There are several different types of resistance tests. Resistance tests differ from voltage and current tests because they are rarely performed on a dynamic basis, that is, while the equipment is operating. Resistance tests are usually performed with the power off and usually with the component disconnected to make sure that there are no short circuits to cause misleading readings.
Resistance testing of a component that has pure resistance is straightforward. The component resistance reading should be the same as it would be in both ac and dc circuits. But many components used in ac circuits have both a pure resistance and an impedance, which affect circuit operation. Any component containing coils or capacitances has an ac impedance. Ohmmeters measure only the DC or pure resistance of a component. There are, however, AC component testers, to measure impedance.
Wire Continuity Tests
Many electrical troubles are caused by breaks, or opens, in wire continuity, as well as increased resistance in a connection. Continuity testing is merely a test to show that there are no complete breaks in a wire, whereas resistance testing generally measures for a more specific reading in ohms. Since a reading is not always necessary in a continuity test, many continuity testers sound an audible to notify the user when continuity exists, while an analog or digital meter would show zero or very low resistance.
Testing the continuity of a wire requires connections to both ends of that wire. With a short wire, this is not difficult, but with a long wire, where one end might not be accessible, that end might have to be tied to another return wire so that the continuity of both wires can be tested in series.
Wire Resistance Tests
Continuity tests are general indicator tests that are useful in most cases. But, in some cases, particularly where a stranded cable is used, a wire can have continuity but too high a resistance if one or two or more strands are broken. Also, a bad connection will allow continuity, but a high resistance reading. Resistance tests are made the same as continuity tests, except ohm readings are obtained on a meter. For long wire lengths, the normal resistance of the length of the wire should be known.
Because of some long-line testing, and the fact that wiring is often buried or hidden from view, many ordinary continuity and resistance tests are challenging. There are several infrared scanners available that allow the user to test for high resistance effects while the equipment is operating. The effect of high resistance in a circuit carrying current is that the resistance dissipates power and generates heat. The infrared testers are also called thermal testers. The tests are made by first aiming the tester at a known normal temperature target to get an ambient reference reading; and then aiming the tester at various suspected targets in a circuit, usually connections. An audible tone on some units may guide the user to a suspected target, while visual displays, such as lights, show the severity of the heat. Other testers can give the actual temperature readings.