Learning to Check the Resistance

When you check loads and switches by measuring resistance, make sure that the power to the unit is turned off and that the components being tested are isolated (as shown in the illustration on this slide). All functional loads have some resistance. Typically, the larger the load is, the lower the resistance will be. To determine the “normal” resistance of a component, you can consult the manufacturer’s specification sheet, or compare the resistance of the component under test with that of a similar component that is known to be good.

A resistance reading of 0 Ω in a load indicates the presence of a short circuit—a condition that generally opens a fuse or trips a circuit breaker. An infinite (“OL”) resistance reading is an indication of an open circuit or an incomplete path. Be aware that you will also get an infinite reading if a motor has an open internal overload.

An excerpt from “Preparing for the NATE Gas and Oil Heating Exam” available from the online store of the Refrigeration Service Engineers Society. RSES, the HVACR Training Authority.

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