hi can anybody explain,why and how a resistance breaksdown in any electrical circuit? i will appreacite your help.thanks

 

Your question is hard to interpret. If you mean a literal resistance, like a resistor, then there are a couple of failure modes. The most common is too much current, which causes heating and eventual destruction of the device. Too much voltage can cause an arc across the terminals which will destroy the resistor.

If you meant "insulation" rather than "resistance", then this link will give you some information - http://www.engineersedge.com/instrumentation/electrical_insulation_testing.htm.