Answer to Question #95627 in Electric Circuits for jane montgomery

Question

Household circuits are wired with GFCI outlets near sinks and other sources of water. Why is this
necessary for safety? How does a GFCI outlet work?

Answer 1

It is necessary because it increases safety in dangerous places such as bathrooms, kitchens and other places with access to water and metal pipes. For a human body AC currents higher than 30 mA within more than a seconds are dangerous, because heart fibrillation can occur. That is why GFCI devices turn off a circuit when the difference between the current in the line conductor and the N conductor is 5 to 30 milliamps within just 0.3 seconds. That current passes through a human body and may cause death if not turned off.

How GFCI works: in normal mode the currents in phase conductor and N-conductor are equal and opposite, so the resultant magnetic field around these conductors is 0. If someone touches a metal tap or freezer or washing machine with their case under some voltage (this may happen if the wire insulation is damaged and the wire touches some conductive surface), in TN-S and TN-C-S earthing systems the current from the tap will flow to the ground through a human body and PE conductor, this is a residual current. This will cause the decrease of current in N conductor (remember Kirchhoff's current law) and the resultant magnetic field around the phase and N wires will not be zero. If we pass these wires through a coil, the non-zero magnetic field will cause some current in the coil. The GFCI outlet breaks the circuit when this current flows in the coil connected to GFCI. If the insulation isn't damaged, the currents in phase and N are equal, and the GFCI has no impulse to break the circuit.