ISA Certified Automation Professional (CAP) program

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Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

In a 24-VDC open-collector output sinking configuration, which of the following could occur?

A. Configuration increases the risk of electrostatic discharge (ESD) from static electricity.
B. Short circuit from the load device to ground can cause unintended actuation.
C. The triode for alternating current (TRIAC) could remain on when the current (but not voltage) is zero unless a snubber network is used.
D. Inductive devices, such as motors, can generate back electromagnetic fields when turned on.

CAP Answer

The correct answer is B, “Short circuit from the load device to ground can cause unintended actuation.” An NPN (sinking) open-collector, when “energized,” switches the load side of the relay (at terminal “A”) to ground (terminal “B”). In the drawing, the NPN open-collector is energized at its base with a light-emitting diode in an optically isolated programmable logic controller output card.

If the circuit below “short circuits” from the load device (relay) to ground, it would have the same effect as switching on the NPN open-collector, and the relay (load) would be actuated.

Note: Resistors that are typically installed in these circuits to regulate or limit current have not been shown for simplicity of illustration.
Jul/Aug CAP Illustration Main image

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.


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