Sphere Gap Meter

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Sphere Gap Meter

Q. Explain how a sphere gap can be used to measure the peak value of voltages. What are the parameters and factors that influence such voltage measurement ?

Sphere Gap Meter

  1. It is one of the standard method for measurement of peak value of DC, AC and impulse voltage and used for checking the voltage and other voltage measuring devices used in high voltage test circuits. Breakdown of sphere gap occurs within a few micro seconds once the applied voltage exceeds the static breakdown discharge voltage.
  2. Two identical metallic sphere separated by certain distance form a sphere gap.
  3. Electrode : are made up of Al, bronze, brass, steel, copper, light alloys of Diameter, 6.25, 10, 125, 15, 25, 50, 150, 200 cm. Care should be taken not exceed gap length over a sphere radius.
  4. A series resistance is connected between sources and sphere to Limit the breakdown current
  5. To suppress the unwanted oscillations in the source voltage under breakdown occurs. Resistance of 100 to 1000 K for AC and DC and not more than 500 for impulse voltage is connected.

Advantages of Sphere Gap Meter :

  1. For peak value of ac and dc, calibration of voltmeter.
  2. Method is simple and up to 2 to 2500 kV.
  3. Can be used for measurement of voltage in stage (impulse) tests.
  4. Method of voltage measurement with spring gap is cheap, simple and reliable.

Disadvantages of Sphere Gap Meter :

  1. Does not give a continuous record of voltage.
  2. Voltage measurement carried out by sphere gap are accurate within 2-3% only with precautions taken.  Accuracy not so high compared to other electrical apparatus but acceptable.
  3. Takes in to accurate humidity pressure, temperature, proximity of earthed object to the testing side etc. which are of statistical nature. These statistical factors introduce some errors in voltage measurements.

Precautions to be taken :

  1. Usual separation of two spheres should not exceed the diameter of sphere.
  2. No measurement should be taken with spacing less than 5% of radius.
  3. The surface of sphere should be free from dust particles, shining surface, with scratching should be avoided.
  4. The spark over interval should be such that pitting and overheating of surface is avoided.
  5. No conducting surface should be in the region.

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