Last week my roommate was watching TV and saw a few sparks fly out of the outlet the TV was plugged into. More specifically, the TV was plugged into a power strip which for some reason she had plugged into a three-to-two prong adapter. This adapter was plugged into the wall (which was three-prong anyway).

She kept watching TV and an hour later there was a loud bang and sparks/fire blew out of the wall for a second. This melted a hole in the two-to-three-prong adapter and the TV went dead.

When I discovered what happened I ran out the breaker panel and discovered that the breaker had never tripped, so I turned it off manually.

My question is, should the circuit breaker have tripped when the first sparks appeared? Or not necessarily?
Also, what would cause a fire like this? The circuit was not overloaded at the time. Only the TV, a VCR, and perhaps a light were on. It is possible that the adapter was loose in the socket. Are there other possible causes?

The breakers in the box are all standard-style, not the ones that are specially made to recognize arcing.

Thanks!

An ARC Fault breaker would have sensed this type of fault. This is a high impedance short, which acts as a load , and the fault current did not ries to a level to trip the circuit breaker.

So your roommate saw sparks fly, and continued to watch TV? What does it take to get someone's attention?

Most likely caused by a loose connection, whether it was loose and worn.

The loose connection causes heat, which allows the plastic melt, and even begin to carbonize. Once the carbon begins to collect, this will allow current to flow as if it is a load. Finally the current flows as the carbon breaks down, and a short developes, the arc flash burns away some material, stopping the current from flowing, thus stopping the short. This is an ARC Fault.

I want to add, that the 2/3 prong adapters can defeat the circuit breaker. A short to to the metal in a device is more likely to trip the breaker if the housing or chassis of it is grounded through a 3 prong plug. In this case, likely the TV had a plastic case and a 2 prong cord. Plugging a 2 prong adapter into a 3 prong outlet, defeats an important safety feature unless the little tab is secured with the screw in the cover plate. I don't ever recall seeing one that was.