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Type of Switch |
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A switch is an electrical device used to enable or disable flow of electrical current in electrical circuits. Switches can be defined by
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- 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.
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The later two terms are similar and often cause confusion. The following diagrams should be of help. |
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| SPST - Single Pole Single Throw |
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| SPDT - Single Pole Double Throw. Also called changeover contact |
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| SPTT - Single Pole Triple Throw |
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| DPST - Double Pole Single Throw |
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The above configurations are same for manual as well as for automatically actuated switches like relays, contactors, thermostat. |
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Two more terms often used with automatically actuated switches are |
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- Normally Open contact
- Normally Closed contact
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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. |
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