CCST Troubleshooting & Maintenance of Control Systems 2

Question 1

A 4-20 mA transmitter is reading 3.8 mA with no process variable change. What is the most likely cause?

Accepted Answer

Open circuit in the signal loop

Answer

A reading below 4 mA (typically 3.6-3.8 mA) indicates a broken or open circuit in the loop, which is the NAMUR standard for wire-break detection.

Question 2

During preventive maintenance on a pneumatic control valve, you notice the diaphragm casing is dented. What is the primary concern?

Accepted Answer

Reduced actuator force due to altered diaphragm area

Answer

A dented diaphragm casing distorts the diaphragm, reducing its effective area and therefore the force the actuator can develop at a given pressure.

Question 3

A PID controller is hunting (oscillating continuously) around the setpoint after a process load change. Which tuning adjustment is most appropriate first?

Accepted Answer

Decrease proportional gain (increase proportional band)

Answer

Continuous hunting usually indicates excessive proportional gain; reducing it (widening the proportional band) is the first corrective tuning step.

Question 4

A thermocouple reading drifts low over several months despite stable process conditions. What maintenance action should be taken first?

Accepted Answer

Inspect and replace the thermocouple if oxidized or contaminated

Answer

Gradual drift in thermocouple readings is most often caused by wire oxidation, decalibration from repeated thermal cycling, or contamination of the junction.

Question 5

A technician is troubleshooting a loop where the control valve fails to move even though the I/P converter output is changing. What should be checked next?

Accepted Answer

Air supply to the I/P and actuator

Answer

If the I/P transducer output changes correctly but the valve does not move, the most likely cause is loss of instrument air supply to the I/P or actuator.

(CCST) Certified Control Systems Technician Practice Test

(CCST) Certified Control Systems Technician Practice Test

CCST Troubleshooting & Maintenance of Control Systems 2