Issue:
User wants to know the abbreviations for common relay types.

Resolution:

• Normally Open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when inactive. This type is also called Form A or a make contact.
• Normally Closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when inactive. This type is also called Form B or a break contact.
• Change-over (Double-Throw) contacts control two circuits: one NO and one NC contact with a common terminal. This type is called Form C or a transfer contact. It is often “break-before-make.” The opposite, “make-before-break,” is Form D.

Other common relay types:

• SPST (Single Pole Single Throw): Two terminals that can be connected or disconnected. With coil terminals, there are four total. Contact can be NO or NC; abbreviations SPNO and SPNC clarify this.
• SPDT (Single Pole Double Throw): One common terminal connects to either of two others. With coil terminals, there are five total.
• DPST (Double Pole Single Throw): Two pairs of terminals, equivalent to two SPST relays operated by one coil. Six terminals total.
• DPDT (Double Pole Double Throw): Two rows of change-over terminals, equivalent to two SPDT relays operated by one coil. Eight terminals total.
• 4PDT (Four Pole Double Throw)/Quadruple Pole Double Throw (QTDP) : Four rows of change-over terminals, equivalent to four SPDT relays or two DPDT relays operated by one coil. Fourteen terminals total.