--- title: "Series impedance voltmeter and its drawbacks" subject: "High Voltage Engineering" chapter: 7 key: "semester_8/high_voltage_engineering:series-impedance-voltmeter" source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/series-impedance-voltmeter site: BCT Engineering --- # Series impedance voltmeter and its drawbacks High Voltage Engineering · Chapter 7: Introduction to High Voltage Testing - Asked in: 2071 Magh, 6+2 marks - Video: [High Ohmic Series Resistance with Microammeter - High Voltage Engineering](https://www.youtube.com/watch?v=24yKoweYHdY) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/series-impedance-voltmeter - Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-7 ## Question Describe the construction and working principle of series impedance voltmeter used in measurement of high AC voltages. What are its drawbacks? ## Short answer A high AC voltage is applied across a known series impedance, and the resulting small current, a few microamperes to a milliampere, is read by an earthed ammeter; voltage is V = IR for resistance or V = I/(ωC) for capacitance, with the Chubb–Fortescue rectifier version measuring peak. Drawbacks are stray capacitance and inductance, heating and temperature drift, corona and leakage errors, harmonic and waveform dependence, plus bulk and cost at very high voltages. ## Answer A **series impedance voltmeter** measures a high AC voltage by passing a small current through a known high impedance (resistance or capacitance) in series with a low-range ammeter. The ammeter is at earth potential, and the voltage is calculated from the current. #### Construction ```text HV (V) ---[ Z: high R or C ]---+ | ( A ) microammeter | with protective gap / zener across it | earth --------------------------+ ``` - **Series impedance $Z$:** either - a very high **resistance** (wire-wound or carbon/metal-film resistors in series, housed in an insulating tube filled with oil or SF₆, with corona-free end caps), or - a **capacitor** (compressed-gas standard capacitor) for AC. - **Ammeter:** a moving-coil microammeter (with rectifier for AC) or a sensitive AC ammeter at the earthed end. - **Protection:** a spark gap, neon glow tube or zener diodes across the ammeter so that a flashover of $Z$ does not put full voltage across the meter. #### Working principle **(a) Series resistance:** current $I = V/R$, so $$ V = I\,R $$ With residual inductance $L$ the impedance is $Z = R + j\omega L$ and $V = I\sqrt{R^2 + \omega^2L^2}$; $R$ is chosen so that $\omega L \ll R$. **(b) Series capacitance:** current $I = j\omega C V$, so $$ V = \frac{I}{\omega C} $$ In the **Chubb–Fortescue** method, two diodes are connected in anti-parallel in the capacitor arm, and a moving-coil meter is placed in one diode branch, so it carries the charging current of one half-cycle only. The charge per cycle is $C\cdot 2V_m$, so the meter reads $I_{av} = 2fCV_m$, and the **peak** voltage is $V_m = I_{av}/(2fC)$. (If a full-wave bridge rectifier is used instead, $I_{av} = 4fCV_m$.) The scale is calibrated directly in kV. Typical currents are a few μA to about 1 mA, so the impedance is very high, e.g. about $10^{9}$–$10^{11}$ Ω for resistive arms at hundreds of kV. #### Drawbacks 1. **Stray capacitance and inductance:** the long resistor has distributed capacitance to earth and residual inductance, so its impedance changes with frequency; errors increase for harmonics and high frequency. 2. **Heating and temperature coefficient:** power loss $I^2R$ heats the resistor; resistance changes with temperature, causing drift. 3. **Corona and leakage:** corona at terminals and leakage over the insulating surface add to the measured current, giving wrong readings at very high voltages. 4. **Harmonics (capacitive type):** current in a capacitor is proportional to frequency, so harmonics in the waveform cause large errors; also depends on exact supply frequency. 5. **Waveform dependence:** reads rms or mean, not peak (except the rectifier version), while breakdown depends on peak. 6. **Bulky and costly** for voltages above a few hundred kV; risk of damage to the meter on flashover without protection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a series impedance voltmeter find the high AC voltage? A. It passes a small current through a known high impedance and calculates V from the ammeter reading B. It divides the voltage with a potential divider and reads the low-voltage arm with a voltmeter C. It measures the spark-over distance of a gap D. It measures the electric force between two electrodes Answer: A. A known high impedance Z in series with a low-range ammeter at earth potential carries a small current, and V is calculated from that current (V = IZ). (Key point: Basic principle) 2. Why is a spark gap, neon tube or zener diode connected across the ammeter? A. To increase the sensitivity of the ammeter B. To reduce the stray capacitance of the resistor C. To stop a flashover of Z from putting full voltage across the meter D. To convert AC into DC for the meter Answer: C. If the series impedance flashes over, the full voltage could appear across the ammeter. The protective device limits the voltage across it. (Key point: Protection of the meter) 3. In the Chubb-Fortescue arrangement, the meter reads the mean rectified charging current I_av = 2fCV_m. What does this give? A. The rms value of the voltage B. The peak voltage V_m = I_av/(2fC) C. The frequency of the supply only D. The mean value of the voltage, which is zero Answer: B. Since I_av = 2fCV_m, the peak voltage is V_m = I_av/(2fC). This is useful because breakdown depends on the peak value. (Key point: Capacitive arm with rectifier) 4. Which is a drawback of the series impedance voltmeter? A. The resistor has no heating, so it needs no temperature correction B. Harmonics cause little error in the capacitive type C. Stray capacitance and residual inductance make the impedance change with frequency D. It can be used cheaply for any voltage above a few hundred kV Answer: C. The long resistor has distributed capacitance to earth and residual inductance, so its impedance varies with frequency. The capacitive type is also sensitive to harmonics because its current is proportional to frequency. I^2R heating causes drift, and the instrument is bulky and costly at very high voltages. 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