The Meaning of Relay Contact Form

Updated Jan 30, 2019

Reported In


  • Relay

Issue Details

When mentioning the types of relay, I find Form A, Form B, Form C, and Form D over there. What do these mean?


Form A:
Contacts are normally open. When relay is on, contacts are closed and configure closed circuit.

Form B:
Contacts are normally closed. When relay is on, contacts are open and configure open circuit.

Form C:
A set of contacts includes a normally open contact (Form A), a normally closed contact (Form B) and a common ("K") contact. Before a "Form C" contact makes the normally open contact closed, it breaks the normally closed contact. This method is called "break-before-make".

Form D:
It is similar to "Form C". Differently, before a "Form D" contact break the normally closed contact, it makes the normally open contact closed. This method is called "make-before-break".


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