AC & DC

hi gents,

there is something i saw and it start turning in my mind and i would like to get explaining from you guys if you dont mind.

we do test for over current relay of GE752 DC generator, we do it by shorting the output leads +,- coming out from the generator which passing through DC contactor which will be off when the over current relay function.

we see the amps meter readings and increase the output from the generator untill the amps readings reach 1300DC amps.

my question is why it goes ok when we short DC currents? can we do the same for AC volts generator?

is that because the power goes in one direction from + to - and back to the generator 'one direction' and so on? while on AC it goes from phase A then B phase comes positive before A phase comes negative and the two lines become positive and cause short?

your help is appreciated

thanks