I was asked the other day by some co-workers what happens to current that is flowing down a line or wire when a breaker or other switching device is suddenly opened. The example they were questioning was a large amount of current 2000A at 4.16kV when a breaker was suddently opened. Some of their questions were weather or not there was some shunting device to get rid of the excess currents on the lines or if the powerlines or cables moved back and forth as kind of a reaction to the sudden interruption of the current. It seemed as if they were equating the sudden interruption of current to water hammer in a pipe.
My explaination to them was that AC current crosses zero twice every cycle at 60Hz. In a perfect world if the device was somehow able to open itself at exactly the zero crossing then it would be almost as if there was no current flowing down the line at all when the device interruped the line. In other words it would be the same as the device opening with no current flowing.
Since opening at zero crossing is not always the case I explained for all other times there is usually arcing in the interruption device until the zero crossing where the arcing is able to estinguish. As an example if a molded case breaker or switch is opened when current is flowing at the peak of the wave then most likely there will be acring between the contacts of the switch/breaker unti the current waveform crosses zero and the arc is able to distinguish iteslf. In some cases there may be enough of a voltage difference that the Arc is able to restrike and arc again for another half cycle until the next zero crossing where it is estinguished.
As a third case I mentioned an example as a vaccum breaker or contactor where there is not much arcing and therefore the current is immediately forced to zero. In such a case the effect of instantly forcing the current to zero usually results in a transient effect of some kind, and in most cases this transient effect is a voltage transient in the form of a voltage spike or voltage rise. This voltage transient can sometimes be followed by ringing or some other transient. So in this case the result of stopping current suddenly is a voltage reaction or transient.
I'm not sure if I addressed this question correcly but responded to the best of my knowledge. I wanted to share my response here, in order to hear what others had to say either in addition to or in correction to my response.
I appreciate any input.
My explaination to them was that AC current crosses zero twice every cycle at 60Hz. In a perfect world if the device was somehow able to open itself at exactly the zero crossing then it would be almost as if there was no current flowing down the line at all when the device interruped the line. In other words it would be the same as the device opening with no current flowing.
Since opening at zero crossing is not always the case I explained for all other times there is usually arcing in the interruption device until the zero crossing where the arcing is able to estinguish. As an example if a molded case breaker or switch is opened when current is flowing at the peak of the wave then most likely there will be acring between the contacts of the switch/breaker unti the current waveform crosses zero and the arc is able to distinguish iteslf. In some cases there may be enough of a voltage difference that the Arc is able to restrike and arc again for another half cycle until the next zero crossing where it is estinguished.
As a third case I mentioned an example as a vaccum breaker or contactor where there is not much arcing and therefore the current is immediately forced to zero. In such a case the effect of instantly forcing the current to zero usually results in a transient effect of some kind, and in most cases this transient effect is a voltage transient in the form of a voltage spike or voltage rise. This voltage transient can sometimes be followed by ringing or some other transient. So in this case the result of stopping current suddenly is a voltage reaction or transient.
I'm not sure if I addressed this question correcly but responded to the best of my knowledge. I wanted to share my response here, in order to hear what others had to say either in addition to or in correction to my response.
I appreciate any input.
