Relays play an important role in protection and control of electrical equipment in nuclear power plants. Thousands of relays perform various protection and control functions in each reactor unit. Plants normally replace relays “like for like,” especially in safety related applications due to the high cost of modifications. Relay life-limit determination is often based on time in service, which has been conservatively developed and determined from the most limiting and severe environmental service
applications. Based on industry testing, relay life is frequently significantly longer than currently used bases would predict.
Structural Integrity has supported several industry projects that provided key insights on relay performance, failure mechanisms and ultimate service life limits. For non-EQ (Environmental Qualification) relays, replacement intervals can be optimized based on actual service conditions and applications such as in-cabinet, in-room, and component specific parameters.
Various models can also have differing service lives as some models are more tolerant than others. Energized versus
non-energized applications also make a difference in relay aging rates as they significantly impact relay internal operating temperatures. Organic components within the relay, particularly the relay coil insulation material, are the most limiting
sub-component parts for normally energized relays. The heat rise associated with energizing the coil provides a natural aging stressor within the relay. This coil heat can also affect other relay subcomponents such as the bobbin, or armatures, or promote internal gassing that can impact the contacts.
Temperature effects can be magnified when relays are placed in cabinets with nearby heat sources or clustered together in a single enclosure. Through application of Arrhenius methodologies, temperature correction factors can be developed and applied for a variety of applications to estimate relay life.
For timing relays, set point drift can be particularly challenging. Set point drift is also aggravated by high thermal aging stresses. On the other hand, lower thermal aging conditions can help maintain set point with lower levels of drift. For lower temperature applications, less frequent calibrations may be justified. For higher temperature applications, such as in warmer cabinets or with relay clusters, more frequent calibration should be considered.
Due to the sheer number of relays in a plant, substantial savings can result from extending relay service life, or from longer calibration intervals or both. Specifically, savings exist in:
Integrating the application specifics and relay type/model information results in a graded approach to relay calibration and replacement. The outputs from grading the relays can support an overall optimization pilot based upon model-specific performance, application environment (particularly temperature), and energized state of the relay (normally energized or normally de-energized). Pilot application across a variety of relays at a plant can serve to validate savings from a larger plant wide implementation effort. With thousands of plant relays, the integrated savings on labor and parts can be substantial and provide a good return on investment.