Operator Interfaces, Switches & Relays


SIL 2 safety switching over fibre-optic cable

August 2022 Operator Interfaces, Switches & Relays

SIL 2 switching applications are often required for emergency stop (E-Stop) systems or mission-critical stop/start applications where safety is a key issue. These are normally hardwired connections, but what happens when you have a large distance between the E-Stop and the device needing to be controlled (stopped)?

Modern safety systems need to conform to the IEC61508 standard, which must be satisfied to ensure a safe system. In a case where, for example, a few kilometres separate two points, a hardwired connection is impractical and a communications network does not meet the IEC61508 requirements, so this becomes a real challenge to implement.

Omniflex Remote Monitoring Specialists has used its experience in IEC61508 and signal conditioning to provide a solution to this problem. Using fibre-optic cable, a fibre-optic transmitter and a corresponding receiver to transmit the status of a switch contact over the fibre cable, the Omniterm FCT and FCR provide SIL 2 safety level and enable contact switching over a long distance safely. This has been embraced by the nuclear industry, mining, and oil and gas, where remote switching is required over long distances.

Forming part of the extensive Omniterm range of industrial instrumentation equipment, the FCT and FCR modules are DIN-rail mounted, fibre-optic contact transmitters and receivers designed for use in safety-critical applications. The pair of modules provides the ability to send a digital contact signal up to 10 km over a single optical fibre, making it ideal for monitoring of safety-critical alarms, emergency stops or mission-critical stop and start applications.

The relay contact input from the field is connected to the FCT module (Fibre-optic Contact Transmitter); this can be either normally open or normally closed (NC for failsafe operation). The state of this relay contact is then continuously transmitted to the FCR (Fibre-optic Contact Receiver) module over fibre-optic cable. In an alarm condition, the relay contact at the FCR module will be energised or de-energised (failsafe), placing its changeover contact output into the alarm condition. LEDs are provided on each of the modules to display the state of both the input and output contacts and also the presence of instrument power.

Some applications for this solution are large, long-distance mining conveyor belts, underground pumping and reservoir/sump controls, and critical alarms from remote locations.


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