Safety Weidmüller

Weidmüller’s safety relay ‘SCS 24V DC SIL3’

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With its new ‘SCS 24V DC SIL3’ SAFESERIES safety relay Weidmüller offers a module designed to shut down process industry plant and plant components in the event of an emergency. TUV NORD certified the safety relay fulfils SIL3 requirements in accordance with EN 61508. The relay features a safety and an additional monitoring circuit. Three relays are connected in series in the safety circuit to guarantee safe isolation of the output circuit at all times. The typical break time is 14 and the make time 7 ms. An internal 5 A fuse guarantees short-circuit protection in the SIL circuit. A special circuit arrangement in the input circuit ensures the module is immune to test pulses, which are generally undertaken in circuitry of this kind.

The additional monitoring circuit is designed as a wide voltage input. The typical make time in the monitoring circuit is 21 ms. These design features in a single device provide safety-oriented shutdown functionality (SIL3) as well as electrically isolated 24-230 V AC/DC field signals to the higher-level controls. Features that allow the user to deploy the relay to suit individual requirements, such as in back-up systems with high DC voltages or overfill protection solutions for tank systems. Two yellow LEDs integrated in the safety and monitoring circuits are located on the front of the device to simplify relay function and fault analysis. Easy accessibility ensures users are able to replace the fuse on site. Measuring just 113.6 mm x 22.5 mm x 117.2 mm (length/width/height) the safety relay is compact by design, meaning it requires only little space on DIN TS35 mounting rails in the electrical cabinet. Screw connection technology ensures vibration-resistant connection of conductors with cross-sections between 0.14-2.5 mm².

Safety shutdown is afforded significant importance in the process industry because it serves functional safety, for instance in backup systems with high DC voltages. EN 61508 specifies that all devices utilized in safety circuits must fulfil particularly strict conditions. SIL (Safety Integrity Level) makes it possible to classify corresponding devices according to the reliability of their safety functions or rather safety-oriented design principles must be maintained in order to minimize the risk of malfunctions. A member of the SAFESERIES the new safety relay ‘SCS 24V DC SIL3’ now offered by Weidmüller is designed to guarantee emergency shutdown of systems and system components operating in the process industry. TUV NORD certified the safety relay fulfils Safety Integrity Level specifications in accordance with EN 61508.

The standard EN/IEC 61508 states that functional safety is given when safety systems and organizational measures minimize risk to an acceptable level. In this case SIL 3 relays are deployed to ensure emergency shutdown. It is not envisaged to implement an event of this nature on a permanent basis, but merely once a year in an emergency. Accordingly, operation in low demand mode is sufficient. However, it should be noted that the high demand operating mode also has a central role to play in creating functional safety. Overfill protection solutions in tank systems is just one example, because their control is a permanent process. Weidmüller also tackles demands of this nature with its new safety relay.

The new safety relay satisfies the highest demands with proven technology and sophisticated features. For instance, status indicators indicate that the relay is operating flawlessly: two yellow LEDs, one ‘Relay Output’ (safety circuit) and one ‘Monitor’ (monitoring circuit) simplify error analysis considerably. Three relays are connected in series in the output of the safety circuit (SIL circuit). This circuit arrangement guarantees reliable isolation in the safety circuit. One challenge to be faced when deploying the safety relay in process engineering and control systems is to accommodate different supply voltages. For example backup systems are operated with very high DC voltages. For that reason Weidmüller has additionally added a monitoring circuit to its safety relay; this is equipped with a wide voltage input, and as a consequence is able to meet this requirement. These design features in a single device provide safety-oriented shutdown functionality (SIL3) as well as electrically isolated 24-230 V AC/DC field signals to the higher-level controls.

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A special circuit arrangement makes the input circuit immune to test pulses, which are usually used in distributed control systems (DCS) to monitor the status of signal circuits. The max. permissible switching voltage is 250 VAC/30 VDC; the max. permissible switching current is limited to 5 A. A standard internal fuse provides short-circuit protection. The typical break time is about 14 and the make time 7 ms. The input of the safety circuit of the ‘SCS 24V DC SIL3’ is rated as follows: rated control voltage 24 VDC (+/- 20%) and 35 mA current consumption. To ensure rapid on-site replacement the fuse is easily accessible.

The design of the monitoring circuit is a further compelling feature: a wide voltage input from 24 V AC/DC to 230 V AC/DC forms the basis for individual application solutions.

The safety relay is designed for use in a wide range of temperatures from -25 °C to +50 °C without restrictions. Thanks to its compact dimensions of just 113.6 mm x 22.5 mm x 117.2 mm (length/width/height) the SAFESERIES safety relay requires just a small amount of space on DIN TS 35 mounting rails in electrical cabinets (IP20).

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