CIRCUITS ON OVERLOAD : HERE, KNOWLEDGE TRULY IS POWER, ESPECIALLY WHEN IT COMES TO INS, OUTS OF HOME ELECTRICAL SYSTEM.

Byline: Edward R. Lipinski The New York Times

Insert a plug into a socket and there's enough power to run an appliance. Most homeowners take electricity for granted because the power is available, convenient and safe. But should the light or the appliance fail, then the frustrated homeowner wonders what went wrong. Sometimes the failure can be traced to the power company when, for example, a power cable goes down or a generator malfunctions. More often, however, the problem is within the home.

Overloaded circuits are the cause of most home power failures. This happens when an appliance draws too much current and causes a fuse to blow or a circuit breaker to trip.

Each house has a network of wires that carry electricity from the service panel (the point where electricity enters the house) to outlets throughout the house. These wires are designed to carry a fixed amount of current - generally 15 or 20 amps. A number of appliances plugged into the same circuit can draw more current than the wires can carry. The overloaded wires heat up and could cause a fire.

Sometimes, the problem may occur within an appliance. This can happen when two worn wires, the hot and neutral wires, contact each other. The new path is called a short circuit, and it allows a large surge of current to overload the wires.

Fortunately, safeguards, in the form of fuses or circuit breakers, are built into the system. Fuses and breakers are designed to sense power overloads and interrupt the flow of electricity.

Fuses have an internal metal link that melts to cut off the power. A blown fuse must be replaced with a new one with the same amperage rating. Some people, frustrated by buying and replacing blown fuses, attempt to install a new fuse with a higher rating. Now the circuit designed to carry 15 amperes is ``protected'' by a 20-ampere fuse. The new fuse will take longer to blow, but in the meantime, the wires within the walls will heat up. Obviously, this is an unsafe practice.

Service panels with fuse protection have plug fuses. They also may have cartridge fuses. The cartridge fuses have higher amperage ratings and are used to protect circuits that supply large appliances. Other cartridge fuses may protect the main power circuit.

Plug fuses (also called Edison base fuses) screw into a socket in the panel. A small window in the face allows you to inspect the condition of the fusible metal link and pinpoint the malfunction. An overloaded circuit will melt the link but leave the window clear. The sudden power surge of a short circuit will cause the link to vaporize and discolor the window.

Cartridge fuses are plastic or fiber cylinders with a metal cap at either end. The cap may be either a plain ferrule or a flat knife-blade. The fusible metal link is in the center of the fuse body. Unfortunately, there is no way to tell if the fuse has blown simply by looking at it. You can, however, test it with a continuity tester. Remove the fuse from the panel, then touch the test probes to the fuse ends. If the tester lights, the fuse is good.

Before removing any fuse, turn off the main power (have a flashlight handy when you do this). Some fuse boxes have a lever switch that controls the power. Other fuse boxes have pullout blocks (boxes that hold cartridge fuses) that can be removed to shut off power. To remove the pullout block, grab it by the metal handle and pull it straight from the panel.

To remove a plug fuse, grasp it by the rim and turn counterclockwise. To be on the safe side, use only one hand, keep the other hand in your pocket (in this way you avoid making a ground contact). Make sure that you stand on a dry surface. As an extra safety precaution, you can keep a board (a 16-inch length of 2-by-6 is ideal) on the floor by the panel. Standing on this will provide additional insulation when you replace fuses.

Do not attempt to remove cartridge fuses with your fingers. Instead, use a nonconductive fuse puller (available at hardware and electrical supply stores). Grip the body of the fuse with the puller and yank it from the spring clips.

In comparison to fuses, circuit breakers are much easier to work with. There is nothing to buy or replace because breakers do not burn out. Inside the breaker, a bimetallic element heats up and bends when an overload occurs. This trips the switch and opens the contacts that allow electricity to flow. Some breakers have a small window that exposes a red flag when the switch is tripped. To reset a circuit breaker, flip the switch to the ``off'' or ``reset'' position, then push it to ``on.'' With some breaker models, it's only necessary to push the tripped switch to the ``on'' position.

Of course, replacing a fuse or resetting a circuit breaker may restore power, but it will not correct the overload problem. So, before attempting to restore power, unplug all lamps and appliances from the circuit. If the circuit fails immediately after power is restored, the problem may be in the circuitry itself. In this case, it's best to leave the power off and consult a professional electrician to trouble-shoot.

Drawing: (Cover--Color) Current events

With the right connections, you needn't blow a fuse when your power fails

Francois Duckett/Knight-Ridder Tribune Graphics Network

Photo: no caption (Electricity)

Terri Thuente/Daily News

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