--- title: "Types of HV tests; sphere gap measurement" subject: "High Voltage Engineering" chapter: 7 key: "semester_8/high_voltage_engineering:sphere-gap-measurement" source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/sphere-gap-measurement site: BCT Engineering --- # Types of HV tests; sphere gap measurement High Voltage Engineering · Chapter 7: Introduction to High Voltage Testing - Asked in: 2073 Magh, 2+6 marks - Video: [Sphere Gap Method for High Voltage Measurement](https://www.youtube.com/watch?v=6jx2uxf7DoA) — Electrogyan - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/sphere-gap-measurement - Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-7 ## Question What are the types of tests carried out on high voltage equipment? Explain with neat and clean circuit diagram the method of measurement of high voltage using sphere gaps. ## Short answer Tests include type, routine, sample and by voltage: power-frequency, DC, high-frequency, impulse. A sphere gap of diameter D and spacing S measures peak voltage: flashover occurs at a known value from IEC tables, corrected with air density factor δ = 0.386b/(273+t). Used for AC, DC, impulse. ## Answer #### Types of tests on high voltage equipment - **By purpose:** type tests (on one sample of a design), routine tests (on every unit), sample/acceptance tests, and special tests. - **By voltage:** power-frequency withstand and flashover tests (dry and wet), DC tests, high-frequency tests, and impulse tests (lightning 1.2/50 μs and switching 250/2500 μs); plus non-destructive tests such as $\tan\delta$ and partial discharge. #### Measurement of high voltage using sphere gaps A **sphere gap** consists of two identical polished metal spheres separated by an air gap. The uniform field between spheres gives a breakdown voltage that is accurately known (IEC 60052 / IEEE Std 4 tables) for a given sphere diameter $D$ and spacing $S$. By noting the spacing at which flashover occurs, the **peak value** of AC, DC or impulse voltage is found. ```text HV test R (series source ----/\/\/\----+ (AC/DC/ | impulse) ( HV ) upper sphere | | S gap spacing | ( E ) lower sphere | | (earthed) +-------earth---+ ``` **Construction and arrangement** - Two equal spheres of copper, brass or aluminium, of standard diameters (2, 5, 6.25, 10, 12.5, 15, 25, 50, 75, 100, 150, 200 cm). - Small spheres are mounted horizontally; large spheres (about 50 cm and above) vertically with the lower sphere earthed. - Shafts, supports and nearby objects must keep the clearances given in the standard so that the field is not disturbed. - A **series resistor** limits the current at flashover, protects the sphere surfaces from pitting and damps oscillations: about 100 kΩ to 1 MΩ for AC and DC; a low, non-inductive value (below about 500 Ω) for impulses so the wave is not distorted. **Procedure** 1. Set the spacing $S$ larger than expected; apply the voltage. 2. For AC/DC, raise the voltage slowly (or reduce the gap) until flashover occurs; repeat several times and take the mean. 3. For impulses, find the 50% flashover voltage: the spacing at which half of the applied impulses cause flashover. 4. Read the standard breakdown voltage $V_s$ (kV peak) for that $D$ and $S$ from the table. **Correction for air density** Tables are for 20 °C and 760 mm Hg. Relative air density $$ \delta = \frac{b}{760}\times\frac{293}{273 + t} = \frac{0.386\,b}{273 + t} $$ ($b$ in mm Hg, $t$ in °C). The actual breakdown voltage is $V = kV_s$, where $k$ is a correction factor; for $0.95 < \delta < 1.05$, $k \approx \delta$. **Factors affecting accuracy** - Spacing should not exceed $0.5D$ for ±3% accuracy (up to $0.75D$ with lower accuracy). - Nearby earthed objects and the shaft affect the field. - Dust, fibres and humidity cause erratic flashovers; spheres must be clean and smooth. - For gaps below about 1 cm (and impulse tests) the gap is **irradiated** with UV light or a radioactive source to provide initiating electrons and reduce scatter. **Advantages:** measures peak of AC, DC and impulse voltages; simple, rugged; used to calibrate other instruments. **Limitations:** the measuring process causes flashover, so it cannot monitor voltage continuously, and corrections are needed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a sphere gap measure? A. The rms value of AC only B. The peak value of AC, DC or impulse voltage from the flashover spacing C. The average value of DC only D. The frequency of the source Answer: B. The breakdown voltage for a given diameter and spacing is tabulated, so flashover at spacing S gives the peak voltage. (Key point: What a sphere gap measures) 2. What is the role of the series resistor, and how is it chosen for impulses? A. It limits current and prevents pitting; for impulses use a low non-inductive value (below about 500 ohm) B. It raises the voltage; use a high value for impulses C. It measures the current; use 1 Mohm for impulses D. It acts as the divider; value does not matter Answer: A. A high resistor would distort the impulse wave, so a low non-inductive one is used. (Key point: Series resistor) 3. How is the sphere gap result corrected for air conditions? A. V = k Vs, where k is about the relative air density B. V = Vs/delta^2 C. No correction is needed D. V = Vs + b Answer: A. Tables are for 20 deg C and 760 mm Hg; for 0.95 < delta < 1.05, k is about delta. (Key point: Air density correction) 4. What spacing limit gives about +-3% accuracy? A. Spacing not more than 0.5 D B. Spacing at least 2 D C. Spacing equal to 5 D D. Any spacing Answer: A. For gaps beyond 0.5D the field becomes non-uniform and accuracy falls. 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