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What is wrong with an electrical circuit that pops the 15A breaker each time a light bulb fails?


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2011-05-15 10:08:11
2011-05-15 10:08:11

Circuit breakers can often trip when a light bulb fails just at the point when you switch on the light. The reason it happens is that, in normal use, the bulb's filament gets thinner and thinner and more and more brittle over time.

Eventually it is about to fail - also known as "burn out" - but it doesn't usually do it whilst it is still burning because it's resistance is at its highest when it is hot and "lit", so it is taking the least amount of current whilst it is "on".

But then, if you switch that bulb off as normal, when you next come to switch it on its filament will be cold and its resistance will be lower than when it was hot. At the point of switching it on it draws a higher starting current for the last time but then, as the filament finally "snaps", the "flying ends" of the snapped filament can sometimes momentarily short-out the hot and neutral filament feed wires, taking a very high current which trips the circuit breaker!

Any incandescent light bulb will draw a higher starting current when you first switch it on because, when cold, the filament resistance is at its lowest point. The bulb soon heats up to its normal running temperature at which point the current settles to its normal running current which is lower than its cold starting current.

But in the case of a bulb that has been used for near to - or more than - its designed number of hours, then it is much more likely to fail in a way that causes the circuit breaker to trip, as has been described above.

Provided they have not been "knocked or dropped" at some time during their life - which would significantly shorten their actual expected "life" because of mechanical damage to the filament - ordinary incandescent bulbs are rated for only about 1000 hours of actual use. More expensive "long-life bulbs are available that are rated at 2000 hours.

A different answer

If your home is new and has the new type of arc-fault breakers, this occurs from time to time when the filament in the bulb blows because the failing bulb causes an arc and the breaker does what it was designed to do: stop the arc.


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