electrical engineering construction

To see why grounding is necessary, look at the drawing, which shows a circuit during normal conditions. Now let's take that same circuit and add a metal ceiling fixture. If the hot wire accidentally became dislodged from the fixture terminal and came into contact with the light fixture's metal canopy, which is highly conductive, the fixture would become electrically charged, or "hot." If you were to touch the fixture under those conditions, a current leakage, or "ground fault," could occur in which you would provide the path to ground for the electric current, and you would get a shock.

The same result could occur in any number of places where electricity and conductive materials are together, such as in power tools and appliances with metal housings, in metal housing boxes, and in metal faceplates. In our example, shock could have been prevented if the circuit had had a grounding system. A grounding wire connecting the neutral bus bar in the service entrance panel to the metal housing of the light fixture would provide an auxiliary electrical path to ground. This grounding wire would carry the fault current back to the distribution center, where the fuse or circuit breaker protecting the circuit would open, shutting off all current flow.

In a typical house circuit, the wiring method dictates how grounding is done. When a home is correctly wired with armored cable, metal conduit, or flexible metal conduit, the metal enclosure can itself serve as the grounding path. But most modern construction uses nonmetallic sheathed cable (type NM), so a separate grounding wire must be run with the circuit wires. Running a separate grounding wire isn't as complicated as it may sound because NM cable contains a grounding wire.

In any of these systems, the end result is the same: an auxiliary path for fault current is provided leading to the neutral bus bar in the service entrance panel, which is tied to ground via the grounding electrode conductor.

In the drawing, the bare grounding wire of the NM cable provides the grounding continuity. The final grounding connection to the receptacle is made through a short piece of wire called a jumper that is bonded to the metal box with either a grounding screw or a grounding clip. If a nonmetallic box were used instead, the grounding wire would be connected directly to the receptacle because that kind of box needs no grounding.

The ground fault circuit interrupter (GFCI or GFI) also protects against electric shock. Whenever the amounts of incoming and outgoing current are unequal, indicating current leakage, the GFCI opens the circuit instantly, cutting off the power. GFCIs are built to trip in 1/40th of a second in the event of a ground fault of 0.005 ampere.

There are two types of GFCIs, both shown. The GFCI breaker is installed in the service panel; it monitors the amount of current going to and coming from an entire circuit. A GFCI receptacle monitors the flow of electricity to that receptacle, as well as to all devices installed in the circuit from that point onward (called "downstream").

The electrical code now requires that receptacles in bathrooms, kitchens, garages, and outdoor locations (in other words, any potentially damp location where the risk of shock is greatest) be protected by a GFCI. You can use either type to serve these areas.