Circuit Breakers and Fuses – Over-Current Protection

Inside an electrical panel there are ‘over-current protection devices’ or OCPDs. These are either ‘circuit breakers’ or ‘fuses’. These devices protect individual branch circuits from having too much current, or electricity, flow through them. Branch circuits feed electricity to specific devices in a home like the dryer, outlets in master bedroom, etc.

We need OCPDs because if you run too much electricity through a wire it will become hot. If the wire gets too hot it can start a fire. Preventing overheating and fires is the most important design aspect of an electrical system.

Fuses

Fuses limit the electricity flowing through them by melting a part of the fuse when too much electricity is detected. The part that melts, the filament, is encapsulated in a housing to protect other components from damage. The filament works like this: when electricity flows through metal the metal heats up. If you run too much electricity through a piece of metal the metal can heat up so much that it actually melts. So the filament is designed to carry just enough electricity that that everything works under normal conditions. When there’s too much electricity the filament melts and electricity stops flowing. This protects the home and devices in it from overheating and potentially causing fires or damaging components.

The main problem with fuses is that once they hit an over-current situation or ‘blow’, they are no good anymore. They have to be replaced. This fact is likely what drove the development of the circuit breaker. It is also the reason that we see very few fuses being installed in electrical systems today. The main places I see fuses today is at a furnace shut-off switch, and in junction boxes next to air conditioners.

Most main electrical panels installed in the last 7 or more decades haven’t had fuses in them. So if you have a main panel in your home that still has fuses it may be time to upgrade your electrical system.

Circuit Breakers

A circuit breaker is and OCPD that is easily reset after it has tripped from an over-current event like plugging too many hair dryers into the outlets of a room and running them all at the same time. It’s very much like a switch that can turn itself off when too much electricity flows, but it can be turned back on instead of needing to be replaced like a fuse.

The way a circuit breaker works is that it has a piece of metal in it that electricity flows through. As more electricity flows through it, it heats up. As it heats up the piece of metal bends. When it bends too far the connection inside it is disconnected and electricity flow stops. Now you might ask, why does the piece of metal bend when it heats up instead of just getting bigger in all dimensions? The answer is that this piece of metal is actually two different types of metal laminated together. They each have different coefficients of thermal expansion which means that when one piece heats up, say 100 degrees, it grows by 0.1 inch whereas the other piece when it heats up 100 degrees it grows 0.2 inches. So when these two pieces of metal that are laminated together heat up the same amount the lamination has to bend to accommodate the difference in thermal expansion.

When a circuit breaker trips it often moves the switch to a position between on and off. This helps you distinguish between a circuit breaker that is off intentionally and one that has tripped. To reset these types of circuit breaker you have to turn it all the way to the off position and then turn it back on.

There are different types of circuit breakers like GFCI and AFCI. I’ll discuss that in a different article here.

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