Protective Relay Testing Explained

Doble F6150So why do engineers and technicians test protective relays? Most people understand that regularly testing electrical equipment is essential to keeping these vital instruments functioning correctly and maintaining their longevity. But the reality is that protective relay testing actually maximizes the protection availability and minimizes the potential risk of incorrect relay operation. To accomplish this goal, adequate testing methods must be used in order to fully realize the benefits of testing diagnostics of relaying equipment.


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Traditional Relays
Failure of traditional relays can lead to false trips, prevent fault operation and/or alter the operational characteristics of the relay. The traditional relay does not self test or monitor its own status, thus making routine testing a must in order to verify that the equipment is operating correctly. When problems exist in traditional relays, that issue may go undetected until maintenance or complete failure. Therefore, traditional relay reliability is highly dependent on a regularly scheduled maintenance routine.

Digital Relays
Digital relays can fail or prevent fault operation in the same way that traditional relays do; however, failures in digital relays do not typically affect relay characteristics. These types of failures have a tendency to generate an indication during self testing or notify the user that there is a problem in the normal operation of the relay. This, in fact, is why digital relays can be much easier to test and maintain. They utilize periodic self testing to monitor relay status and will close alarm contacts if a failure is detected. These devices also disable trip and other control signals for certain test failures, and component failure is often discovered at this time.

Testing Types
When deciding on new relay designs, testing of the relay is essential to make sure that it will operate properly for the intended application.

  •        Type testing – is often performed on single representative relays from the manufacturer so that personnel can ask questions and clearly understand all the protective functions of the equipment. These tests will detail certain characteristics such as time overcurrent plots, mho circle plots and relay element accuracy in order to verify algorithms and characteristics of the relay.
  •        Commission testing – prior to placing protective relays into service, installation or commissioning tests are also performed using digital relay tests. These help to verify effectiveness of relay element and logic settings.
  •        Routine maintenance testing – primarily used to detect traditional relay failures and verify that protective relays will operate when necessary.

We understand the relay test set is vital to performing accurate and reliable protective relay testing. Doble testing equipment and Megger are two manufacturers that offer some of the best relay testing solutions available in the marketplace today. That’s why we have test equipment rental programs designed to work for your project and budget for short or long term rental, lease or even purchase. 



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Editor’s Note: This post was originally published June, 2012 and has been updated freshness, accuracy and comprehensiveness.