A Method for Online Diagnostics of Electromagnetic Relays Against Contact Welding for Safety Critical Applications

In a nuclear power plant, electromagnetic (EM) relays are often used to communicate TRIP signal configured in fail-safe mode. Online diagnostics is an important part of fault tolerant design. Relay contact welding is a failure mode of concern. Current methods do not allow for reliable online diagnostics of this failure mode without actually opening/closing the relays or without making measurements at contact side. In this paper, a technique for online diagnostics of EM relay without affecting the contact status is proposed. The method uses the differences in characteristic decay of coil current during deenergization process between a healthy and a relay whose contacts got welded. A test circuit is designed to verify the concept of deenergizing, followed by quick reenergization of relay coil. Diagnostic information is extracted, and the method is amenable for online automated execution. Results are satisfactory, and a welded contact is successfully detected and indicated by the circuitry. Reliability improvement achieved by deploying the proposed method is highlighted.