1.A 4-20mA signal reads 12mA. If the range is 0-100 PSI, what is the pressure?
- A.50 PSI✓
- B.30 PSI
- C.75 PSI
- D.12 PSI
Why: 12mA is at the midpoint of 4-20mA range: (12-4)÷(20-4) = 8÷16 = 50%. 50% of 0-100 PSI = 50 PSI. Formula: Value = ((mA-4)/16) × span + zero.
2.A Wheatstone bridge circuit is used in instrumentation to:
- A.Amplify signals
- B.Precisely measure small changes in resistance (strain gauges, RTDs)✓
- C.Generate AC power
- D.Convert AC to DC
Why: Wheatstone bridges detect tiny resistance changes with high precision. When balanced, output = 0. Small resistance changes create a proportional voltage output.
3.A current transformer (CT) with a 200:5 ratio on a conductor carrying 150A outputs:
- A.3.75A✓
- B.7.5A
- C.30A
- D.150A
Why: CT output = Primary current × (Secondary ratio ÷ Primary ratio) = 150 × (5÷200) = 3.75A. CTs step down current proportionally for metering.
4.A capacitive proximity sensor detects:
- A.Only metals
- B.Metals, liquids, plastics, granular materials — most materials✓
- C.Only ferrous metals
- D.Only moving objects
Why: Capacitive sensors detect changes in the dielectric field — any material that changes capacitance: metals, plastics, glass, liquids, powders. More versatile than inductive sensors.
5.A SCADA system is used for:
- A.Motor starting only
- B.Supervisory control and data acquisition — monitoring and controlling industrial processes remotely✓
- C.Email communication
- D.Lighting control only
Why: SCADA collects data from remote instruments and PLCs, displays process status, allows operator control, logs data, and generates alarms. Used in utilities, oil/gas, water treatment.
6.Short circuit current in an industrial facility is determined by:
- A.The size of the motor
- B.The impedance of the entire system from the utility transformer to the fault point✓
- C.The length of the building
- D.The number of lights
Why: Available fault current depends on utility contribution, transformer impedance, conductor impedance, and motor contribution. Calculations determine required equipment AIC ratings.
7.Impedance grounding of a transformer neutral limits:
- A.Voltage
- B.Ground fault current to a safe level while maintaining ground detection✓
- C.Power consumption
- D.Harmonics
Why: Impedance (resistance or reactance) grounding limits ground fault current to reduce arc flash energy and equipment damage while still allowing ground fault relaying to detect and clear faults.
8.A control valve positioner ensures:
- A.The valve stays closed
- B.The valve position accurately follows the control signal despite friction and process forces✓
- C.Maximum flow always
- D.Minimum pressure
Why: Positioners compare the control signal to actual valve position (feedback) and adjust air pressure to the actuator until the valve reaches the exact demanded position. Eliminates hysteresis and stiction.
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