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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.
Below would be the same contacts in their energized state.
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).
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.
