We're in the electrical room of one of Duke's buildings. Outside of this building, a distribution line for one of the university's substations enters a transformer which steps the voltage of electricity down from 12 and a half kilovolts to 480 volts. This lower, more usable voltage is rooted into this room, where it enters a switchboard with a busbar that splits the electricity among a number of circuits that run to electrical outlets and fixtures throughout the building. These electrical circuits are controlled by the circuit breakers that you see here on my right. These can be used to shut a circuit down manually or to automatically break the circuit if it experiences an inadvertent electrical surge caused, for example, by a short circuit. >> Once electricity has been stepped down to a usable level outside a building by the building's external transformer, the electricity enters the building either through 3-phase current, 2-phase current, or 1-phase current. 3-phase current is used almost exclusively to power large electric induction motors and so is primarily drawn by industrial customers. The induction motors are connected directly to three power lines carrying AC current, with three different phases, to provide a nearly continuous level of current needed to drive the motors. At the other end of the spectrum is the electricity from single-phase current, which is what all residential and most commercial customers receive. Again, the external transformer delivers the electricity to the residents or commercial building through three lines. However, in this case only two of the lines are hot. Meaning that the current in them has a significant voltage difference with respect to ground. The third line is the neutral line, which taps into the midpoint of the transformer and connects directly to ground and so has essentially zero voltage. Using this design, the external transformer creates a difference of 120 V between each hot line and the neutral line. And a difference of 240 V between the two hot lines. This then is how a single phase current can be used to drive both lower power 120 V appliances like lamps, toasters, and TVs. And higher power, 240 V, appliances such as washing machines and electric furnaces. We'll focus on the more common 120 V appliances. In a properly wired building, current will flow into a building when an appliance that is plugged into a building outlet is turned on and thus creates a complete circuit between the hot line and the neutral line. As the voltage of electricity is essentially fixed that 120 V, Ohm's law dictates that the appliance will only draw as much current as is needed to over come the appliances internal resistance and allow the current to move to hot line to the neutral line and thus power the appliance. This current is the same in both the hot and neutral lines, but whereas the hot line is 120 V with respect to ground, the neutral line is essentially 0 V for it runs the ground. In actuality, the voltage in the neutral line is greater than 0 because the current in the neutral line is not 0. Remember, it's current is the same as the current in the hot line. However, the resistance of the neutral line is very low, so the voltage in the neutral line driving the current into the ground will also be very small. This voltage is typically on the order of several volts or less which, if you touch the neutral line, will give you no more than a slight tangle. However, never, ever touch the neutral line. This is because if the neutral line is not properly grounded or has a break in it, then the current passing through the neutral line may have a voltage difference with respect to ground of up to 120 V. If you touch the neutral line under these circumstances, electricity will be redirected from the neutral line to and through you because you now become the shortest path to ground. And given the human body's low electrical resistance, the electricity may move through you with so much current as to seriously hurt if not kill you. This type of misdirection of current is known as an electrical fault. Which is any inadvertant electrical connection between an energized part of the electric system and something at a different electric potential, like you, that creates a voltage difference. To protect against faults, appliances and building electric systems have a number of safeguards and redundancies built into them. For example, in addition to having openings for one hot line and the neutral line, a standard 120 V electrical outlet also has a third opening for a ground line. This ground line connects directly to the ground and so provides a shorter alternative path for electricity to flow in the event there is something wrong with the neutral line and an electrical fault occurs. Standard 120 V electrical outlets also have different size openings for the hot and neutral parts of an appliance plug. This is to ensure that the hot wire coming into the appliance is always the same. And, thus, the switch for the appliance works properly. If an appliance plug is put in backwards, it may result in a situation where the switch, when it is in the off position, only stops electricity after electricity has passed through the whole appliance. This could result in the appliance having an excess voltage with respect to ground when it is off and thus make the appliance dangerous to touch. Electrical outlets can also have a fourth type of protection in the form of a Ground Fault Circuit Interrupter or GFCI. The benefit of a GFCI is that it will almost instantly break the circuit in an outlet if it detects a fault in which electricity is going to ground outside the outlet and near the plugged in appliance. For example, if someone is using an electric saw and cuts through the cord. The GFCI will trigger the shutdown when the current being drawn from the outlet hot wire by the appliance does not equal the current in the outlet neutral wire on the other side of the circuit. Indicating that current has broken out of the circuit. A whole additional set of protections occur in the electric box where the external power lines connect to wires in the building that lead to each outlet. The two external hotlines that come into the building attach to metal strips, or buses, that run down either side of the electrical box. Switches are also attached to these strips. And when on or closed direct current from one or both hot wires into different circuits throughout the building each of which leads to one or more outlets. These switches are designed to remain on, or closed, and will only open, and thus switch off, when the current passing through their circuit exceeds its designed capacity. The switches are of two general types, one being fuses. Fuses shut down a circuit when the current passing across a filament in the fuse exceeds the fuse's capacity and causes the filament to melt, break and thus break the circuit. The other more modern type of switch is a circuit breaker. These, too, will open and thus break the circuit when the current in the circuit exceeds a designed threshold. In this case, however, the circuit is broken by the automatic opening of a switch in the breaker. Thus unlike a fuse, the breaker does not have to be replaced to get current flowing back through the circuit once the electrical fault has been extinguished. All that needs to be done is to flip the breaker on so that it once again closes and the circuit is re-established. One last thing. Before connecting to the electrical box, the external power lines coming into the building pass through an electric meter, which is typically mounted on the outside of the building. This meter measures current drawn by appliance use within the building and converts the current into electric power usage over time. Which it typically records in units of kilowatt hours. Hence, the electric meter measures the building's electric energy consumption, which the electric utility then charges the building owner for.
