Relay Forms

Relays are classified by their number of poles and number of throws. The pole of a relay is the terminal common to every path. Each position where the pole can connect is called a throw. A relay can be made of n poles and m throws. For example, a single-pole single-throw (SPST) relay has one pole and one throw, as illustrated in the following figure.

A single-pole double-throw (SPDT) relay has one pole and two throws. Based on the default position of the pole, one throw is considered normally open (NO) while the other is normally closed (NC). The following figure illustrates a SPDT relay.

A double-pole double-throw (DPDT) relay has two poles, each with two simultaneously controlled throws, as illustrated in the following figure.

An RF transfer switch (DPDT) has four ports (1–4) and two states (reset and set). In the reset state, port 1 is connected to port 2 and port 3 is connected to port 4. In the set state, port 1 is connected to port 3 and port 2 is connected port 4. The following figure illustrates the reset and set states of an RF transfer switch.

Relays are then classified into forms. Relay forms are categorized by the number of poles and throws as well as the default position of the relay. The following table lists three common relay forms.

Form	Symbol	Description
Form A		SPST relays with a default state of normally open.
Form B		SPST relays with a default state of normally closed.
Form C		SPDT relays that break the connection with one throw before making contact with the other (break-before-make).