Contact Performance in Relays


5.1 Applications

Electrical contacts for relays fall into three broad applications categories-logic (low signal), control, and power. The contact requirements in each of these area have similarities but also some differences.

Contacts used on logic relays, for example, need to provide high reliability in making and maintaining a circuit with a consistent and usually low contact resistance. These contacts are energized from low power logic supplies and work with relatively low contact forces and in addition need to "make" and "break" a circuit reliably for many millions of operations.

Control relay are supplied form control system power sources-24Vdc for aircraft systems and 120-600Vac for commercial and industrial systems-and are used to operate other switching devices such as solenoids or motor control contactors. While the contact resistance requirements are not as severe as with logic relays, they must have a low mv drop, be capable of interrupting the circuit, and have an electrical life on the order of several million operations.

Power relays operate from control system power sources and are used to directly control power equipment from control system power sources and are used to directly control power equipment such as solenoids or motors. They must carry the power current reliably with low contact temperature rise, they need to be able to close and open a circuit reliably and require a lifetime of hundreds of thousands of operations.
Taken as a whole all classes of relay contacts must perform four functions reliably albeit with different emphasis on performance determined by the application. They must:

1. "Make": a circuit through any contamination present on the contact surface.
2. Maintain circuit continuity consistently over a period of time.
3. Interrupt the circuit by effectively breaking any welds formed and interrupting any arcs formed.
4. Provide an acceptable operational life governed by the application.
Contact theory and its application to the design of relays constitute a complex subject that is treated comprehensively in texts. Since this chapter is intended only to acquaint the reader with the factors that influence contact performance in service, the discussion is largely limited to presenting the practical aspects of contact application. Reference (1) presents a comprehensive discussion of these factors such as: constriction resistance, plastic flow, arc initiation glow discharge, tunnel effects, film breakdown and the like.