Check Contact Resistance to Diagnose Relay Problems

Digital multimeter method (DMM): As its name suggests, DMM uses a multimeter to directly measure resistance across the contacts. While it's commonly used, DMM can produce misleading results whenever the contacts' surfaces aren't clean. For example, oxidation films that build up on the contact surfaces produce DMM readings that are unstable or that exaggerate the contact resistance.

6V1A method: This method applies 1A through the contacts and derives a resistance value using Ohm's Law. The 6V1A method produces a more accurate contact resistance value than DMM because the heat going through the contacts removes oxidation and other contaminants.

Overload: This occurs when the relay is used beyond its design specifications. High inrush currents and voltages can cause overload conditions, as well as excessive relay switching. Overload conditions ultimately trigger electrical arching, which generates heat that degrades the contact material. In overload conditions, contact resistance can vary depending on how completely overload conditions have degraded the contact material. Mildly degraded contact materials may produce resistance values ranging from very low to near normal. If the contact material is severely degraded, resistance measurements will likely indicate an open-contact condition.

Contamination: In industrial environments, contamination routinely interferes with the operation of the relay's contact. Contaminants, which can include oxidation films or foreign particles, tend to produce contact resistance readings that are either high or unstable. Contamination commonly happens during extended storage periods, use at high temperature and humidity environments, and low load conditions.

End of life: As electromechanical relays reach the end of their lifecycles, they frequently experience a degradation of their contact materials. Contact resistance measurements offer a way to predict when the relay is likely to wear out. As relays exceed their maximum cycle count, contact resistance values can become unstable or read as an open contact.

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