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What is the definition of snap-action contacts on limit switches?

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 positive 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.

This provides a clean break, giving very positive signaling action and avoids arcing and false signaling in the in transition position.

A possible disadvantage is that this mechanism inevitably creates a hysteresis effect, where the closing and opening of the switch have considerable overlap. Please see attached timing diagram example.

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 signaling at the crucial make/break position, where actuator movement is slow and/or erratic.

Sequence of moving contacts

Timing Sequence for "Snap Action" Note overlap of switching positions for different directions of travel

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


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