Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

There is no way that the broken filament can arc internally to the
bulb's bayonet/earth is there?

In article ,
David J wrote:
Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

I presume you mean the RCD rather than switch?

There is no way that the broken filament can arc internally to the
bulb's bayonet/earth is there?

Any other lights on the same circuit at the same time? If something with a
capacitor etc it's possible that caused a current imbalance when the
single pole MCB trips.

--
*Hard work pays off in the future. Laziness pays off now *

Dave Plowman London SW
To e-mail, change noise into sound.

On Mon, 28 Sep 2009 13:11:29 +0100, "Dave Plowman (News)"
wrote:

In article ,
David J wrote:
Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

I presume you mean the RCD rather than switch?

I probably do mean that. It looks like a 'switch'.

There is no way that the broken filament can arc internally to the
bulb's bayonet/earth is there?

Any other lights on the same circuit at the same time? If something with a
capacitor etc it's possible that caused a current imbalance when the
single pole MCB trips.

Just a chandalier type of light fitting with 4 of those candle-shaped
bulbs. I can understand a spike might get generated when the filament
ruptures, but why should that trip the entire house supply. It's not
the first time this has occurred.

David J

On Sep 28, 11:19 pm, David J wrote:
Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

There is no way that the broken filament can arc internally to the
bulb's bayonet/earth is there?

Was the bulb pointing upwards or downwards? If it's pointing upwards,
bits of broken filament could short out the two solid wires going to
the base, and that could melt a wire outside the bulb and it might
touch the earthed base. I'm sure I've had bulbs blow a fuse even when
hanging downwards.

When a filament is simply broken, I have a party trick where I plug
the bulb in and rotate it so the broken ends touch, and magically the
bulb works again! It works for quite a few hours, good for an
emergency when there are no new bulbs.

On Mon, 28 Sep 2009 12:19:07 +0100, David J wrote:

Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

What do you exactly mean by "main circuit switch" and "lighting
circuit protection"?

The arc in a expiring tungsten light bulb is very low impedance so
there is quite a surge in current, this can easyly trip an MCB.

RCD trips are also possible depending on how high the existing
leakage is. These days of everything having SMPSU's and filter
components between all three mains wires the standing leakage through
an RCD serveing a couple of rings etc can be quite high and thus the
RCD only needing a little push to trip it.

--
Cheers
Dave.

On Mon, 28 Sep 2009 12:19:07 +0100, David J
wrote:

Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

When the filament fails it usually does so at one point only. As it
fails the gap is initially small enough for an arc to form and be
maintained for a very short time drawing a very high current.
Incandescent bulbs have a built in fuse (a thinner section of wire)
designed to burn out if this happens..

During the fractional time this takes to happen the current drawn by
the bulb can often be over 100A for a few cycles and very unstable.
This is not of sufficient duration to trip the 5A MCB but the very
fast rise in current and rapid fluctuation in current causes a variety
of normally insignificant capacitive and inductive effects in the
wiring to generate a current imbalance in the supply wires for a
fraction of a second.

As an RCD will trip in 2 cycles (40mS - 10 times or more faster than
an MCB will react) the RCD will therefore often trip when a bulb
blows. Some light fittings will usually be more susceptible to this
than others depending upon the wiring layout in the house so it isn't
unusual to find one lamp fitting that will nearly always trip an RCD
when a bulb fails.

On Mon, 28 Sep 2009 13:58:27 +0100 (BST), "Dave Liquorice"
wrote:

On Mon, 28 Sep 2009 12:19:07 +0100, David J wrote:

Can anyone explain why a failing (tungsten) bulb should trip the main
circuit switch, and not just the lighting circuit protection?

What do you exactly mean by "main circuit switch" and "lighting
circuit protection"?

Sorry - I'm using technical terms very loosely here. The box appears
to have individual circuit protection (just like the old fusewire
days) plus one device that covers everything. I guess that is the
RCD...


The arc in a expiring tungsten light bulb is very low impedance so
there is quite a surge in current, this can easyly trip an MCB.

RCD trips are also possible depending on how high the existing
leakage is. These days of everything having SMPSU's and filter
components between all three mains wires the standing leakage through
an RCD serveing a couple of rings etc can be quite high and thus the
RCD only needing a little push to trip it.

Thanks..