At the beginning of this lab your instructor will demonstrate for you a Ground Fault Circuit Interrupt (GFCI). This circuit can be found in the wall outlet of most bathrooms and garages. Its purpose is to prevent electrocution. Recall that (metal) plumbing is electrically connected to earth ground in a house. A dangerous situation that can occur is when a person accidentally makes contact with the line voltage. For example, a hair dryer may have a loose wire and a person could touch it by accident. If the person is electrically isolated there is no problem. However, if that person were to touch a (metal) plumbing fixture, the person would be connected to earth ground, causing the completion of a circuit through the person's body. Electric current causes muscle contractions in human beings when as little as 6 mA for women and 9 mA for men goes through the body. This may cause a person to be unable to release anything he or she is holding. Such a small current is usually not enough to kill a person. However, once you make contact with electricity, you begin to sweat which reduces the body's surface resistance so that more current flows through you. Once this current reaches 50-100 mA through the heart, it will likely stop beating (called fibrillation). The GFCI is a circuit breaker that produces an open circuit in the power line if the difference between the currents in the hot and neutral wires (see the residential power system diagram in the EE61 Coursepak) in the GFCI is greater than 5 mA. This occurence would indicate that 5 mA or more had found an alternate route to ground, possibly through a person. The GFCI senses the difference in current between the hot and neutral wires by mutual inductance. Three coils are utilized in the sensor. One coil is wrapped around the hot wire and the other is wrapped around the neutral wire. The current flowing in the wires induces a voltage across each of the two coils. These two coils are wrapped so that if the currents are approximately equal, the net flux will be zero. However, if the currents are not equal, the net flux will not be zero. This non-zero flux will induce a voltage across a third coil wrapped around the other two. This voltage is then used to open the circuit breaker, cutting off the current flow.