Certified Control Systems Technician (CCST) Questions

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Certified Control Systems Technician (CCST)

This post covers the Certified Control Systems Technician (CCST) Questions and answers which are useful for the exam preparation.

CCST QUESTION

The main purpose of an optical isolation circuit on a digital input card in a PLC I/O system is:

A. to provide a common reference point for DC signals.

B. to isolate the low-voltage circuitry on the digital input card from the field device voltage.

C. to block light from the surroundings in order to prevent the digital input card from overheating.

D. to isolate the positive and negative terminals on the digital input card.

CCST answer

The correct answer is B , “to isolate the low-voltage circuitry on the digital input card from the field device voltage.” An optical isolation circuit is used to provide a barrier between the field wiring and associated field wiring issues (shorts, ground loops, transients, etc.) In most PLC systems, this is accomplished with a special high-precision LED as a light source and a phototransistor as a receptor. The dielectric barrier between the two provides physical separation of the LED circuit (connected to the field device in a digital input circuit) and the phototransistor (connected to the PLC card circuit.)

Answer A is not correct. This choice describes a feature of PLC cards, which do not have channel-to-channel isolation.

Answers C and D do not have any relation to the “optics” or “isolation” that is provided by an optical isolator. The “optics” in answer C refers to environmental light, which is not a factor in a PLC input circuit, and answer D refers to the normal isolation of positive and negative terminals in an electrical circuit.

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

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CCST question

Which of the following flow measurement devices does not require square root extraction?

A. magnetic flowmeter
B. venturi flowmeter
C. orifice plate
D. pitot tube

CCST answer

The correct answer is A, magnetic flowmeter. Square root extraction is a mathematical method that is required for flowmeter readings where a measured differential pressure, or pressure drop, is used to infer the flow rate. From the choices in the problem statement, the magnetic flowmeter is the only sensor on the list that does not depend upon a differential pressure to be developed to quantify flow rate, and therefore, does not require square root extraction.

Answer B, venturi flowmeter, infers flow rate by measurement of the differential pressure across a tapered tube.

Answer C, orifice plate, infers flow rate by measurement of the differential pressure across a thin plate with an orifice bored through it.

Answer D, pitot tube, infers flow rate by measurement of the differential pressure between a reference pressure (static pressure of the flowing fluid) and the pressure measured at the impact tube.

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

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CCST question

A schematic that is a representation of a complete hydraulic, electric, magnetic, or pneumatic circuit is called a:

A. process and instrumentation diagram (P&ID)

B. logic diagram

C. loop diagram

D. process flow diagram

CCST answer

The correct answer is C, loop diagram. This diagram shows complete details of a hydraulic, electric, magnetic, or pneumatic circuit, showing all of the interconnections, components, and devices that participate in that circuit.

Answer A, P&ID, is a diagram that shows the interconnection of process equipment and the instrumentation used to control the process.

Answer B, logic diagram, is a diagram used to define on/off control.

Answer D, process flow diagram, is a diagram that defines a process schematically and shows how much of each product a plant might make.

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

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CCST question

If a controller’s output increases when the controlled variable increases, the controller is said to be:

A. in a reset windup condition

B. direct acting

C. reverse acting

D. in a feed-forward control scheme

CCST answer

The correct answer is B, direct acting. The controller output acts in the direction of the change of process variable, hence the name “direct acting.” This type of controller would be used with a fail-closed valve in a pressure control scheme. As the pressure increases (controlled variable), the valve output would increase to open the valve in order to relieve the pressure.

Answer A is not correct, since reset windup is caused by allowing a controller’s integral action to continue to operate after a valve has reached an output limit. This can occur with direct- or reverse-acting controllers.

Answer C is not correct, since a reverse-acting controller will decrease its output as the controlled variable increases in value. This would be typical of a steam valve in a heating loop with a fail-closed valve. As the temperature rises, the output would decrease to reduce the amount of heat being transferred to the system.

Answer D is not correct. Feed-forward control can be configured for both forward- and reverse-acting loops and is concerned with the magnitude of control output changes based on process dynamics and transport times.

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

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CCST question

Which of the following features is unique to a pneumatic differential pressure sensor/transmitter?

a. high pressure inlet

b. d/p cell diaphragm

c. low pressure inlet

d. feedback bellows

CCST answer

The correct answer is D , feedback bellows. In a pneumatic transmitter, the nozzle faces a baffle, so, as the measured pressure increases, the baffle moves toward the nozzle. This causes the backpressure within the nozzle to rise. The rising pressure is amplified by a pneumatic relay, with the output pressure applied to both the feedback bellows and to the sensor output port…

As the bellows expands, it draws the nozzle away from the baffle. There is a balance that is achieved by matching the motion of the baffle with the motion of the nozzle. This balance keeps the distance between the baffle and nozzle, and, hence, the output pressure, constant for a constant input pressure. Only pneumatic differential pressure transmitters have this feature. There are other classes of devices, such as electro-pneumatic relays (I/P transducers) that also use this principle.

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

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