# Why circuit breaker shuts off when lamp burns out?

by Charles123
Tags: breaker, burns, circuit, lamp, shuts
 P: 140 Isn't the case that when the lamp filament melts the resistance of that part of the circuit goes to infinite? So why does the circuit breaker turn off current? Thank you Regards
PF Gold
P: 1,827
 Quote by Charles123 Isn't the case that when the lamp filament melts the resistance of that part of the circuit goes to infinite? So why does the circuit breaker turn off current? Thank you Regards
The answer to your first question is yes, when a lamp filament melts the lamp's resistance goes to infinity (open).

Now, to answer your second question will you please describe where is the circuit breaker? What equipment is this? We can't know why the CB trips until you tell us the details.
P: 3,002
from http://www.megavolt.co.il/Tips_and_i...andescent.html

 Why does my breaker jump when my bulb burns out? Sometimes the bulb explodes too. When the filament in the bulb breaks, an arc sometimes forms, expanding across the entire broken filament. While drawing a few hundred amperes of current, the arc starts to get unstable and melts what is left of the filament, in a bright blue glow. This will usually cause the bulb to burst and the circuit breaker to trip. Most good quality bulbs have a small built-in fuse inside the base that will normally burn before your breaker has a chance to trip. Cheaper bulbs have poor quality control and no built-in fuse in the base.

http://www.dumet.net/lamps.html

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## Why circuit breaker shuts off when lamp burns out?

Thank you, Old Jim. Never imagined that arcing scenario drawing hundreds of amps and tripping the breaker!

Us old dogs CAN learn new tricks!

Bobbywhy
 Sci Advisor P: 3,002 Thanks for the kind words Bobbywhy ! I have to think the hundreds of amps scenario is the exception. I've only seen two lamps blow their tops off. But we all know that "flash-thud" when they go. I didnt know about the internal fuse until found that link. A good reason to buy the $1.59 lamp instead of the$0.49 for locations where falling glass shards could be an annoyance.. but the point is, an arc conducts well. Only after it goes out do you see infinite resistance. That should be at next zero crossing of sinewave.

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