Circuit Protection

Because the amperage available from the utility grid is almost unlimited, a 120V household system is powerful and dangerous. The electrical current could easily melt all the wiring in your home. Special devices that limit current are located in the main service panel. If you open up the door of your electrical panel, you will find either fuses or circuit breakers (and sometimes both), each rated to withstand a certain amount of current, usually 15 A. If the current exceeds the listed amount, the fuse will burn out (blow) or the breaker will trip, shutting off the current and protecting the wiring system from an overload. When this happens, it is a signal that you are trying to draw too much power through the wires.

Overloaded and short-circuited currents can result in overheating and fires. Circuit protective devices protect insulation, wiring, switches, and other equipment from these dangers by providing an automatic way to open the circuit and break the flow of electricity.

Fuses and Circuit Breakers

If too much current flows in a wire, it can get hot enough to set fire to surrounding material. Fuses and circuit breakers protect against this possibility by cutting off power to any circuit that is drawing excessive power. They provide an automatic means for opening a circuit and stopping the flow of electricity.

The key element in a fuse (Fig. 29-2) is a strip of metal with a low melting point. When too much current flows, the strip melts, or blows, thereby interrupting power in the circuit. When the fusible strip of metal is installed in an insulated fiber tube, it is called a cartridge tube. When encased in a porcelain cup, it is a plug fuse.

A circuit breaker (Fig. 29-3) is an electromechanical device that performs the same protective function as a fuse. A circuit breaker acts as a switch to protect and disconnect a circuit. A strip made of two different metals in the circuit breaker becomes a link in the circuit. Heat from an excessive current bends the metal strip, as the two metals expand at different rates. This trips a release that breaks

Screw-in (plug) fuse

Cartridge fuse

Figure 29-2 Fuses.

the circuit. Commercial and industrial applications use solid-state electronic tripping control units that provide adjustable overload, short-circuit, and ground fault protection. Circuit breakers can be reset after each use, and can be used to manually switch the circuit off for maintenance work. They shut off the current to the circuit if more current starts flowing than the wire can carry without overheating and causing a fire. This may occur when too many appliances are plugged in at once, or when a short circuit occurs. Circuit breakers are easily installed as needed for various circuits in the building.

Both fuses and circuit breakers are rated in amperes and matched to the wiring they protect. Plug fuses are screwed in and are rated from 5 to 30 A, and 150V to ground maximum. Cartridge fuses are used for 30 A up to 6000 A and 600V. Cartridge fuses often show no sign of having blown. A blown screw-in fuse can generally be spotted by a blackened glass or break in the metal strip. Unlike circuit breakers, which can be reset after they trip, fuses must be replaced when they have been used to break a circuit. In neither case, however, should the circuit be reactivated until the cause of the problem has been located and fixed.

A demand for too much power, called an overcur-rent, occurs when too many devices are connected to a circuit or when a failed device or loose wire causes a short circuit. Overloading the circuit with too many appliances or lighting fixtures is the commonest cause of fuses blowing repeatedly or circuit breakers tripping again and again. An overcurrent also may occur when high-wattage fixtures and appliance motors are turned on, because they momentarily need much more electricity to start than they draw when operating. If a cir cuit is near capacity, a start-up overcurrent can blow a fuse, even though there is no real danger to the system.

Circuit breakers are built to withstand these momentary surges, but standard fuses are not. When a circuit often blows a fuse when an appliance such as a refrigerator or room air conditioner is turned on, a time-delay or slow-blow fuse can help cope with brief surge demands. Both plug and cartridge fuses are available in slow-blow designs that safely allow temporary overloads. Whether a fuse or a circuit breaker is the better choice depends on the application and on other technical considerations.

Ground Fault Circuit Interrupters

As we mentioned earlier, the NEC recognizes a ground fault circuit interrupter (GFCI or sometimes GFI) (Fig. 29-4) as a way to protect against shocks when a building's wiring is not grounded. GFCIs are actually designed for another primary use, which we now look at in more detail.

Even after ground wires were commonly installed in buildings, researchers found that shocks were still common, especially in damp or wet areas, including kitchens, bathrooms, basements, and outdoors. Plug-in devices such as hair dryers, power tools, and coffee makers that are in common use around sinks, in the basement, or out in the garage are part of the problem.

Water and electricity don't mix. Dampness in the soil or in concrete that rests in the soil makes either surface a good electrical conductor and a good ground. Metal faucets and drains are also excellent grounds, because the water supply lines and sewers that they connect with are usually underground. Shutting off a faucet with one hand while holding a faulty hair dryer with the other could be fatal.

Figure 29-4 GFCI receptacle.

Figure 29-4 GFCI receptacle.

Unfortunately, the fuses or circuit breakers in the main service panel will not protect you from a lethal shock in such circumstances. Fuses and circuit breakers protect the wires in your house from overheating, melting the insulation, and causing a fire. They don't protect you against faults in the electrical ground.

Fortunately, the ground fault circuit interrupter, which is a special type of circuit breaker, was invented around 1970. The role of the GFCI is to protect you from a potentially dangerous shock. A GFCI device can be part of a circuit breaker or can be installed as a separate outlet.

