Understanding Form A, Form B, Form C Contact Configuration


#1

What does it mean in relays or reed switches when the contacts are listed as Form A, Form B or Form C?

This is a short hand way of describing contact configuration.

A “Form A” would mean the contacts are normally open when the coil of the relay is not energized or the there is no magnetic field nearby in a reed switch.

A “Form B” would mean the contacts are normally Closed when the coil of the relay is not energized or the there is no magnetic field nearby in a reed switch.

A “Form C” would have 3 leads and would have 1 normally open and 1 normally closed circuit. This is also called a “changeover” device because the common contact changes from the normally closed position to the normally open position when the coil is energized in a relay or a magnetic field is nearby in a reed switch.

Here is an example of the contacts in there non-energized state.
Contact Configuration
Below would be the same contacts in their energized state.
    Energized contact Configuration

You will also find many relays that have multiple contacts and some will have a variety of configurations. You can select the option you need under the contact form section(shown below).

Contact Forms

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