Types of Switches
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 - Centre 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.
