In machine safety it is important to take steps to reduce systematic and random hardware failures, in order that a safety function remains available.

To reduce the likelihood of Common Cause Failure (CCF) where two identical components (e.g. limit switches) are used, two different methods of operation can be used in combination to monitor a single function. These methods are:

Negative mode operation - The limit switch uses a Normally Open contact and sends a signal when it is activated
Positive mode operation - The limit switch uses a Normally Closed contact and sends a signal when it is deactivated

In the case below the use of these two distinct operating modes means that if one limit switch fails due to external influence, the other should not because it is operating in a different or diverse mode.

Combined mode is a form of diversity by which common cause failures can be reduced, and hence the integrity of the safety function stands a better chance of being preserved.

Released for: Schneider Electric UK