Question

air is a bad conductor of electricty . so, when the air become a good conductor of electricity​

Answer 1

Air becomes a good conductor of electricity when it is ionised.

Answer 2

Explanation:

Air does not conduct electricity in the way that metals do. We normally think of conductors as metals with free electrons that move easily throughout the whole metal. Small voltages move the electrons and a current can flow.

In the case of air and many other materials there are electrons present, but they are firmly bound to individual atoms and molecules and cannot move about the way the free electrons do in metals.

Now if we use a very very high voltage - from about 200 V to 1000 V (or higher) mostly - then we can get sparks formed, lightning or other sorts of plasmas where there are free electrons moving in the gas that can conduct electricity. A significant amount of energy is required to generate these plasmas because electrons from atoms and molecules must be removed against large energy barriers. High voltage is required to drive this 'ionization' process in the gas. Have a look at Paschen's Law and note the Paschen curves showing minimum breakdown voltages for different gases. (Note that energy is required to maintain free electrons in a gaseous environment like air, whereas in a metal conductor the free electrons are always present. Every free electron must be generated by ionization of gas atom/molecule or electron emission from a surface - so for every free electron in the plasma there must have been 5 to 10 eV or more energy used to generate the free electron ... and then electrons will be lost, for example in recombination with positive ions, so energy must be continually used to maintain a significant number of free electrons)

So with large voltages it is possible to generate electrons that can move freely in a gas and conduct electricity - as in the example of lightning.