19 Sep 2011
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United Kingdom
Hello all
Just trying to increase my knowledge!

In a house that i own there are a few circuits ending back at the Consumer unit.
There are

3 Ring mains, protected by 32Amp MCB
3 Lighting radial circuit Protected by 6 AMP MCB
1 cooker Radial curcuit protected by 32 amp MCB
1 Garage supply Protected by 32 Amp MCB (4mm swa cable)

There is no RCD protection...The instalation is 15 years old!!
My question for an explanation is that on the odd occasion i will turn a light on, and the lamp "blows" this causes a 6 amp MCB to trip
As i understand a MCB is Amp rated so that if this rating is exeeded it will trip

An example may be running 14 no 100 watt lamps on a 240 volt supply on a 6amp protected radial curcuit would be ok, but running 15 no would draw 6.25 Amps causing the MCB to trip (hope this is a correct assessment)

so my question is, What exactly happens when i turn on that single light and the 6 AMP breaker trips, ???
Probably basic stuff!!

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when the filament melts and vaporises it causes a momentary surge. Running 15 lamps would not cause an immediate trip, there is some tolerance.

better quality light bulbs should have an internal fuse that reduces the frquency this happens, but modern MCBs are very fast-acting and will often trip when an old-fashioned filament bulb blows. It seems to happen much more with spotlights.

When you change over to CFLs (energy-saving lamps) this will no longer occur.
I believe its just a function of the surge in current that occurs when a lamp blows, and its fairly common.
There you go, simply a current surge!!

Just to extendthis a bit further!, any idea what sort of rating this surge may be??
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Hello all
Just trying to increase my knowledge!
Starting with the humble light bulb. These should have a fuse built in as the BA22d lamp holder (in the main) is only rated at 2A. However in the real world often these fuses are not there or set too high to rupture before the MCB trips.

A B6 MCB will trip and 5 x 6 = 30A on its magnetic part so even if 30A is only exceeded for a fraction of a second the MCB will trip. This was not the case with the humble fuse. Using a C6 (10 x 6 = 60A) or a D6 (20 x 6 = 120A) may stop the trip opening but much would depend on the earth loop impedance to if these could be fitted.

The problem is what is called "ionisation" in this case inside the bulb but it can also happen outside the bulb and is the biggest worry for electricians. This is why for live working so much PPE has to be worn. The most noticed example of ionisation of the atmosphere is the thunder storm. But this is why some times when a bulb blows we hear it go pop.

The fuses should also be fitted in CFU but it seems often they are not. I had one from Ikea weld to the BA22d contacts when it failed supplied from a 16A MCB. As a result using a C or D rated MCB may stop it tripping but it could also result in having to change bulb holder with the bulb.

In the main the CFU does not take the MCB out when it fails so the problem will in the main go away as we move over to the environment unfriendly mercury filled new type of lamp.
mercury filled new type of lamp.
the old CFLs held about enough mercury to make this dot ------> .

The new ones contain considerably less.

Interestingly, coal-fired power stations emit more incidental mercury, in generating the power used in the life of a filament lamp, than a CFL contains.
The problem is what is called "ionisation" in this case inside the bulb ...
That probably needs explaining.

When the filament melts there will be a spark between the ends of the broken wire. Some of the metal will vapourise and form a nice conductive cloud. Given the right circumstances, you can get quite an ark going inside the light as a conductive path forms an effectively unrestricted connection between the two terminals - so a "sort of short circuit" across the supply and "click" goes your breaker.

For a somewhat more dramatic demonstration, go to this video of a deliberately induced arc. The spacing between the busbars is several orders of magnitude more than the voltage could possibly jump on it's own - so they created a conductive path with a fine copper wire.
My thanks to those who have expanded on what I had said. As to mercury I would agree the coal releases mercury and if we assume all power comes from coal powered power stations then that's a valid argument.

However all power does not come from coal powered power stations. And also the heat produced by the standard bulb does not in the main go to waste. In fact the radiated heat unlike the convected heat from a radiator heats the body rather than the air. So the heat from the tungsten bulb could reduce the ambulant air temperature by at least 2 degrees needed to make a comfortable temperature to live in. So in real terms the old tungsten bulb could well be more environmental friendly than the CFU.

With tungsten lamps often 18 deg C is a comfort zone but with CFU often this is raised to 20 deg C the energy required to lift the room temperature this 2 degrees is far more than the energy so called wasted running a tungsten rather than CFU lamp.

So in many cases the tungsten lamp saves more energy than the CFU during the winter months. I did ask the energy saving trust for info on this and they admitted that tests has not been completed and the outcome would depend on the number of air changes per hour. So the whole idea of saving energy by using CFU's is flawed.

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