It's one half of a C-H double 20 A (two circuits in one breaker position) and it failed in such a way that there's no way to switch it off. Any idea what could cause this? I've seen lots of breakers fail closed (ie no current) but never seen one fail open like this.
Possible hint: The circuit (AWG 12) had an outlet added recently for a one-room A-C. Plate says 10 A. Three other outlets on the circuit with floor fan, radio, clock, computer so with A-C going the circuit is getting plenty of work.
and it failed in such a way that there's no way to switch it off
This condition is failed closed not failed open.
This is a very rare failure, replace the circuit breaker immediately.
one-room A-C. Plate says 10 A. Three other outlets on the circuit with floor fan, radio, clock, computer so with A-C going the circuit is getting plenty of work.
The continuous operating budget for your 20A circuit is 16A. Your floor fan, radio, clock, and computer most likely do not exceed the remaining 6A.
While it is ideal for the AC to have a dedicated circuit, your existing circuit is completely safe with a new breaker and checking the condition of the wiring (tight connections, outlets that tightly grab plugs.)
If your panel is C-H, be sure to use C-H breakers.
Christopher -- Thanks. I don't know why I said open, closed makes much more sense, They've bought a new one and should have it in by now.
What's really wierd is that even the bad one (upper) will open (shut off current) if you open BOTH switches and the bottom operates correctly regardless, but if you open only the upper one, current still flows through that circuit. Gotta be a mechanical problem inside the breaker. As I said, never seen this before. I was astounded!