1769-IF4 A-B  Analog Input Module

The figure below shows a typical control circuit with a master control relay. This circuit works fine, if we can assume that CR(b) never welds. If CR(b) were to weld, pushing the E–Stop button will not remove power from the motor (M). If the motor drives a blade or cutter, the hazard can not be disabled. If the operator assumes the blade has stopped and enters the area, he could suffer serious injuries.

If a process can cause serious harm to an operator, it is a likely candidate for a redundant, self–monitoring control circuit, like the one shown below. This is similar to the internal circuit found in an Allen–Bradley safety relay. It is redundant because it uses three relays (redundancy requires two or more of a given component). It is self–monitoring or self–checking because it allows power to be removed from a load, even if a contact welds, but will not allow a subsequent restart until the fault is eliminated.

Normal Operation When the E–Stop is pulled up, power flows through CR3(a) and CR1(b) energizing the coil CR2. When this coil is energized, the NO contacts CR2(a), CR2(b), and CR2(c) close and energize coil CR3. The NO contact CR3(b) closes and holds the coil CR3 energized. The device is now “armed and ready”. When the operator pushes the start button, coil CR1 energizes. CR1(a) closes to hold in CR1. Then the NC contact CR1(b) opens and disconnects power from coil CR2. The final state is: coil CR1 on, coil CR2 off, and coil CR3 on. This condition allows the contacts of the output line [CR1(d), CR2(d), CR3(c)] to be closed and the load energized. The output line is also called the “monitored output” or the “safety output”. The terminology is the same for safety relay outputs.

The structure and details of the product：

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