# Sets of NO (Normally Open) and NC (Normally Closed) Contacts

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Each relay will have some number of NO, NC, or a combination of both NO and NC contacts. These sets of contacts have become standards and they each have names to indicate the types and arrangement of contacts. Ill. 1 shows examples of four arrangements of contacts. The contacts in Ill. 1a show a relay with two sets of NO and two sets of NC contacts. Each of these sets of contacts is isolated from the others so that one can control four separate circuits if necessary. When the coil is energized, the two open sets move to the closed position, and the two closed sets move to the open position.

Above: Ill. 1: (a) Electrical diagram of a relay with two sets of NO and two set: of NC contacts. (b) Electrical diagram of a relay with two sets of NO and NC contacts. Each set of NO and NC contacts is connected to a common terminal. This configuration is called double-pole double-throw (DPDT). (c) Electrical diagram of a relay with five sets of NO contacts. (d) Electrical diagram of a relay with five sets of NC contacts.

The relay in Ill. 1b shows a double-pole, double-throw (DPDT) set of contacts. The term double pole means that each set of contacts has a common terminal connected to both an open and a closed set of contacts. When the coil is energized, the circuit between common and NO is closed, and when the coil is de-energized the circuit between common and NC is closed.

The relay in Ill. 1c shows five sets of NO contacts. Each of these sets of contacts is isolated so that it can be used in five separate circuits. The relay in Ill. 1d shows five sets of NC contacts. Each of these sets of contacts is also isolated so that it can be used in five separate circuits.