Glossary · Automation fundamentals, platforms and components
Forced-guided contacts
Also known as: Mechanically linked contacts, Positively guided contacts
German: Zwangsgeführte Kontakte
In safety engineering, forced-guided contacts are relay or contactor contacts that are mechanically linked so that normally open and normally closed contacts can never be closed at the same time, even if one contact welds. This allows a welded contact to be detected via the opposite contact.
- Automation components
- Functional safety
- Standards
In one sentence
Forced-guided contacts are mechanically linked so NO and NC contacts are never closed at once, letting a welded contact be detected.
Example
The safety relay monitors the NC auxiliary contacts of two contactors in its feedback circuit and blocks a restart if one main contact has welded.
How it applies
- Engineering: Forced-guided contacts are used in feedback circuits (external device monitoring) of safety functions. Relays with forcibly guided contacts are specified in IEC 61810-3; for contactors, mirror contacts are described in IEC 60947-4-1 and mechanically linked contacts in IEC 60947-5-1.
- Commissioning: The feedback circuit is tested by simulating a welded contact and checking that the safety device blocks the restart.
- Documentation: Circuit diagrams should mark forced-guided and mirror contacts. Spare parts information must state that only equivalent components may be used as replacements.
Forced-guided vs. mirror contacts
Forced guidance applies within a relay's contact set. Mirror contacts are auxiliary NC contacts of a contactor that cannot be closed while a main contact is closed. Both support fault detection; neither makes a circuit safe on its own — the achieved PL or SIL depends on the whole architecture.