--- title: "General HV tests; resistance divider with micro-ammeter" subject: "High Voltage Engineering" chapter: 7 key: "semester_8/high_voltage_engineering:resistance-divider-microammeter" source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/resistance-divider-microammeter site: BCT Engineering --- # General HV tests; resistance divider with micro-ammeter High Voltage Engineering · Chapter 7: Introduction to High Voltage Testing - Asked in: 2075 Bhadra, 8 marks; 2074 Bhadra, 8 marks; 2070 Bhadra, 8 marks (asked 3 times) - Video: [POTENTIAL DIVIDERS FOR HIGH VOLTAGE MEASUREMENT](https://www.youtube.com/watch?v=7Tnyj34zyMY) — Simplified EEE Studies - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/resistance-divider-microammeter - Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-7 ## Question What are the general tests that are carried out on high voltage equipment? Explain with clean circuit diagram the method of measurement of high voltage using resistance divider and micro-ammeter. ## Short answer General tests: sustained low-frequency (withstand/flashover), DC, high-frequency, and impulse (lightning/switching) tests. Measurement: series resistor with micro-ammeter reads V=IR; micro-ammeter at earth potential, protected by gap. Alternatively, a resistive divider (R1, R2) yields V = V2*(R1+R2)/R2, measured by a high-impedance voltmeter. ## Answer #### General tests on high voltage equipment High voltage equipment is tested to prove that its insulation can withstand the stresses it will meet in service. By their nature, the **general tests** are: 1. **Sustained low-frequency (power-frequency) tests** – withstand and flashover tests at 50/60 Hz, dry and wet (rain), usually for 1 minute; done on insulators, bushings, transformers, switchgear, cables. 2. **High-voltage DC tests** – for cables, capacitors and rotating machines, where AC testing would need very large charging current; also leakage current measurement. 3. **High-frequency tests** – at kHz frequencies, for insulators used with carrier/radio frequencies and to check behaviour under high-frequency transients. 4. **Surge or impulse tests** – lightning impulse (1.2/50 μs), switching impulse (250/2500 μs) and chopped wave tests, to prove the basic insulation level (BIL/BSL). By purpose, tests are grouped as **type tests** (on one design sample), **routine tests** (on every unit), **sample/acceptance tests**, and **non-destructive tests** (insulation resistance, $\tan\delta$, capacitance, partial discharge measurement) that check condition without damaging the insulation. #### Measurement using series resistance micro-ammeter A very high resistance $R$ is connected in series with a micro-ammeter between the high voltage terminal and earth. The current is proportional to the voltage: $$ V = I\,R $$ ```text HV (V) | [R] very high R, in oil/SF6 tube | ( uA )----+ | | protective gap | === or zener | | earth ----+ ``` - $R$ is built from many wire-wound or metal-film resistors in series, inside an insulating tube filled with oil or SF₆, with corona-free end caps. It is chosen to pass a small current (a few μA to about 1 mA at full voltage). - The micro-ammeter is at earth potential, so it can be read safely; it is calibrated directly in kV. - A **protective device** (paper spark gap, neon tube or zener diode) across the meter bypasses the current if $R$ flashes over. - Used mainly for **DC**; also for AC when the resistor's stray capacitance is small. #### Measurement using resistance potential divider ```text HV (V) ---+ | [R1] high-voltage arm | +-------> V2 (to electrostatic | voltmeter / CRO) [R2] low-voltage arm | earth ----+ ``` $$ V = V_2\,\frac{R_1 + R_2}{R_2} $$ The low-voltage output $V_2$ is read by a high-impedance meter (electrostatic voltmeter, digital voltmeter or CRO). For impulses the divider is connected by a matched coaxial cable to an oscilloscope. #### Sources of error and remedies | Source of error | Remedy | |---|---| | Heating changes $R$ | Low-temperature-coefficient resistors, low current, oil cooling | | Stray capacitance to earth | Shielding / guard rings around the resistor column | | Corona and leakage currents | Smooth end caps, enclosure in oil or SF₆ | | Resistor inductance | Non-inductive (bifilar) winding | | Meter damage on flashover | Protective gap or zener across meter | Accuracy of about ±0.2% to ±2% is obtained with careful construction. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the series resistance micro-ammeter method, how is the voltage found? A. V = I/R B. V = I R, with the meter at earth potential and calibrated in kV C. V = I^2 R D. V = I R only if the meter is at HV Answer: B. A very high R in series carries a current proportional to V, read safely by the earthed micro-ammeter. (Key point: Series resistor method) 2. Why is a protective gap or zener diode connected across the micro-ammeter? A. To increase the sensitivity of the meter B. To bypass the current and save the meter if R flashes over C. To reduce the resistance R D. To convert AC into DC Answer: B. If the resistor flashes over, the full current would destroy the meter, so the protective device shunts it. (Key point: Protective device) 3. For a resistance divider with arms R1 (HV) and R2 (LV) and output V2, what is the measured voltage V? A. V = V2 R1/R2 B. V = V2 (R1 + R2)/R2 C. V = V2 (R1 + R2)/R1 D. V = V2 R2/(R1 + R2) Answer: B. V2 = V R2/(R1 + R2), so V = V2 (R1 + R2)/R2. (Key point: Divider formula) 4. Which remedy is correctly matched to its error in resistive HV measurement? A. Heating of R: use a high-temperature-coefficient resistor B. Stray capacitance: use shielding or guard rings C. Resistor inductance: use ordinary single-layer winding D. Corona: use sharp end caps Answer: B. Shielding and guard rings control stray capacitance to earth. Low-temperature-coefficient resistors, bifilar winding and smooth end caps fix the other errors. 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