# Contacts



## Big John (May 23, 2010)

They're all single pole.

Form A is normally open.
Form B is normally closed.
Form C is double-throw.


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## JRaef (Mar 23, 2009)

Form A: aka NO- (Normally Open) or SPST( Single Pole Single Throw) but must be used with NO.









Form B: aka NC (Normally Closed) or NC-SPST







(Don't really like this drawing too much)

Form C: aka NO/NC, CO (COmmon) or SPDT (Single Pole Double Throw)









There is also technically a "Form D" contact, which is a Form C, but the NC opens AFTER the NO closes, also called a Late Break Changeover switch. That term is hardly ever used any more though.

Form D contact.










That may be related to what your alarm guy was thinking.


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