Trip The Breaker
To return this fault current directly to its source, many devices have a third wire connecting exposed metal to the safety ground pin of their plugs.
The outlet safety ground is routed, either via the green wire or metallic conduit, to the neutral conductor at the main breaker panel.
This low-impedance connection to neutral causes a high fault current to flow, quickly tripping the circuit breaker that removes power from the circuit. To function properly, the safety ground must return to neutral. (Note that the EARTH connection had nothing to do with this process!)
Never, ever use devices such as three- to two-prong AC plug adapters, a.k.a. “ground lifters,” to solve a noise problem! (Figure 3) Such an adapter is intended to provide a safety ground (see the fine print) in cases where three-prong plugs must be connected to two-prong receptacles.
Figure 3: The GFCI (above) has a retractable ground pin that allows it to be used with a two-prong outlet. Below - it’s tempting, but don’t use “ground lifters” to eliminate system noise.
If a proper safety ground isn’t available, always use a ground-fault circuit interrupter (GFCI). A GFCI works by sensing the difference in current between the line and neutral conductors.
This difference represents current in the live conductor that is not returning in the neutral - the assumption is that the missing current is flowing through a person.
If the difference reaches about 5 mA (milliamps), an internal circuit breaker is tripped, removing power from the circuit. The GFCI shown in Figure 3 is unusual because it has a retractable ground pin that allows it to be used with a two-prong outlet.
Also consider two devices connected by a signal cable, each device having a three-prong AC plug. One device has a ground “lifter” on its AC plug and the other doesn’t.
If a fault occurs in the “lifted” device, the fault current flows through the signal cable to get to the grounded device. It’s very likely that the cable will melt and burn. Defeating safety grounding not only is both dangerous and illegal, it also makes you legally liable!
In a typical recent year in the U.S., consumer audio and video equipment electrocuted nine people and started 1,900 residential fires that caused 20 deaths, 110 civilian injuries, and over $30 million in property damage.
Current determines the severity of electric shock. At 1 mA or less, it’s simply an unpleasant tingle. But at about 10 mA, involuntary muscle contractions can result in a “death grip” or suffocation if the current flows through the chest.
Currents of 50 mA to 100 mA through the chest usually induce ventricular fibrillation that leads to death.
The resistance of dry human skin is high enough to safely allow lightly touching a live 120-volt conductor, but normal skin moisture allows more current to flow as does increased contact area and pressure.