Diagnostics-enabled process safety relays

04 September 2018

The KFD2-RSH safety relays from Pepperl+Fuch  are designed to overcome the weaknesses of force-guided contacts. The device combines diagnostics, line fault transparency, and triple redundancy in a one-out-of-three architecture.

The function of the switching contacts is automatically checked during each switching operation which reduces the effort required for proof tests. The relay is immune to test pulses so is compatible with digital output cards for all common control panels.

For DTS (de-energiSed-to-safe) applications, there are three serial contacts. For ETS (energiSed-to-safe) signal circuits, there are two groups, each of three contacts arranged in parallel. The safety function is ensured, even if two contacts in a group fail. The integrated diagnostics function switches 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 operations. 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 rather than during shutdown. 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.

In conventional solutions, control-side line fault detection to the field device via the galvanic isolation of the module is not possible. The specific line fault can only be detected with additional wiring of a fault indication output on the module. In contrast, KFD2-RSH safety relays provide comprehensive line fault transparency (LFT). Short circuits and lead breakages are also detected on the control side and assigned to a specific signal circuit. No additional wiring is required. The relay module monitors the connected load in the field. If there is a fault in the field wiring (short circuit, lead breakage), the resonance of the test pulse filter on the input side is incorrect. The diagnostics-enabled DO (digital output) card of the control system can detect the fault on the field side from it. In addition, the relay modules provide an auxiliary contact, which allows freely selectable feedback of line fault information.

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