Difference of potential. If no difference of potential exists, you don't get electrocuted. A bird sitting on a hot wire is at the same electrical potential, until it touches, ie. references ground, or another phase, thus creating a difference of potential between, gnd. or the other phase. That is how you are able to work on a hot circuit, as long as you avoid touching gnd. and/or a hot leg, or two hot legs, at the same time. All "Hot" work, should be left to the "Hot" shots.
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