Contact conigurations and actions
Switch contact configurations are primarily described in terms of the number of poles and number of throws or ways. A switch pole is a moving contact, and the throws or ways are the fixed contacts against which the moving pole can rest. The term throw is usually reserved for mains switches, mostly single- or double-throw, and way for signal-carrying switches. A single-pole, single-throw switch (SPST) will provide onjoff action for a single line, and is also described as single-pole onjoff. Such switches are seldom used for AC mains nowadays and are more likely to be encountered on DC supply lines. For AC use, safety requirements call for both live and neutral lines to be broken by a switch, so that double-pole single-throw (DPST) switches will be specified for this type of use. Double-throw switches are not so common for mains switches in electronics use – their domestic use is in two-way switching systems, and they have some limited applicability for this type of action in electronics circuits. A double-throw switch will also be described as a change-over type. The word throw, used to describe the number of fixed contacts, is a reminder of the snap-over action that is required of this type of switch.
Vhere a single or multiple contact is required to make connection to more than two fixed contacts per pole, then the fixed contacts are referred to as poles. This type of configuration is much more common in signal switches, and switches of up to l2 ways per pole are available in the traditional wafer form, as noted in Chapter l2.
The configuration of a double-throw switch also takes account of the relative timing of contact. The normal requirement is for one contact to break before the other contact is made, and this type of break-before-make action is standard. The alternative is make-before-break (MBB), in which the moving pole is momentarily in contact with both fixed contacts during the change-over period. Such an action is permissible only if the voltages at the fixed contacts are approximately equal, or the resistance levels are such that very little current can flow between the fixed contacts. Once again, this type of switching action is more likely to be applicable in signal- carrying circuits. There may be a choice of fast or slow contact make or break for some switches.
A switch may be biased, meaning that one position is stable, and the other, off or on, is attained only for as long as the operator maintains pressure on the switch actuator. These switches are used where a supply is required to be on only momentarily – often in association with the use of a hold-on relay – or can be interrupted momentarily. A biased switch can be of the off-on-off type, in which the stable condition is off, or the on-off-on type, in which the stable condition is on. The off-on-off type is by far the more common. The ordinary type of push-button switch is by its nature off-on- off biased, although this is usually described for such switches as momentary action (see, however, alternate action below), and biased switches are also available in toggle form.