Measurement, Testing and Instruments
Measurement, Testing and Instruments
This domain tests instrument types, range extension, bridge methods, oscilloscope reading and a systematic approach to fault finding.
Core rules
- PMMC instruments respond to the average value and suit DC; moving-iron instruments respond to RMS and work on AC and DC.
- Ammeter range extension: shunt Rsh = Rm/(m − 1), where m = required current / full-scale current. Voltmeter: multiplier Rs = Rm(m − 1), where m = required voltage / full-scale voltage.
- Energy meter: energy in kWh = revolutions / meter constant (rev/kWh).
- CRO: voltage = divisions × volts/div (× probe attenuation); time = divisions × time/div; frequency = 1/period; peak amplitude = peak-to-peak ÷ 2.
- Bridges: Wheatstone for medium resistance, Kelvin double bridge for low resistance, Maxwell for inductance, Schering for capacitance and loss angle.
- CTs and PTs step current and voltage down for measurement; never open-circuit a live CT secondary.
- Fault finding: verify the symptom, work from supply to load or split the circuit in half, test continuity and voltage, then confirm against the circuit diagram.
Worked example
A CRO trace spans 4 divisions vertically at 2 V/div, and one cycle spans 5 divisions at 1 ms/div. Peak-to-peak voltage = 4 × 2 = 8 V, so peak = 4 V and RMS ≈ 2.83 V for a sine wave. Period = 5 × 1 ms = 5 ms, so frequency = 1/0.005 = 200 Hz. With a ×10 probe the voltage would read 80 V peak-to-peak, but the frequency is unchanged.
Mark-losing traps
- Reading peak-to-peak as peak amplitude.
- Ignoring probe attenuation on a scope.
- Connecting an ammeter in parallel with a supply, or a voltmeter in series.
- Overlooking voltmeter loading on high-resistance circuits.