ISA Certified Control Systems Technician (CCST) program


Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST 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 load device to ground can cause unintended actuation.
C. The 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 EMF when turned on.

CCST Answer

The correct answer is C: "Short circuit from load device to ground can cause unintended actuation".

In a sinking output configuration, power is connected to the load and the digital output card channel switches (or sinks) the load to ground in order to actuate the device.

If there is a short circuit between the load and ground, it effectively creates same condition as the digital output switching the load to ground. Therefore, the device will actuate.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition (2005), ISA Press