A switch is an electrical device used to enable or disable flow of electrical current in electrical circuits. Switches can be defined by
method of actuation - manual, electrical, mechanical, pneumatic, hydraulic etc.
number of poles or number of electrical circuits it controls. -One, Two, Three. Multiple way
number of positions.
The later two terms are similar and often cause confusion. The following diagrams should be of help.
SPST - Single Pole Single Throw
SPDT - Single Pole Double Throw. Also called changeover contact
SPDT - Center off
SPTT - Single Pole Triple Throw
DPST - Double Pole Single Throw
The above configurations are same for manual as well as for automatically actuated switches like relays, contactors, thermostat.
Two more terms often used with automatically actuated switches are
Normally Open contact
Normally Closed contact
Both NO & NC contacts are SPST type, the difference being that when the external force is removed the NO type of contact switches to open position whereas the NC type of contact switches to closed position.
