Factors Affecting on Measurement
Influence of humidity : atmospheric condition :- Spark over voltage increases with humidity. The increase in humidity is about 2 to 3 % over normal humidity range of 8 g/m3 to 15 g/m3
- Humidity effect increases with the size of sphere and is maximum for uniform field gaps.
- The spark voltage increases with partial pressure of water vapour in air and for a given humidity condition, the change in spark over voltage increases with the gap length.
- The breakdown voltage increases with partial pressure of water vapor and increases in gap length is due to the relative values of ionization and attachment coefficient in air. The water particles readily attach free electrons, forming negative ions. Theses ions slow down and are unable to ionize neutral molecules under field conditions in which electrons will readily ionize.
- Variation is about 3% and no correction is usually given.
- Correction factor for humidity is calculated from dry and wet bulb temperature and finding absolute humidity and from it correction factor h.
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| Figure A |
If T - Temperature
P - Pressure in tort
V0 - Spark over voltage under standard condition at T 20°C and P = 760 torr.
V - Spark over voltage under tester specified condition Then V = KV0
Where K is function of air density factor d which is given by
d = p / 760 (293/273 + t)
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| Figure B |
- The effect is observed by kuffel. He observed that breakdown voltage reduces when gap is surrounded by metal cage cylinder especially when gap length exceeded a sphere radius.
- The reduction in voltage is given by,
△V = % reduction an
Where,
B - Diameter of earthed enclosing cylinders
D - Diameter of spheres
S - Spacing and m and care constants.
- The variation of breakdown voltage with A / D radius is given in Figure D.
- If specifications regarding the clearance are closely observed then error is within tolerance and specified accuracy.
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