5 amp fuse on lighting circuit?

Ionisation of the gases inside a tungsten bulb can cause a near short circuit when the filament ruptures, but there should be a fuse built into the bulb which should rupture before the main 5 amp one does, but since the ceiling rose is used as a junction box and it is rated 5 amp there is no real option but use a 5 amp fuse for lighting, when the fuse is replaced with an MCB/RCBO 6 amp is normally used, and I found in the old days of tungsten the 6 amp MCB would trip before the internal fuse in the bulb ruptured, but as we moved to CFL and LED this stopped. I did have an Ikea 6 watt CFL in an outside lamp powered from a 16 amp MCB, maximum allowed for a BA22d fitting, it went short circuit and welded itself onto the pins of the holder, so would say good reason for type B 6 amp MCB on lighting.

Most BA22d lamp holders are rated 2 amp, and rely on the bulbs internal fuse, but think many imported bulbs don't have the built in fuses any more.
 
I found it used to be non-standard bulbs, like miniature spotlights, most likely to blow fuses.
 
Blown lamps trip MCBs because the arc created as the filament snaps vaporises metal and in the confirmed space of a halogen capsule there is enough concentration of metal vapour to create a plasma discharge. This is effectively a dead short and the current through the plasma becomes very high until the MCB operates.

By contrast the volume inside the old large globe filaments lamps is large enough that the concentration of vapourised metal is far too low to allow a plasma discharge to form.

Sobering fact that with a 6 Amp MCB protecting the circuit the plasma discharge will be creating at least 1.5kW of heat until the MCB trips.
 
EFLImpudence said:
13A sockets on 32A circuits?
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It is the outlet from socket limited to 13 amp, in the same way as outlet from a BS22d limited to 2 amp, the supply to sockets can be 32 amp, in same way as supply to rose can be 5 amp.

I do see your point, is the 5 amp marked on a ceiling rose the limit to the BA22d lamp holder or limit to supply to next ceiling rose, however it is the lamp holder be it BA22d, E14, or E27 which is the outlet, not the ceiling rose, and in essence the ceiling rose is simply a junction box, so if junction box marked 5 amp then supply fused to 5 amp, so if ceiling rose marked 5 amp, again supply fused to 5 amp.

The question does arise if ceiling rose 5 amp can you use a 6 amp MCB, same as if a fig of 8 connector rated at 2.5 amp can you use a 3 amp fuse, in both cases some common sense must be used, getting a 5 amp MCB or RCBO would be very hard, as would a 2.5 amp fuse for a plug, so common sense says 6 amp and 3 amp can be used.
 
bernardgreen said:
Sobering fact that with a 6 Amp MCB protecting the circuit the plasma discharge will be creating at least 1.5kW of heat until the MCB trips.
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I'm not sure how you worked that out, or what you think the rating of the MCB has got (directly) to do with it.

As I need not tell you, an MCB does nothing to limit the amount of current that can flow which, in the case of a 'dead short', will be determined only by the supply voltage and (L-N) loop impedance.

If the circuit has been designed correctly, then the L-E loop resistance (as measured from the place of interest) needs to be low enough for the MCB to trip magnetically in response to a 'dead short' at that location - so, to cater for the worst case, 5 x 6A = 30A for a 6A MCB. However, since CPCs are invariably smaller than the corresponding N conductors, the L-N loop impedance will be appreciably lower than the L-E loop impedance - so, if the circuit (protected by a 6A MCB) is correctly designed (in relation to L-E faults), the current resulting from a L-N 'dead short' will be appreciably greater than 30A, and that current will flow until the MCB trips.

Hence, in that situation, the power generated (primarily in the cables) will be well in excess of 6.9 kW, rather than the modest 'at least 1.5 kW' you suggest. Having said that, the MCB will trip very quickly under those conditions, and 6.9+ kW dissipated (primarily in the cables) for a handful of milliseconds will not amount to very much energy, so probably not even a perceptible temperature rise.

Kind Regards, John
 
John
I should have been more accurate and said the plasma discharge would be extremely low impedance instead of describing it as a short circuit.

The plasma discharges in a halogen capsule can be hot enough and last long enough to melt the quartz envelope before the current is high enough to cause the MCB to operate.

JohnW2 said:
to cater for the worst case, 5 x 6A = 30A for a 6A MCB.
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with about 230 volts ( less drop along the cables ) differential at the ends of the 30 Amp plasma discharge there could be 6.9kW of heat generated in the plasma discharge. That is when the capsule can explode producing a shower of molten quartz.
 
bernardgreen said:
John, .... I should have been more accurate and said the plasma discharge would be extremely low impedance instead of describing it as a short circuit. .... with about 230 volts ( less drop along the cables ) differential at the ends of the 30 Amp plasma discharge there could be 6.9kW of heat generated in the plasma discharge.
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As I said, I agree with the 6.9+ kW figure. However, I'm not quite sure where you have moved the goalposts to. As I said, if the arc represented "an extremely low impedance" (which both of us described as 'short circuit') then almost all of the circuit's impedance (hence 'voltage drop', and hence {2} power dissipated) would be in the cable (all the ay back to substation), not the arc - in which case most of the 6.9+ kW would be dissipated in the cable - which, as I said, sustained for just a few milliseconds would probably not represent enough energy to produce a perceptible temperature rise.

The only way in which most of the 6.9 kW could be dissipated in the arc would be if, far from being "extremely small", the impedance of the arc was considerably greater than the impedance of the wiring (all the way back to the substation, which might well be 'a few ohms' in a lighting circuit). Are you really suggesting that? (I thought that, as you say, arcs were effectively of "extremely low impedance").

Kind Regards, John
 
The lamp holders used in pendant sets are usually rated at no more than 2A.
 
securespark said:
The lamp holders used in pendant sets are usually rated at no more than 2A.
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Even if that were true (and I find it hard to believe it is), I would have to wonder what 'rated at' would actually meant in that context, since it's very hard to think of how, even if one wanted to, one could design a lampholder which could not safely 'tolerate' a lot more than 2A!

It is, of course, moot, since even in the days of incandescent bulbs/lamps, there was no way that the 'normal running current' through a lampholder could be anywhere near even 2A - and, as for fault currents, they haver always been (and always will be) many dozens of amps, at least.

Kind Regards, John
 
JohnD said:
this is for your candelabra? No, they all seem to have the thick white base.

apart from the more expensive "imitation filament" ones

https://www.wickes.co.uk/search?text=candle
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Strange concerning the prices quoted on your reference for "imitation Filament" LEDs, since prices in Australia are usually higher than in the UK or USA.

The following Osram lamps are available here (with different bases) and they all seem to be AUD $5.90 (GBP £3.30)
https://www.bunnings.com.au/osram-4w-470lm-warm-white-frosted-sbc-filament-led-candle_p0122264
https://www.bunnings.com.au/globe-led-candle-fila-osram-4w-frosted-470l-ww-ses-light-globe_p0122272
https://www.bunnings.com.au/osram-4w-470lm-ww-frosted-filament-led-candle-es-light-globe_p0122234
https://www.bunnings.com.au/osram-led-filament-candle-4w-470lm-ww-bc-light-globe_p0122232

It is my understanding that Osrem owns Sylvania - or vice versa.
 
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