Clinton FL-20A - Page 49

FL-20A Instruction Manual - Page 41
Model FL-20A Cable Fault Locator
A ground-fault circuit-interrupter (GFCI) monitors the
dierence in the current owing between the power con-
ductors serving a load. If the dierence exceeds a prede-
termined level, it is assumed that the dierence in current
could be owing through a person’s body, and the GFCI
rapidly trips. e speed of interruption is, by design, fast
enough to avert ventricular brillation. A typical Class A
GFCI trips in approximately one cycle of 60 Hz, and is
intended for use on circuits that have no more than 150
volts to ground. Circuits with more than 150 volts to
ground could cause higher body currents during a ground-
fault that would require a considerably shorter trip time
to avert ventricular brillation. Class A GFCIs used in the
United States for electric shock protection have a dierential
current trip rating of 5 mA. As such, these devices protect
consumers from ventricular brillation, as well as muscle
tetanization, which prevents them from breaking contact.
Many GFCIs are rated for a 15- or 20-ampere, 60-Hz
load. Many GFCIs have not been designed or tested for use
on circuits involving larger loads, higher frequencies, non-
sinusoidal waveshapes and DC components. New designs of
GFCIs may be needed for use on some of the electric vehicle
charging circuits.
A GFCI discerns load current from possible electric
shock current by where the current ows. Current owing
both to and from the load through the dierential trans-
former is considered by the device to be acceptable. Current
greater that the trip rating that ow outside the dierential
transformer is not acceptable. If a load is congured so that
a current carrier is connected to an accessible part, shock
current might be able to ow and not be discerned by a
GFCI as being dierent from ordinary load current. For
example, if one side of the circuit is connected to the vehicle
chassis, then shock current between an accessible energized
part and the vehicle chassis would appear to the GFCI as
load current. A GFCI would not be able to protect against
this type of fault.
If the system contains more than one source of voltage
that can be hazardous, a single GFCI may not be able to
protect against electric shock. Both sources need to be con-
sidered by the protection scheme.
Ground-Fault
Circuit-Interrupter