Re: who can explain 450.5(3)
Kelly
Do you understand on how a GFCI detects when not all the current that is going out the hot is not returning via the neutral? The GFCI uses a small current coil/ transformer and has both the hot and neutral ran through it this coil will not have any current induced into it as long as both the hot and the neutral have the same amount of current on them (they will be 180 deg out of phase and one cancel's the other). But if there is any leakage of current to any other return path as in earth. EGC, your body. it will cause an imbalance in the two wires thus reduceing the canceling effect. This will allow a current to be induced into the coil and cause the GFCI to trip.
Now to anser your question, useing the same theory we place two of these coils on the phase conductors feeding a auto-transformer and if one phase is lost it will cause a current to be induced into the affected coil and cause a contact to trip the shunt trip breaker to remove all the current feeding this auto-transformer.
Maybe some of the others can be a little more simple in explaining this than me but I hope I helped.
