What are the differences between snap-action and slow-break contacts in limit switches?

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Snap action

The speed of travel of snap-action contacts is not related to the speed of operation of the actuator. The actuator pretensions an "overcentre spring" mechanism and it is the spring tension which operates the contact once a certain tension has been achieved. This results in a very fast and assured operation of the contacts as the spring mechanisms "flicks" (or "snaps") over the trigger point and latches the contacts on. A similar action takes place on the release, i.e. as the actuator is released the spring tension reduces but the contacts do not move until the tension has dropped below a certain level, when the spring mechanism will snap the contacts open again.

Slow Break

A "slow break contact" is one where the physical movement of the actuator is directly linked to the physical movement of the contact. This can give very precise positioning and synchronization of both make and break points but is susceptible to arcing and intermittent signalling at the crucial make/break position, where actuator movement is slow and/or erratic.

Main differences between Snap Action and Slow Break

The snap action contact mechanism inevitably creates a hysteresis effect, where the closing and opening of the switch have considerable overlap. Please see attached timing diagram example. This does not occur with slow break, as the movement of the contacts is directly linked to the movement of the switch / object.

Slow break have a higher minimum actuation speed, to overcome the effect of sustain arcing on the contacts. Note that for applications that cause the switch to open a little and then re-close repeatedly / rapidly, this can reduce electrical durability over time.

Both types commonly have positive opening N/C contacts (always check your switch to confirm), but snap action provide an assured signal at slower speeds, whereas slow break rely on the minimum actuation speed. This is why there is potential for intermittent signalling on slow break, if that minimum speed is not adhered to.

Slow break can often switch bigger loads and offer higher electrical reliability values compared to snap action but please check product tables / data to confirm this on a case-by-case basis.

Sequence of moving contacts

Timing sequence snap action contacts
Timing Sequence for "Snap Action". Note overlap of switching positions for different directions of travel.

Timing sequence slow break contacts

Timing sequence for "Slow Break". Note same position for make and break.