External Over voltages 
This kind of over voltages originates from atmospheric disturbances, specially because of lightning. This takes the  
shape of a surge and has no direct courting with the working voltage of the road. It can be because of any of  
the subsequent reasons: 
i. Direct lightning stroke 
ii. Electromagnetically prompted over voltages because of lightning discharge taking place  
close to the road, called `aspect stroke'. 
iii. Voltages prompted because of atmospheric adjustments alongside the period of the road. 
iv. Electrostatic best friend prompted voltages because of presence of charged clouds nearby. 
v. Electro statically prompted over voltages because of the frictional results of small particles  
like dirt or dry snow withinside the surroundings or because of extrade withinside the altitude of the road. 
Internal Over voltages: 
These over voltages are as a result of adjustments withinside the working situations of the energy gadget. These can be  
divided into  agencies as below: 
Switching over voltages or Transient over operation voltages of excessive frequency: 
This is prompted whilst switching operation is completed below everyday situations or whilst fault takes place withinside the  
network. When an unloaded lengthy line is charged, because of Ferranti Effect the receiving give up voltage is  
extended extensively ensuing in over voltage withinside the gadget. Similarly whilst the number one aspect of the  
transformers or reactors is switched on, over voltage of temporary nature takes place. 
EFFECTS OF OVER VOLTAGES ON POWER SYSTEMS: 
Over voltage has a tendency to strain the insulation of the electric equipment‟s and in all likelihood to motive harm to them  
whilst it often takes place. Over voltage as a result of surges can bring about spark over and flash over between  
segment and floor on the weakest factor withinside the network, breakdown of gaseous/solid/ liquid insulation, failure  
of transformers and rotating machines. 
Switching Impulse or Switching Surge: 
When a no load transmission line is abruptly switched on, the voltage on the road turns into two times of everyday  
gadget voltage. This voltage is temporary in nature. When a loaded line is abruptly switched off or  
interrupted, voltage throughout the road additionally turns into excessive sufficient modern reducing withinside the gadget specially  
at some stage in establishing operation of air blast circuit breaker, reasons over voltage withinside the gadget. During insulation  
failure, a stay conductor is abruptly earthed. This can also prompted unexpected over voltage withinside the gadget. If  
emf wave produced with the aid of using alternator is distorted, the problem of resonance may also arise because of fifth or better  
harmonics. Actually for frequencies of fifth or better harmonics, a crucial state of affairs withinside the gadget so  
appears, that inductive reactance of the gadget turns into simply identical to capacitive reactance of the gadget. As  
those each reactance cancel every different the gadget turns into in simple terms resistive.