An SPST relay has one pole and one throw, and can switch one circuit at a time. In contrast, an SPDT relay has two poles and can switch two circuits at once. There are many different types of relays, and you can use one for one circuit or several for different circuits.

The basic design of an SPST relay is to have three contacts on one side and two contacts on the other. The center of the three contacts on the left side is the common contact. The other two contacts are the coils, which are isolated from the common contact and the other connections. This allows a relay to operate in both directions at once.

SPST relays can switch low and medium-current signals. They can also be used for automated failure-injection testing. Solid-state relays are also silent and do not require a minimum load current. They do not require a power supply, and they are perfect for low and medium-amperage applications.

The state of the SPST relay determines when it makes and breaks connections. A state of 1 indicates that the connection has been made, while a state of 0 means it is disconnected. The delay between making and breaking a connection can be controlled via a physical signal input port or through an electrical conserving port. A delay in the making or breaking of a connection introduces a behavioral fault and can cause the connection to become stuck or degraded.