2. If you do not find the trouble, or open, by a visual check, use a voltmeter to determine whether the circuit is live (operating) up to the point of the component.
A SHORT CIRCUIT results when two bare conductors of different potential come into contact with each other. If a conductor inadvertently contacts a metallic part of a wiring system, such as a motor frame or conduit, the system is sometimes said to be GROUNDED instead of having a short circuit. Grounds or short circuits can be (1) solid, (2) partial, or (3) floating. This situation presents a serious safety hazard because the machinery may be in operation, even though it has a short circuit. This condition is especially true in motors and some appliances.
A solid ground or short circuit is one in which a full- voltage reading is obtained across the terminals of a blown fuse when the load is disconnected from the circuit. The circuit resistance, in this case, is quite low, and the current is quite high so that the fuse will blow.
A partial short or ground is one in which the resistance between the phase wires, or between the phase wire and the ground, is partially lowered. However, enough current still remains to blow the fuse. Grounds of this type are generally more difficult to locate than are solid grounds.
A floating ground is a condition in which the resistance of the defect in a system varies from time to time. Grounds of this type may be present in an electrical system for some time before their existence becomes known. A floating ground is indicated when fuses are blown on the phase side of a circuit a number of times, and a circuit test shows no defects in the system. In grounds of this type, fuse trouble may not occur for several days. Then the ground recurs, and the fuses are blown again.
The procedures used to repair the troubles mentioned thus far are usually fairly simple. In the case of an open, short, or ground in NM cable, the bad section from box to box can simply be replaced using the same procedures outlined for installation. Another method would be to cut the cable at the trouble spot, install junction boxes, and add a short piece of cable to replace the bad section. Although the latter is the cheapest, it may not be possible if the trouble is concealed. Remember, you must have at least 6 inches of free conductor in a junction box to make the splices.
Once you find out where the trouble is in a conduit system, the repair procedure is even easier. All that is required is to pull the open, shorted, or grounded conductor out and replace it with a new one. This replacement can be done by attaching the new conductor to the one that is to be removed. In any case, a little common sense and knowledge of the NEC requirements will dictate the action you should take.
Many times a visual inspection does not uncover an apparent problem; therefore, you must advance to troubleshooting with meters. In electrical troubleshooting, you will use voltmeters, ohmmeters, ammeters, and the meter that incorporates many meters - the multimeter.
When using a voltmeter, you have to connect the power to the circuit before testing. On the other hand, you cannot use the ohmmeter on an energized circuit. You need to start voltmeter tests at the power input end of the circuit, whereas you start ohmmeter tests at the ground end
Electrical circuit troubles develop either in the wiring or in the operating unit. If you analyze the problem carefully and take systematic steps to locate it, not only will you save much time and energy, but you will also prevent damage to expensive equipment.
Either dead circuits or live circuits can be tested with instruments. Circuit defects can sometimes be located more easily by one method than the other, depending upon the type of circuit and the trouble.
To test a dead circuit, disconnect the device from the outlet or disconnect switch. Equipment for this method of testing includes such units as ohmmeters and battery-powered test lamps. Asuitable continuity tester can be made easily from a flashlight in an emergency. An ohmmeter that contains its own batteries is excellent for continuity testing. A basic factor to consider in choosing continuity test equipment is to use relatively low-voltage instruments, reducing the danger of sparking.
WARNING When connections are made in the presence of combustible vapors, sparking is a serious fire hazard.
When you test live circuits, energize the circuit under test from the power source. Generally, you will test with a voltmeter. Make certain that the voltmeter is designed for the type of current to be tested and has aContinue Reading