Safety chains are intended for monitoring critical processes. Their role is to detect dangerous process events and to deliver fail safe information to a control system. Examples range from escalator protection to flow monitoring. The new JAQUET T420 safety relay, when used with an IQ sensor, forms a measurement chain that fulfils the IEC 61508 standard for Safety Integrity Level 2.

The IQ sensor is the device that manages the monitoring process and delivers an OK/Alarm signal. The alarm or trip signal can then be easily integrated into a control system using the single change-over relay stage offered by the T420. Process monitoring can be based on any IQ-sensor limit function, e.g. over-speed / under-speed, speed regularity limits, direction of rotation, temperature limits or others are achieved with IQ operating in a fail safe mode.

Continuous integrity testing is required to meet SIL 2 requirements. Integrated self tests, e.g. sensor monitoring, RAM, ROM tests and system Watchdog are complemented by the ability to execute a periodic test routine which is initiated by maintenance personnel to check the integrity of the complete monitoring process. During periodic testing the IQ sensor delivers a reference signal to the T420 through its normal output line. T420 then verifies the levels and frequency of that signal. During this test, the time-bases of both devices are compared and CPU availability of both devices is also checked. The whole internal circuitry of the sensor, the wiring between the sensor and T420, the T420 internal circuitry and the relay output are thereby tested.

The open collector output of T420 is defined as system alarm. As long as the integrated self check routines confirm proper measurement chain operation, the alarm output has low impedance. If a fault is detected, the open collector output has high impedance.

A front panel LED also indicates system integrity.