266 Electrical Power Systems Technology
pital applications and residential applications are designed to protect peo-
ple from excessive shock. The motor and electrical power applications are
designed for protecting electrical equipment.
Another classification method is according to the amount of current
required before an alarm system sounds, or the disconnect of an electrical
circuit occurs. Typical current values that will cause alarms or disconnects
to activate are 0.002 amperes (2 mA) for hospital applications, 0.005 am-
peres (5 mA) for residential applications, 5 to 100 amperes for motor-pro-
tective circuit applications, and 200 to 1200 amperes for electrical power
distribution equipment applications.
Need for Ground-fault Protection
In order to understand the need for a ground-fault circuit interrupter
(for the protection of people), certain basic facts must first be understood.
These facts relate to people as well as to ground faults.
One important fact is that a person’s body resistance varies with
the amount of moisture present on the skin, the muscular structure of the
body, and the voltage to which the body is subjected. Experiments have
shown that the body resistance from one hand to the other hand is some-
where between 1000 and 4000 ohms. These estimates are based upon sev-
eral assumptions concerning moisture and muscular structure. We also
know that resistance of the body (hand to hand) is lower for higher volt-
ages. This is because higher voltages are capable of “breaking down” the
outer layers of the skin. Thus, higher voltages are more dangerous.
We can use Ohm’s law to estimate that the typical current resulting
from the average body resistance (from hand to hand) is about 115 mA at
240 volts AC, and about 40 mA at 120 volts AC. The effects of a 60-Hz AC
on the human body are generally accepted to be as given in Table 10-1.
Ventricular fibrillation is an abnormal pattern of contraction of the
heart. Once ventricular fibrillation occurs, it will continue, and death will
occur within a few minutes. Resuscitation techniques, when applied im-
mediately, can save a victim. Deaths caused by electrical shock account
for a high percentage of the deaths that occur in the home and in indus-
try. Many of these deaths are due to contact with low-voltage circuits (600
volts and under), mainly 120- and 240-volt systems.
Ground-fault Protection for the Home
Gr ound-fault interruptors for homes are of three types: (1) circuit
breaker, (2) receptacle, and (3) plug-in types. Ground-fault protection de-