WINDING AND CONTACT RESISTANCE TEST BASIC INFORMATION AND TUTORIALS

What are winding and contact resistance testing?

Winding and contact resistance are similar in that both are looking for a very low ohmic value, since they are measuring the "resistance" of a component that is supposed to conduct electricity.

A Kelvin Bridge has long been a standard method of measuring low values of resistance, and is still in use today.

With the advent of electronics, there are digital meters available that also are capable of measuring very low values (milliohms or microohms) of resistance.

The typical low-resistance ohmmeter uses four terminals (to eliminate lead resistance) in which a dc current is injected into the conductor to be measured and the voltage drop across the conductor is measured.

Contact resistance test sets can be used to measure the resistance of bus joints and cable joints, as well as the closed contacts of a circuit breaker or motor starter.

In many cases, it is a comparative type test in which the resistance of one set of contacts is compared to the readings obtained from the other two phases of the same, or a similar, piece of equipment.

Winding resistance differs from contact resistance in that the inductance of large windings can interfere with the operation of the test set.

There are test sets, available commercially, that are designed specifically for large transformer and motor windings, for cases in which a standard low-resistance ohmmeter is not adequate.