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Entry type: FAQ Entry ID: 109474538, Entry date: 06/24/2015

3SE / 3SF position switches Hysteresis values when used in safety controllers

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This FAQ describe the hysteresis for slow-action and snap-action contacts



Is the hysteresis the same for slow-action and snap-action contacts?
What contact is more suitable for use in safety controllers (signal change or discrepancy times in the F-CPU)?




The hysteresis describes the difference between the operating point and reset point in the case of snap-action contacts. This difference results from the different geometric positions of the snap-action mechanism during switching and reset operations.
In the case of slow-action contacts, the operating point and the reset point are always the same because the actuator head actuates the contact mechanism linearly. There is therefore no hysteresis with slow-action contacts.
The operating distances are largely standardized, and thus it is possible to compare competitors within the usual tolerances. Siemens is the only manufacturer able to offer short-stroke contact blocks for the types of construction 50041 and 50047, which enable the switching operation to be implemented significantly earlier.

The following always applies:

Snap-action contact:
- Switching time including contact bounce NC/NO - contacts approx. 2-5 ms (i.e. the contacts switch almost simultaneously within this time)
- Operating point independent of actuating speed
- Operating point independent of contact erosion
- Hysteresis is approximately 30% to 40% of the operating travel

Slow-action contact:
- Switching time dependent on actuating speed
- NC contact always opens first. The NO contact closes only after the non-overlap distance has been overcome (except for make-before-break contacts)
- Operating point dependent on contact erosion/aging
- No hysteresis

A snap-action contact block is by far the better suited contact for contact interrogation by controllers.

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