Pump failure signals are fed into the SIS system; wiring issue
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If the pump failure signal is obtained from a thermal relay and fed into the SIS system, is it required that the thermal relay send an open contact signal to the SIS when there is a pump failure? (In other words, when the pump is operating normally, the contact signal sent by the thermal relay remains closed; it only opens in the event of a pump failure. Is this what is meant by fail-safe design?) ) If the fault signal is fed into the DCS, the approach is generally the opposite of that in SIS.
Yes, your understanding is correct. This design approach is commonly referred to as fail-safe design or fail-safe state. When the pump is operating normally, the contacts of the thermal relay remain closed. Once a fault in the pump is detected (such as overheating or overcurrent), the contacts are opened and a fault signal is sent. This signal is received by the SIS system (Safety Instrumented System), which then takes appropriate action, such as triggering an alarm or implementing emergency measures. In DCS (Distributed Control Systems), the logic for handling fault signals may be the opposite of that in SIS. In some designs, the fault condition may be indicated by closed contacts, which requires corresponding handling during wiring. In general, this involves designing both the hardware and software so that the system can handle failures in a safer and more effective manner, thereby reducing the risks associated with those failures. .