1) Why is the breakdown under impulse statistical in nature?
The breakdown is statistical in nature as it depends on the availability of the first free electron necessary for electrical conduction in air.
2) What is the significance of CFO?
It is essential to know the Critical Flash Over Voltage in order to provide adequate clearances in case of air gap insulation and also to design for an appropriate withstand voltage under impulse conditions in the case of solid insulation.
3) What are the corrections applied to the measured values?
Air density and humidity correction factors are applied to the measured values. The Air density factor is ( ) . the humidity correction factor is not applied in the case of sphere gap breakdown due to very little variation with humidity.
4) What is the significance of studying the CFO for a sphere gap ?
The sphere gap is a standard calibrating device in High Voltage Measurements. This is because, the breakdown voltages have been correctly determined and documented for various sphere diameters and gap distances under AC, DC and impulse conditions at standard temperature and pressure. It thus constitutes a good laboratory experiment where the student can verify and cross check these values and be able to calibrate test sources.
5) For a sphere gap, what is the critical gap spacing upto which the field is uniform?
For a sphere gap with a symmetric configuration, the field remains uniform upto a distance ‘d/2’ , where d is the diameter of the sphere.
6) What gap configurations are possible?
Most of the sphere gap assemblies are used in the vertical configuration. However, they can also be used in horizontal configuration.
7) How is the gap distance varied?
Normally one end of the sphere gap, the top end is fixed while the bottom end is connected to a movable insulating spindle which can be moved by a motorized arrangement in a vertical plane to adjust the gap distance.
8) Why is a hollow sphere used?
Hollow sphere is used so as to reduce the weight and provide a perfect conducting surface. The spheres are normally made of copper.
9) How are spheres maintained??
Due to repeated breakdowns, the surface of the sphere may get pitted and have black carbon deposits. These can affect the breakdown. The spheres are periodically cleaned by using a soft cloth. The use of a chemical is never recommended to clean the spheres.