each with 4 GU10 lamps. For some reason (I am now suspecting either over-voltage and spikes) when tungsten GU10s, they never lasted that long, but inevitably when one failed, usually at switch on, it would take the 6A MCB with it. Seems to suggest that as the filament failed it arced across, but is that the reason? From Tungsten went to compact GU10 florescents and now LEDs. But the LEDs do not have the expected life, nor did the compact florescents. Their electronics should have handled the over-voltage (we are not far from the sub-station, whilst within limits, generally around 243 volts. So perhaps spikes?
Anyway, my experience is that when a lamp fails, it causes a current surge such as to blow the MCB. In your case, I am assuming that the MCB was not supplying anything else?
I have seen recently a sparks drilling a hole in trunking hit a cable inside and blow the MCB on the board and the MCCB back at the main panel. He also needed a new drill bit.
Please allow me to rephrase that.
I stood there with my hands in my pockets watching a sparks drill through a live cable. ?
Concerning selectivity, that is all for faults of negligible impedance, so perhaps faults of not so negligible impedance behave differently?
Concerning GU10s, filaments blew much more often than standard (non-halogen) ones. CFLs seemed to go dim before failing far too soon. LEDs have been much the best - I don't think that I have had to replace one yet.
Looking on the bright side you only had to reset the MCB rather than find a spare fuse.
Andy Betteridge
This was part of an answer to a question on the part 1 236 C&G electrical technology paper. I now feel that the click of an MCB tripping out is really only half the story. A jolly good bang rams home the seriousness of the situation. and other indicators such flash overs may indicate the point of the fault.
I still think the use of BS1361 cartridge fuses is still vaild.....
Chris. Yes, My compact florescent certainly went dim prior to failing, But LED's - well despite buying what I believed were a decent brand "Crompton" I have been through about two dozen GU10s in our study/home office. It certainly is overheating. Imagine a glass plate about 4-inches away from the ceiling with 4 holes drilled through the glass. In each hole is a spider supporting a GU10 holder. Plenty of air circulation. Also three wall lights. However not just the GU10s but also 4 foot and 5 foot flourescent replacement LED tubes. Have had 2 of those fail, one in the kitchen and one in the bathroom, so that is three different circuits/MCBs.
Our supply is underground TN-S So whilst not as likely as TT for weather related surges am wondering whether I should have a SPG device fitted?
