Switch Terminology
Data sheet
The purpose of a switch is to make and break electrical circuits, usually by means of a mechanical movement. In general a switch comprises
of two parts, the contacts and the actuator.
Contacts can either be normally closed, normally open or changeover. This is typically abbreviated as NC, NO and CO, respectively. The
term ’normally’ generally refers to the switch when it has not been actuated.
Current/Power Switching Rating
The maximum steady current/power which may be interrupted or prospective current/power which may be made, with a purely resistive
load. It is the usual Contact rating (A) quoted for all switches.
Current Carrying Rating
The maximum steady current through a closed contact. The limit is imposed by heating effects (I2R) and is generally greater than the
current/power switching rating.
Voltage Rating
The maximum working voltage which is determined by insulation materials, contact separation, rate of separation and by safety considerations. Again, operation with a purely resistive load is
assumed.
Contact Configurations
SPST
Single Pole Single Throw
Single Pole On/Off Switch
Double Pole Double Throw
Two Pole Changeover Switch
(Sometimes expressed as DPCO)
DPDT
DPST
SPDT
Double Pole single Throw
Two Pole On/Off Switch
Single Pole Double Throw
Single Pole Changeover Switch
(Sometimes expressed as SPCO)
NC
NC
NO
NO
Typical Push-button Switch Configuration
(Sometimes expressed as Z-form)
Momentary Action
When an actuator is depressed to change the contact state (e.g. OFF to ON), this change of state only remains whilst the operating force is
applied. This would typically be expressed as ’(on) – off’, where the brackets indicate the momentary state. This is sometimes described as
’biased’.
Alternate Contact Action
When a push button switch is depressed to change its state (e.g. OFF to ON), the button or lens will return to its original position once the
operating force has been removed, but the
contacts will remain in the new state.
Latching Action
This is similar to the Alternate action, but when the operating force is removed, the actuator will also remain in the new position. The
actuator and contacts will revert to their original position, once an operating force is applied again.
General
Switches are usually of the break before make style, but in certain applications it is desirable to make the connection before breaking the
initial connection, i.e. make before break. (mbb)
In some cases a manufacturer will quote two current ratings, one for resistive loads and one for inductive loads.
If a switch was marked 10A (4A), it has a 10Amp resistive rating and a 4Amp inductive rating. The inductive rating is particularly important
when selecting a switch for a motor or transformer load.
Switch Terminology - Data sheet
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