GETCO Exam Paper (30-11-2008)
Corona usually occurs when the electrostatic stress in the air around the conductor succeeds

30 kV (maximum value)/cm
22 kV (maximum value)/cm
11 kV (rms value)/cm
6.6 kV (rms value)/cm

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GETCO Exam Paper (30-11-2008)
The function of a breather in a transformer is

to control the level of oil in the tank
to suppress harmonics
to arrest flow of moisture into the tank
to provide protection against over currents

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