Figure 10.54 Structure of power line power scavenging. (a) General view, (b) axial cross section
10.24 Ground Fault Circuit Interrupt (GFCI). An important safety device is the GFCI (also called a residual current device
(RCD)). It is intended to disconnect electrical power if current flows outside of the intended circuit, usually to ground, such
as in the case when a person is electrocuted. The schematic in Figure 10.55 shows the concept. The two conductors
supplying power to an electrical socket or an appliance pass through the center of a toroidal coil. Normally the currents in
the two conductors are the same and the net induced emf due to the two conductors cancel each other, producing a net zero
output in the current sensor. If there is a fault and current flows to ground, say a current Ig, the return wire will carry a
smaller current and the current sensor produces an output proportional to the ground current Ig. If that current exceeds a set
value (typically 30 mA), the induced emf causes the circuit to disconnect. These devices are common in many locations
and are required by code in any location in close proximity to water (bathrooms, kitchens, etc.).
Consider the GFCI shown schematically in Figure 10.55. The device is designed to operate in a 50 Hz installation
and trip when the output voltage is 100 μV RMS. For a toroidal coil with average diameter a ¼ 30 mm and a crosssectional
diameter of b ¼ 10 mm:
(a) Calculate the number of turns needed if an air-filled toroidal coil is used and the device must trip at a ground
current of 6 mA.
(b) Calculate the number of turns needed to trip at a current of 6 mA if a ferromagnetic torus with relative
permeability of 2,000 is used as a core for the coil.