Page 42 - FL-20A Instruction Manual
Model FL-20A Cable Fault Locator
Shielding can be used to limit voltages that can appear
on accessible conductive parts during fault conditions when
products generate high voltages internally. A properly con-
nected shield will prevent voltage on the accessible con-
ductive parts from exceeding line voltage during fault
conditions. This can help a GFCI function within its design
capabilities and protect people effectively from electric shock
from products that would otherwise demand a faster trip
speed of the GFCI for shock protection.
Fire hazards resulting from short-circuits involving the
shield and internal high-voltage supplies can be controlled
by overcurrent devices, temperature-sensitive devices and
similar products.
Polarization is a form of shielding. If the physical layout
of a product is such that parts connected to one side of the
line of a grounded system are more likely to be touched or
fault to accessible parts, then the line connections should be
such that the grounded side of the line is connected to those
more exposed parts. This can involve the use of plugs and
connectors that permit mating with only one polarity.
Interlocks and “smart” circuits can be used to keep
potentially hazardous parts de-energized unless specific safety
conditions are satisfied. Some of these “safety” conditions
include specific covers that must be closed, specific connec-
tors that must be fully mated with the proper receptacles, or
a power source that “handshakes” with the intended load,
and nothing else but the intended load.
“Smart” circuits may involve waveshaping and recognition
networks that permit current of recognizable traits to flow,
but that also de-energize the circuit if the current is not
shaped by the load in precisely the expected way. The addi-
tion of a human body in the circuit would add a load
of characteristics that are different from expected, and the
source would be rapidly de-energized.
The protective mechanisms that should be required
may be different for each product design. In general, the
system of protection against electric shock should consist
of one or more of those mechanisms that will effectively
Shielding
Polarization
Interlocks and
“Smart” Circuits
