A lot of green apprentice electricians ask about how panels work, and more specifically what the roll of the circuit breaker is. Some guys think that breakers "put out" electricity, or that they "send" a certain amount of amperage to a load from the panel. This is not how it works.
To start out, an electrical panel is simply a place where all of the many loads throughout the house meet up with the main feeders or service entrance conductors coming into the building. It's essentially a large junction box. But rather than just joining all of the branch circuits and feeders to the service entrance wires, we put breakers in between each circuit to protect that circuit from any problems that may arise.
To understand how electricity flows we must understand the idea of "difference of potential." When a difference of potential energy exists between 2 wires, connecting those wires together will allow current to flow through them. It will be extremely dangerous due to the amount of electrons flowing through the circuit, and you will most likely see an explosion happen in front of you. So don't do this. Electricity is extremely dangerous in a circuit without a load or impedance in it. So to make electricity useful, we have to introduce a load to the circuit. A load is essentially a resistance. It is something that slows current down. Loads can also be inductive/capacitive and therefore reactive rather than resistive, but we won't get into the semantics of this topic, in this post. For now just understand that for electricity to be useful it must be slowed WAY down.
When a circuit has a proper resistance on it (say a toaster or lightbulb), the current slows down to a usable level and does not allow an explosion to happen. But sometimes parts wear out and when this happens current can rise to dangerous levels (essentially speeding back up). When this happens we want to have a protective device in place to disconnect the circuit so it doesn't melt or cause a fire. Enter the circuit breaker. (By the way...fuses are very similar to breakers and serve the same function. More on that in another episode)
A circuit breaker does not "output" any power. It simply allows current to flow through it until that current gets to a dangerous level, in which case it automatically opens or "trips" the circuit. A breaker is essentially an automatic switch. It can be manually opened or closed (turned on or off), or it can automatically trip by sensing how much current is flowing through it.
Breakers "sense" current flow by a small electromagnet inside of the device. For instance, a 40 amp breaker will have an electromagnet inside of it that when more than 40 amps flows through it, magnetically (mechanically) pulls the contacts apart - stopping current from continuing to flow through the device.
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**Disclaimer - These videos are for training purposes alone, all work done on electrical systems should be done by a licensed and insured electrical contractor. If you are not an electrician, do not attempt any of the work you are seeing in these videos.**