MCB type B or C

I had a cap-up bulb fail with a blue flash, then fall, capless, onto the floor, having melted its own wires. Didn't blow the fuse funnily enough.

Andy.

Reply to
andrewpreece
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Every time a lamp 'pops' my Hager type 6B MCB trips, can I change it for a Type 6C?

Peter

Reply to
Peter Andrews

Probably -- need to check the tables in the on-site guide against max circuit length which I don't have on me.

I always fit 6C MCBs for lighting when installing a CU. It doesn't prevent trips when a lamp blows, but it might reduce the incidence of them.

Other possibilities are to fit a BS1361 cartridge fuse carrier in place of the MCB, or to change to using compact fluorescents instead of filament lamps.

Reply to
Andrew Gabriel

I suspect your lamp that "pops" is cap-down? o The bulb screws down into a light fitting o The bulb thus has the cap at the bottom o So when the element blows, bits fall onto the bottom feed o Thus creating a momentary surge high enough to trip a Type-B

Is the bulb in question is that produced by General Electric? :-) o They have taken the price of darkness contract from Lucas o Their minimum life bid was found to put them in the dark

Yes, solved it for me - although it didn't always trip on Type-B. With a refit they are back on Type-B, doesn't always trip.

In most instances ok, check the curves if worried (RS has them). Commonly used if you fit halogen/spot-transformers & such.

Reply to
Dorothy Bradbury

This is the wrong mechanism, and wouldn't trip the fault current (magnetic) detection in an MCB.

What happens is that as the filament breaks, a small spark jumps the gap. The spark starts an arc in the gas fill. The arc has a much lower impedance than the filamant and the two ends of the arc very rapidly run in opposite directions along the filamant until they reach the lead-in wires. You now have an unballasted discharge lamp, whose impedance rapidly drops as the arc current increases, limited only by the supply impedance, so this is basically a short circuit. This all happens in a tiny fraction of a mains cycle. You will get the characteristic flash from the arc discharge and pop sound from the rapidly heating gas fill. In very rare cases, the bulb envelope can break too, but I have only experienced this once in a regular GLS lamp. The in-fine fuse in the lamp base will normally blow when this arc happens, but an MCB has a faster trip curve in the fault current zone than a fuse, so often the MCB will beat the lamp base fuse, or at least it's a draw.

Some lamp types have no space to include a fuse (halogens in some forms particularly). Halogens tend to be less likely to suffer this because the higher gas fill pressure and thicker filamant found in what are typically higher power lamps reduces the chance of the spark turning into an arc, but when they do, they are more likely to explode.

Reply to
Andrew Gabriel

Ah, and as such takes type-B breaker with it v internal fuse, but not the older cartridge fuses re trip characteric.

I've had a 500W large clear heating lamp explode on failure, however as one instance it might have been something else. It was in a very cold environment, just at turn-on.

Thanks for the correction. I noticed that cap-up bulbs failed in the same flash & pop manner.

Thus defeating the falling-metal hitting feed wires theory :-)

I've seen photo studio lights explode - both flash & continuous.

Thanks :-)

Reply to
Dorothy Bradbury

What a pity !

DG :-)

Reply to
Derek *

The other reason for lamps exploding at switchon is that since last switchoff, the seal has failed due to a small crack and the bulb gradually filled with air to atmospheric pressure. At switchon, this air rapidly heats and expands, far too quickly to get back through the crack, and causes the bulb to shatter (possbly helped by the crack already present). Often the failure is quite a neat removal of the envelope from the lamp base.

This was a significant problem back in the days of carbon filament lamps. In addition to the expansion due to heating, you had the rapid production of carbon dioxide as the filament burned up, and these two effects together cause quite an explosion of the glass envelope, rather than the shatter you might see today. Combine this with rather less well made bulbs than we have today, and this was not at all uncommon. This caused many people be be quite scared of actually switching a light on, as there was a not insignificant chance it would explode at switchon.

Reply to
Andrew Gabriel

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