Diagnostics-enabled safety relays for the process industry

Jon Lawson

The new KFD2-RSH safety relays from Pepperl+Fuchs combine a diagnostic function, line fault transparency, and dual redundancy in a one out

of three architecture. During each switching operation, the function of the switching contacts is automatically checked. This drastically reduces
the effort required for proof testing. Thanks to line fault transparency, the entire signal circuit can be monitored for line faults without the
need for additional wiring. The relays are immune to test pulses and are therefore compatible with digital output cards for all common control

The architecture with dual redundant switching contacts is at the core of the new modules. For DTS (de-energised-to-safe) applications, there are three serial contacts. For ETS (energised-to-safe) signal circuits, there are two groups of three contacts each, arranged in parallel. The safety function is ensured, even if up to two contacts in a group fail.

The integrated diagnostics function moves 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. Another relay is checked during every switching cycle.

The relays provide comprehensive line fault transparency. As a result, short circuits and lead breakages can also be detected on the field side through the control system's digital output card (DO card) and assigned to a specific signal circuit. No additional wiring is required.