# Circuit Breakers & Resources

 Circuit breakers Circuit breakers are absolutely essential devices in the modern world, and one of the most important safety mechanisms. Whenever electrical wiring in a building has too much current flowing through it, these simple machines cut the power until somebody can fix the problem. Without circuit breakers (http://circuit-breakers.aaker.com/circuit_breakers.html), electricity would be impractical because of the potential for fires and other mayhem resulting from simple wiring problems and equipment failures. Circuit breakers monitor electrical current and cut off the power when current levels get too high. The circuit breaker is an incredibly simple solution to a potentially deadly problem.

 Electricity Basics Electricity is defined by three major attributes: Voltage Current Resistance Voltage is the "pressure" that makes electric charge move. Current is the charge's "flow" - the rate at which the charge moves through the conductor, measured at any point. The conductor offers a certain amount of resistance to this flow, which varies depending on the conductor's composition and size. Voltage, current and resistance are all interrelated - you can't change one without changing another. Current is equal to voltage divided by resistance (commonly written as I = v / r). This makes intuitive sense: If you increase the pressure working on electric charge or decrease the resistance, more charge will flow. If you decrease pressure or increase resistance, less charge will flow.

 Why You Need a Circuit Breaker The power distribution grid delivers electricity from a power plant. Inside the electric charge moves in a large circuit, which is composed of many smaller circuits. One end of the circuit, the hot wire, leads to the power plant. The other end, called the neutral wire, leads to ground. Because the hot wire connects to a high energy source, and the neutral wire connects to an electrically neutral source (the earth), there is a voltage across the circuit - charge moves whenever the circuit is closed. The current is said to be alternating current, because it rapidly changes direction. The power distribution grid delivers electricity at a consistent voltage (120 and 240 volts in the United States), but resistance (and therefore current) varies in your location. All of the different electrical devices offer a certain amount of resistance, also described as the load. This resistance is what makes the device work. A light bulb, for example, has a filament inside that is very resistant to flowing charge. The charge has to work hard to move along, which heats up the filament, causing it to glow. In building wiring, the hot wire and the neutral wire never touch directly. The charge running through the circuit always passes through a device, which acts as a resistor. In this way, the electrical resistance in devices limits how much charge can flow through a circuit (with a constant voltage and a constant resistance, the current must also be constant). Devices are designed to keep current at a relatively low level for safety purposes. Too much charge flowing through a circuit at a particular time would heat the device's wires and the building's wiring to unsafe levels, possibly causing a fire. This keeps the electrical system running smoothly most of the time. But occasionally, something will connect the hot wire directly to the neutral wire or something else leading to ground. For example, a motor might overheat and melt, fusing the hot and neutral wires together. Or someone might drive a nail into the wall, accidentally puncturing one of the power lines. When the hot wire is connected directly to ground, there is minimal resistance in the circuit, so the voltage pushes a huge amount of charge through the wire. If this continues, the wires can overheat and start a fire. The circuit breaker's job is to cut off the circuit whenever the current jumps above a safe level.

 Basic Circuit Breaker Design The basic circuit breaker consists of a simple switch, connected to either a bimetallic strip or an electromagnet. The hot wire in the circuit connects to the two ends of the switch. When the switch is flipped to the on position, electricity can flow from the bottom terminal, through the electromagnet, up to the moving contact, across to the stationary contact and out to the upper terminal. The electricity magnetizes the electromagnet. Increasing current boosts the electromagnet's magnetic force, and decreasing current lowers the magnetism. When the current jumps to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. The entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit. The electricity shuts off. A bimetallic strip design works on the same principle, except that instead of energizing an electromagnet, the high current bends a thin strip to move the linkage. Some circuit breakers use an explosive charge to throw the switch. When current rises above a certain level, it ignites explosive material, which drives a piston to open the switch.

 Advanced Circuit Breaker Design More advanced circuit breakers use electronic components (semiconductor devices) to monitor current levels rather than simple electrical devices. These elements are a lot more precise, and they shut down the circuit more quickly, but they are also more expensive. Another circuit breaker device is the ground fault circuit interrupter, or GFCI. These sophisticated breakers are designed to protect people from electrical shock, rather than prevent damage to a building's wiring. The GFCI constantly monitors the current in a circuit's neutral wire and hot wire. When everything is working correctly, the current in both wires should be exactly the same. As soon as the hot wire connects directly to ground (if somebody accidentally touches the hot wire, for example), the current level surges in the hot wire, but not in the neutral wire. The GFCI breaks the circuit as soon as this happens, preventing electrocution. Since it doesn't have to wait for current to climb to unsafe levels, the GFCI reacts much more quickly than a conventional breaker.

 Amps, Volts, Ohm & Watts The most basic units in electricity are voltage (V), current (I) and resistance (r). Voltage is measured in volts, current is measured in amps, resistance is measured in ohms. A neat analogy to help understand these terms is a system of plumbing pipes. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size. There is a basic equation in electrical engineering that states how the three terms relate. It says that the current is equal to the voltage divided by the resistance. I = V/r Let's see how this relation applies to the plumbing system. Let's say you have a tank of pressurized water connected to a hose that you are using to water the garden. What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of the hose. The same is true of an electrical system: Increasing the voltage will make more current flow. Let's say you increase the diameter of the hose and all of the fittings to the tank. You probably guessed that this also makes more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases the current flow. Electrical power is measured in watts. In an electrical system power (P) is equal to the voltage multiplied by the current. P = VI The water analogy still applies. Take a hose and point it at a waterwheel like the ones that were used to turn grinding stones in watermills. You can increase the power generated by the waterwheel in two ways. If you increase the pressure of the water coming out of the hose, it hits the waterwheel with a lot more force and the wheel turns faster, generating more power. If you increase the flow rate, the waterwheel turns faster because of the weight of the extra water hitting it. In an electrical system, increasing either the current or the voltage will result in higher power. Let's say you have a system with a 6-volt light bulb hooked up to a 6-volt battery. The power output of the light bulb is 100 watts. Using the equation above, we can calculate how much current in amps would be required to get 100 watts out of this 6-volt bulb. You know that P = 100 W, and V = 6 V. So you can rearrange the equation to solve for I and substitute in the numbers. I = P/V = 100 W / 6 V = 16.66 amps What would happen if you use a 12-volt battery and a 12-volt light bulb to get 100 watts of power? 100 W / 12 V = 8.33 amps So this system produces the same power, but with half the current. There is an advantage that comes from using less current to make the same amount of power. The resistance in electrical wires consumes power, and the power consumed increases as the current going through the wires increases. You can see how this happens by doing a little rearranging of the two equations. What you need is an equation for power in terms of resistance and current. Let's rearrange the first equation: I = V / R can be restated as V = I R Now you can substitute the equation for V into the other equation: P = V I substituting for V we get P = IR I, or P = I2R What this equation tells you is that the power consumed by the wires increases if the resistance of the wires increases (for instance, if the wires get smaller or are made of a less conductive material). But it increases dramatically if the current going through the wires increases. So using a higher voltage to reduce the current can make electrical systems more efficient. The efficiency of electric motors also improves at higher voltages. Based on http://electronics.howstuffworks.com/circuit-breaker.