Check Contact Resistance To Diagnose Relay Problems

October 23, 2012
By Anonymous

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.

When the coil is energized, the COM contact and the NO contact will close. The DVM is set to measure the voltage drop across the contacts. Since current on the external power supply is set to 1A, the voltage measurement on the DVM is equal to the resistance value between the contacts. Therefore a measurement of 20mV represents 20mΩ.

 

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 very 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:

  • Digital Multimeter Method (DMM): As its name suggests, DMM uses a multimeter to directly measure resistance across the contacts. While it is commonly used, DMM can produce misleading results whenever the surfaces of the contacts are not 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 one amp of current 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. Here at Panasonic, we use 6V1A method as our standard measurement method—for our own testing purposes and for the development of our data sheet specifications.

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

Relay Contamination In this example of a relay failure due to high contact resistance, we analyzed the contact surfaces and discovered that a film had formed due to a chemical interaction between the relay’s silver contact surfaces and environmental sulphur. Because this relay had been operated under low loads, not enough heat was generated during switching to keep the sulphur film from forming on the contact surfaces.

 

With contact resistance measurements in hand, you can diagnose the most common causes of relay failure, including:

  • Overload occurs when the relay is used beyond its design specifications. High inrush currents and voltages can cause overload conditions as well as excessive switching of the relay.  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 periods of storage, usage in high temperature and humidity environments, as well as low load conditions of use.
  • End of Life. As electromechanical relays reach the end of their lifecycle, they frequently start to 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.

For more information on how to avoid relay failure and the resulting downtime, visit our online support system: http://pewa.panasonic.com/support/contact/?div=components

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