ok, lets dumb this down so i can understand it,
power surge from your supplier is a worry,
so once the power goes off from a lightening strike, terrorist attack or squirell in the transformer, we don't care, we just want to protect the old Neve desk from failure,
what do we do?
a ups system will keep power going to the equipment by way of a backup battery and inverter, along with some fancy circuitry,
this would seem to be the best solution, as you might be recording a pepsi commercial with barry manilow or something,
GFI is more for human protection or equipment malfunction,
they come in many flavors, the home repo version might install in your bathroom and protect you while you are shaving, same fairchild chip in everyone keeps it cheap, but you do not wnat that,
other GFI models might be a small plastic or metal box that has a small toroid coil on a hi perm core, either powdered iron or 80 permalloy, which has either a single groundl wire or 2 hot and cold wires for 120 VAC or 3 wires for a three phase system, if a wye system, maybe just the neutral, when leakage current flows thru the toroid, say a 500 turn toroid, and the leakage is 5 ma, you will have 5ma/500T = 0.01 ma or 10 microamps, put a 1000 ohm burden resistor across the toroid and you will generate 0.000010 x 1000 = 0.01 volts AC or 10 millivolts.
this is why you need good shielding and a balanced circuit, to avoid noise hits from tripping the hi amp circuit needed to turn 10 mv into 1or 2 volts to switch a transistor base or pull in a relay.
usually a couple of opamps are used, one to jack up the ac signal and another as a comparator to switch on or off depending on the input signal vs a precision ref.
ferrite beads on the supply wires to the GF are a must for good operation, must be high bandwidth, maybe 7 turns of #36 on a 1/2 inch cylinder,
lets say you stick your hand in the toaster while standing in the bathtub, :-\
a current might flow from ground/neutral through your body to the hot side of the 120.
this would create a current in the neutral wire that is not normally there,
you can set the GFI to any trip current you like depending on the model,
they range from 1 ma for human/medical protection, all the way up to maybe 1 amp,
some have a range switch that allows say, 5, 10, 30 ma trip points,
others may have as pot for adjusting the trip point,
GFI's need to be shielded in order to prevent nuisance tripping from external sources,
you can get the output of the GFI in a form C relay of a solid state switch,
or a latching relay which will act like the shunt circuit breaker, otherwise the GFI will simple reset itself when the leakage current stops, so you need to add your own latching ralay or program the PLC to shut off power to the offending circuit.
otherwise you have a situation where the GFI gets tripped, power gets shut off, the leakage current therfore goes away, the GFI reverts to normal state which turns power back on, which trips GFI again, so you havean ugly motorboating going on that will be worse than any power surge you can imagine, thus, the latching rrlay.
so GFI would not be for power surge, UPS would be good or possibly a Furman power conditioner,m i do believe those would have some sort of power surge protection?
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