Diagnostics that require practically no additional effort for proof testing—this is a key feature of Pepperl+Fuchs’ new KFD2-RSH series safety relays. To ensure safe switching, they are also equipped with line fault transparency to monitor the field circuit for line faults without additional wiring. The relays are immune to test pulses and are therefore compatible with digital output cards for all common control panels.
Diagnostics is made possible by a 1oo3 (one out of three) architecture, a redundant structure with three elementary relays. For DTS (de-energized-to-safe) applications, there are three serial contacts. For ETS (energized-to-safe) signal circuits, there are two groups of three contacts each, arranged in parallel. Availability of the safety function is ensured, even if up to two contacts in a group fail. The module’s integrated diagnostics move the three contacts successively with a time delay during each switching operation. In ETS applications, all three relays of both contact groups are initially closed with three consecutive switching events. During the delay period, the device checks whether this contact closes the circuit and detects any faulty circuits.
DTS device diagnosis takes place during the restart process. Initially, two relay contacts are closed simultaneously, then the third contact is closed after a time delay. The idea is that there is no current flowing before the third contact closes, otherwise this relay is defective because it no longer disconnects the circuit. A different relay is checked during every switching cycle.
After switching three times, the relay module is fully tested once without any additional effort.
The KFD2-RSH safety relays provide comprehensive line fault transparency. Short circuits and lead breakages are also detected on the field side and assigned to a specific signal circuit. No additional wiring is required. Digital output cards on control panels often offer built-in diagnostics. The module input effectively filters the test pulses coming from the digital card. This prevents an unintentional switching of the field device as a result of a diagnosis measure or the unintentional display of a line fault in the control panel.
