Testing Guidelines

Resistance Is NOT Futile

Resistance is important when building and testing cables and harnesses. Not only can resistance influence the effectiveness of the cable, but knowing the resistance of a wire is an important part of testing. The concept of resistance can get complicated, but understanding the basics can help you build and test with more efficiency.

What is Resistance?

Resistance is the limitation of current through a wire. Current cannot flow through a wire uninhibited. It’s like obstacles found in a river. In the wide, deep parts of the river, the water can flow unrestrained. When the river narrows, becomes shallow, or is blocked by debris, the water cannot flow as smoothly.

In a cable, material, length, and components are like the depth, width, and debris in a river. These factors can affect the electrical current through a circuit. The more a manufacturer knows about the resistance caused by certain materials and lengths, the better decisions they can make when building a cable.

Why is it important to know a wire’s resistance?

When an engineer is selecting materials to build a cable, they carefully consider how those materials restrict the current. The engineer calculates the amount of expected resistance then brings the specs to the cable manufacturer.

Resistance in a cable can be affected not only by wire gauge and length, but from bad solder joints, bad crimps, and even compression. That is why resistance is measured during testing. If the actual resistance does not match the expected resistance, it could mean an error occurred during construction. If not corrected, the added resistance could affect the cable’s performance.

How is resistance determined?

Calculating a wire’s resistance is as simple as knowing the wire’s length and gauge. Determining the resistance for an entire cable can be more complicated. The cable’s connectors, components, and other pieces will affect resistance. Also important to consider when testing is the fixturing (the mating harness used to connect the device-under-test to the tester). All these pieces add resistance that must be considered in order to test accurately.

Cirris has created calculators to help determine resistance in a wire or for a cable being tested. The wire resistance calculator exists on cirris.com or as a free app.

View Calculator Online


Knowing the correct resistance of a wire will result in more accurate testing. Resistance can be complicated, but when understood it can create deeper understanding of how to test cables and harnesses.


Further Reading:

Are you using the wrong test method?

How Much Time Should You Invest in Testing?

Guidelines for Setting Resistance Test Thresholds


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