Definitions of Commonly Used Terms in the World of Safety Relays

Advantages of mechanical contact systems:

Force Guidance of contacts provides designers of safety relevant circuits with all the fundamental details upon which they can build up their circuit technology consideration. In addition to force guidance, electromechanical contact systems offer string electrical isolation and immunity to external influences or electromagnetic compatibility (EMC). The air gap in the contact system recovers after a dielectric breakdown. A solid-state component does not recover after a dielectric breakdown.

What does Force Guidance achieve?

Force Guided Contacts are used to set up self-monitoring electrical circuits (cyclic self-monitoring). Force guided contacts make it possible to recognize failure within electrical circuits, using the principal of single fail-safe characteristics. Force guided contacts are used to set up electric circuits exhibiting deterministic behavior in event of failure.

What is Self-Monitoring in technology?

Self-Monitoring is a condition in which a system performs a self-test of all functions. If a fault is detected, this information can be used to determine a certain behavior of the system.  Cyclic self-monitoring is a self-test repeated at predefined time intervals or event triggered intervals.

ZH1/281 Section 3.9.1

In the event of a malfunction, it must be ensured that a command is initiated to interrupt a hazardous closing function; or during the next test, a command preventing a hazardous closing function is initiated.

What is Deterministic in physics?

This term is used to describe a system that due to its physical properties passes in an unambiguous way from a defined initial state to any future condition.  Such conditions are generally foreseeable and can be calculated.  In technology, this term is used to describe a system which causes an unambiguous state after having failed.  In safety technology, this state is always a state that prevents possible hazards.

Definition of a Failure Mode Effect Analysis (FMEA):

General Information:

The Failure Mode Effect Analysis (FMEA) is an examination of the effects of faults/failures. This term describes the evaluation of the results of failure conditions in order to perform a risk assessment.

Faults under consideration:

IEC/EN 50205 Section 5.1

Opening failure of NC or NO contacts (fault exclusion is not permissible) Results of wear of parts (fault exclusion is possible) Breakage of parts (fault exclusion is possible) Breakage of a contact spring (fault exclusion is not permissible)