How circuit breakers work

[allanb]

We recently suffered what we thought was a general power cut caused by a storm, but finally realized that it was just our house.  It turned out to be a bad wire in a socket that had caused a short, melting some of the plastic moulding in the process.

But my question is: why would this trip the main breaker for the whole house, rather than just the one for the circuit that was affected?   I would have expected the specific breaker for the circuit to go first.

It happened the same way several times, while I was trying to find the fault by trial and error; it was always the main breaker that tripped.  So it seems that it must be designed that way.  Is this normal?

 

[Théière]

Your "main breaker" does that have the RCD device attached? (interupteur differential)

If yes then that can trip faster than a fuse or circuit breaker can trip, Clever aren't they, so cutting the power and saving lives.

[Anton Redman]

The individual circuit breakers cut the power when too much power is going through them

The interuptuer differential/ main breaker for a row of  on the board cuts the power when the phase and neutral are showing different reading, normally meaning a small but significant current is leaking to earth.

[Punch]

To be slightly clearer on this:

The individual circuit breakers (disjoncteurs divisionaires) in your tableau electrique will only trip off when you are drawing current over the power rating of the breaker (as stated by Anton above) OR it can be caused by a short circuit between either phase and neutral or phase and earth. However in the latter case there has to be a good enough earth for this to happen.

The main EDF trip is a combined unit which combines overload protection AND earth leakage protection. ( this is called a disjoncteur differentielle), and typically is rated at 500 milliamps for earth leakage protection. All EDF installations have this unit which serves as your main switch, and abbonnement rating.

Additionally, all electrical installations should have at least one and typically three individual "interrupteur differentielles" rated at 30 milliamp trip current. An  interrupteur differentielle only gives earth leakage protection and does not protect from overload or short circuit. From what you say, it seems like your installation may not have an "inter diff ", as this would have tripped before your main EDF switch.

The most likely reason for your problem was the EDF switch tripped out before the individual circuit breaker in your fuseboard because you had some leakage to earth that would not be detected by the individual disjoncteur divisionaire.

Without further details on your actual installation, it is difficult to comment more, but the three items highlited in orange above all operate differently. The differentielles are very sensitive and can trip out very quickly from the smallest of earth leakage currents such as moisture and off balanced loads.