CCST Troubleshooting & Maintenance of Control Systems 4

Question 1

A control valve with a positioner consistently stops 5% short of the commanded position throughout its stroke. This is most indicative of:

Accepted Answer

Positioner feedback linkage slippage or incorrect cam

Answer

A consistent gain error across the full stroke (always 5% short) points to a mechanical error in the positioner feedback linkage or an incorrectly adjusted cam.

Question 2

During loop checkout, a technician applies 4 mA to a controller input and the display reads 1.2% instead of 0%. The most appropriate correction is:

Accepted Answer

Perform a zero trim on the transmitter or input card

Answer

A small offset at the live zero (4 mA) with no corresponding process variable indicates a zero calibration error that should be corrected with a zero trim.

Question 3

What does a negative loop gain indicate in a feedback control system during troubleshooting?

Accepted Answer

The controller action (direct/reverse) is set incorrectly

Answer

Negative loop gain (output moves in the wrong direction relative to error) means the controller action (direct vs. reverse acting) is set opposite to what the process requires.

Question 4

A technician is calibrating a RTD temperature transmitter. The measured resistance at ice point is 2 ohms higher than expected. What is the likely cause?

Accepted Answer

Lead wire resistance not compensated (2-wire RTD used without compensation)

Answer

In a 2-wire RTD connection, lead wire resistance adds directly to the measured resistance; a 3- or 4-wire configuration must be used or lead resistance must be compensated.

Question 5

An analog control loop passes loop current through a junction box where several connections are made. Intermittent signal drops to 0 mA are observed. What is the first thing to inspect?

Accepted Answer

Junction box terminal connections for corrosion or loose terminations

Answer

Intermittent drops to 0 mA suggest a momentary open circuit, and the highest-probability location in a loop is corroded or loose terminals inside the junction box.

(CCST) Certified Control Systems Technician Practice Test

(CCST) Certified Control Systems Technician Practice Test

CCST Troubleshooting & Maintenance of Control Systems 4