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Check Contact Resistance to Diagnose Relay Problems
February 11, 2013
3 Min Read
Even the best relays can fail at some point, but what causes them to fail?
Conventional wisdom lays the blame on worn-out contacts. And there is some truth to that view. Every electromechanical relay has a finite number of cycles it can endure before the contacts call it quits.
The truth about relays, however, is that they sometimes fail to last as long as they should because of overload or contamination.
Fortunately, both of these failure modes can be diagnosed by measuring the relay's contact resistance. The same measurement can also help you predict when relays are reaching the end of their expected lifecycle.
Two methods are generally used to measure contact resistance are:
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.
Keep in mind that contact resistance specifications on data sheets represent an initial value. This value can change over time, depending on operating conditions.
Using contact resistance measurements
With contact resistance measurements in hand, you can diagnose the most common causes of relay failure, including:
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.
Dario Torres works with Panasonic Electric Works Corporation of America as a product specialist. His responsibilities are product registration, technical support, failure analysis, and product development.
About the Author(s)
Dario Torres is a graduate of Stevens Institute of Technology with B.E. in Electrical Engineering and currently works with Panasonic Electric Works Corporation of America as a product specialist. His responsibilities range from product registration, technical support and failure analysis, as well as product development.
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