When you leave a hair dryer with a frayed cord in a little spilled water that is in contact with the sink's metal faucet, you have the makings of a shocking situation. You could accidentally touch an exposed hot wire in the frayed cord while at the same time turning off the water faucet with your other hand. Even though the dryer is turned off, an electric current immediately flows from the cord, through your body, through the plumbing system, and eventually to ground. This is called a ground fault. It will not cause the circuit breakers or fuses in the main service panel to break the circuit, and the current will continue to flow through your body.

A GFCI instantaneously senses misdirected electrical current and reacts within one-fortieth of a second to shut off the circuit before a lethal dose of electricity escapes. When it senses a ground fault, the GFCI interrupts the circuit and switches it off.

Another function of GFCIs is to detect small ground faults (current leaks) and to disconnect the power to the circuit or appliance. The current required to trip a circuit breaker is high, so small leaks of current can continue unnoticed until the danger of shock or fire is imminent.

Ground fault circuit interrupters permit the easy location of ground faults. They are required in addition to circuit breakers in circuits where there is an increased hazard of accidental electrical shock, such as near bathroom sinks. If the GFCI senses any leakage of current from the circuit, it will disconnect the circuit instantly and completely. The GFCI does this by precisely comparing the current flowing in the hot and neutral legs of the circuit. If the amount of current is different, it means that some current is leaking out of the circuit.

Ground fault circuit interrupters have a relatively short history in the NEC. The 1971 code initially required them on circuitry controlling lights and other electrical equipment for swimming pools. It required GFCIs in outdoor locations in 1973 and at construction sites in 1974. The 1975 code required GFCI outlets in all new and remodeled bathrooms. Initial GFCI locations were undoubtedly limited to the most dangerous areas by the relatively high price of the device, about $25. As the price dropped below $10, the cost became insignificant compared to the safety gained. More recent versions of the NEC expanded GFCI requirements to include garages and basements.

The 1993 code requires GFCI protection for readily accessible outlets located outdoors, in crawl spaces and unfinished basements, and in garages. The NEC requires GFCIs in all standard 120 V duplex receptacle outlets in bathrooms and kitchens. The code treats spas, hot tubs, and Jacuzzis as if they were swimming pools, and outlets, lights, and electrical equipment within a certain distance of pools all require GFCI protection. Local codes typically also require them in office break rooms, bar areas, and laundry rooms, as well as in outdoor and other damp locations. GFCIs should be used on all appliance circuits. Because lighting fixture circuits are commonly in the ceiling and are switch controlled, they are not usually required to have GFCIs.

Ground fault circuit interrupters can be installed built into a receptacle, or in an electrical distribution center in place of a circuit breaker, to protect that particular circuit. The type of GFCI that plugs into an existing outlet should only be used on a temporary basis, as on a construction site before permanent wiring is installed.

To make sure that GFCIs are working, manufacturers added the "test" and "reset" buttons that you see on them. Pushing the test button creates a small electrical fault, which the GFCI should sense and immediately react to by shutting off the circuit. The reset button restores the circuit. Repeated action by the GFCI to protect a leaking circuit will eventually wear the GFCI out. You should test your GFCIs every week and replace them immediately if they are not working properly.

Each 120 V circuit has one hot and one neutral conductor. The hot conductor originates at the branch circuit overcurrent protective device (fuse or circuit breaker) connected to one of the hot bus bars. A 240V circuit uses both hot conductors, and originates at the branch overcurrent protective device connected to both hot bus bars.

All of the neutral conductors start at the neutral bus bar in the distribution center. All the neutral conductors are in direct electrical contact with the earth through a grounding conductor at the neutral bus bar of the service entrance panel. An overcurrent protective device never interrupts the neutral conductors, so that the ground is maintained at all times. The effect of this arrangement is that each branch circuit takes off from an overcurrent protective device and returns to the neutral bus bar.

In order to decide how many branch circuits to specify and where they should run, the electrical system designer takes into account a variety of different loads. Lighting is the first and often the greatest. Data processing equipment, convenience outlets, desktop computers and their peripherals, plug-in heaters, water fountains, and other miscellaneous electrical power users make up a second group. Heating, ventilating, and air-conditioning (HVAC) and plumbing equipment use electrical energy for motors and switches. Elevators, escalators, and material handling equipment, dumbwaiters, and trash and linen transportation systems are another group of loads. Kitchen equipment in restaurants, most hospitals, and some office, educational, and religious buildings can be a significant electrical load. In addition, some buildings contain special loads such as laboratory equipment, shop loads, display areas and display windows, flood lighting, canopy heaters, and industrial processes.

Once the electrical power requirements of various areas of the building are determined, the electrical engineer lays out wiring circuits to distribute power to points of use. Branch circuits extend from the final over-current device protecting a circuit to outlets served by the circuit. Each circuit is sized according to the amount of load it must carry, with about 20 percent of its capacity reserved for flexibility, expansion, and safety. To avoid excessive drops in voltage, branch circuits should be limited to less than 30 meters (100 ft) in length.

The electrical engineer will specify general-purpose circuits to supply current to a number of outlets for lighting and appliances. Manufacturers specify load requirements for lighting fixtures and electrically powered appliances and equipment, and the interior designer is often responsible for getting these specifications to the engineer. The design load for a general-purpose circuit

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