Understanding Form A, Form B, Form C Contact Configuration

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

See Also:


Thanks for the post!

One suggestion: You should consider adding in Form D switches. Form D are the same as Form C, the differentiation being that Form C is BBM (break-before-make) and Form D is MBB (make-before-break).



Here are a few more electrical contact forms you may encounter. Although forms A, B and C are by far the most common configurations.

A “Form D” is similar to Form C as stated by @NewMC but differs from it by being a make-before-break instead of break-before-make. Essentially, Form D guarantees for a moment that all contacts are closed, while Form C guarantees for a moment all contacts are open. I found that a picture is more useful in understanding the difference.

There is also a “Form K” that is similar to Form C and D but relays are rarely configured this way. It differs from both Form C and Form D by normally open position in the center where neither connection is made. This form is common in SPDT toggle switches.

A “Form X” is basically 2 Form A contacts, forming a double make configuration. This can also be described as SPST-NO.

A “Form Y” is similar to Form X, except it is essentially 2 Form B contacts. This time making a double break configuration, also described as a SPST-NC.

A “Form Z” is a double make, double break configuration similar to Form C. It consists of 2 normally open contacts and 2 normally closed contacts. Form Z can be described as a SPDT configuration.


See Also:
Switch Contact Materials, Functions, & Types