[jdiwnab]

This isn't offical but our 120A circuit breaker broke after dropping about 4 volts. Not good. So we took it apart to find out how it worked and then didn't.

Post-mortem autopsy of a Buss 120A circuit breaker

Operators- David Keagan, Jay Penick, Bryan Hartley

Date of autopsy- November 10, 2004

Writers- Bryan Hartley, David Keegan

Date of Write-up- November 15, 2004

Following the death of a Buss 120A circuit breaker, an autopsy was performed to determine the cause of death. The circuit breaker arrived in a decomposed state. The positive terminal casing was mangled and a large split was found across the side with the positive terminal.
Because of the level of external decomposition, it was necessary to review the inside. An incision was made to remove the front portion of the breaker. Three of the four pins holding it in place were cut. This front portion was rotated away and it revealed a rubber insulating membrane. This was cut away to reveal the inside of the breaker. Unfortunately, the decomposition had continued to the inside in parts. The positive terminal was attached to a metal plate that appeared to be a bimetallic spring. A wide “T” shaped metal object was removed to remove the positive terminal and the metal plate. The red button that reset the breaker and was visible externally was removed along with the power on switch. A broken shaft was found that appeared to have been connected to the power on switch and was part way attached to a small spring. These were removed, along with the positive terminal and the metal plate.
The apparent cause of death at this time was the charring of the metal plate. Upon closer examination of the metal plate, it is found to most likely to be a bimetallic spring. To better determine the cause of death, the circuit breaker needed to be reassembled to find out how it worked.
The bimetallic spring and positive terminal were inserted into the body of the breaker. The negative end the the spring connected to a contact near the negative terminal. However, it did not attach to it. The red reset button was inserted into the front portion, along with the power on switch. Part of the shaft was placed near the negative terminal, centered on the bimetallic spring. This portion had a flange that appeared to overlap some of the spring. It was found that the “T” shaped part could hook another “T” shaped attachment on the bimetallic spring and would rest on two side holders. This would allow the reset button to push down on the “T” , which would make the bimetallic spring be removed from the negative contact. The power on switch would cause the shaft to push down on the spring, creating contact.
With these findings, a consistent cause of death was reached. Because of metal fatigue, the spring lost it’s elasticity. In use, the motors current draw increased at some point, causing heat. This made the spring bend, but not enough to cut off the current, only to created an ‘air resistor’. Because of the loss of elasticity, the spring didn’t reengage fully and so overheated dangerously.

So, don't over use your circuit breaker and have replacements.