CCT CCT Electrical & Electronic Calibration 1 — Questions and Answers
Question 1: Which instrument is used as the primary reference for calibrating digital multimeters in an electrical calibration lab?
- Oscilloscope
- Multifunction calibrator (Correct answer)
- Signal generator
- Power supply
Correct answer: Multifunction calibrator
A multifunction calibrator (such as a Fluke 5700A or similar) generates precise, known voltage, current, and resistance values used as the reference when calibrating digital multimeters.
Question 2: What is the standard unit of electrical resistance in the SI system?
- Farad
- Henry
- Ohm (Correct answer)
- Siemens
Correct answer: Ohm
The Ohm (Ω) is the SI unit of electrical resistance, defined as the resistance between two points when a constant potential difference of one volt produces a current of one ampere.
Question 3: When calibrating an AC voltmeter, which parameter is most important to verify in addition to amplitude accuracy?
- DC offset
- Frequency response across the instrument's rated bandwidth (Correct answer)
- Battery voltage
- Display brightness
Correct answer: Frequency response across the instrument's rated bandwidth
AC voltmeters must be verified for frequency response across their rated bandwidth because accuracy can degrade significantly at frequencies near or beyond the instrument's specified range.
Question 4: What is 'burden voltage' in the context of current calibration?
- The maximum voltage a power supply can deliver
- The voltage drop across a current-measuring instrument due to its internal resistance (Correct answer)
- The voltage required to start an electric motor
- The output voltage of a current calibrator
Correct answer: The voltage drop across a current-measuring instrument due to its internal resistance
Burden voltage is the voltage drop developed across a current-measuring instrument (such as a shunt or clamp meter) caused by its internal impedance, which can affect the circuit being measured.
Question 5: Which four-wire (Kelvin) measurement technique is used in precision resistance calibration to eliminate the effect of lead resistance?
- Two-terminal measurement
- Bridge measurement
- Four-wire (Kelvin) connection (Correct answer)
- Single-ended measurement
Correct answer: Four-wire (Kelvin) connection
The four-wire (Kelvin) connection method uses separate current-source and voltage-sense leads, eliminating the resistance of the test leads from the measurement and improving accuracy for low-resistance values.
Question 6: What does 'loading effect' mean when calibrating voltage-measuring instruments?
- Physical weight of the instrument on the bench
- The change in the measured voltage caused by the instrument drawing current from the source (Correct answer)
- Overloading the instrument with too high a voltage
- The effect of ambient temperature on readings
Correct answer: The change in the measured voltage caused by the instrument drawing current from the source
Loading effect occurs when a voltmeter draws current from the circuit under test, causing a voltage drop and a reading lower than the true open-circuit voltage, which is significant with high-impedance sources.
CCT CCT Electrical & Electronic Calibration 1
Which instrument is used as the primary reference for calibrating digital multimeters in an electrical calibration lab?