Fault clearance in power distribution systems is normally provided by circuit breakers, fuses, or overload relays
in each phase. These devices provide personnel protection only if the fault current is sufficient to trip the
over-current device. They generally however do not have response times which are adequate to protect the,
individual if he happens to be in direct contact with the energized object.
4.1.2 Ground-Fault-Circuit-Interrupter (GFCI). High resistance faults (low and moderate currents of 5
milliamperes or more) can be cleared rapidly with a device called a ground-fault-circuit-interrupter (GFCI).
The GFCI contains an electronic circuit which continuously monitors the difference between the current
supplied to the load and the current returned from the load. If this difference is not zero, some current must be
leaking to ground. When this leakage current exceeds a preset value, the GFCI will act to interrupt the power
to the circuit. GFCI's are so sensitive that they can be set to interrupt power fault currents as low as 2
milliamperes. Experiments with dogs have shown that trip currents of 5 milliamperes or less will prevent
electrocution. (GFCI's have proven so effective as protection against electric shock that the National
Electrical Code requires that all 15 and 20 ampere bathroom, garage, and outdoor receptacles in family
dwelling units and in circuits set up at construction sites be protected with a GFCI. MIL-STD-188-124A also
recommends they be installed on 120 volt single phase 15 and 20 ampere receptacles of C-E facilities.)
4.2 EARTH CONNECTION.
Historically, grounding requirements arose from the need to protect personnel, equipment, and facilities from
lightning strokes and from industrially generated static electricity. Structures, as well as electrical equipment,
were connected to earth, i.e., grounded, to provide the path necessary for lightning and static discharges. As
utility power systems developed, grounding to earth was found to be necessary for safety. All major
components of the system such as generating stations, substations, and distribution systems are earth grounded
to provide a path back to the generator for the fault currents in case of transmission line trouble. The path to
earth should have as low a resistance as possible. A low resistance minimizes the potential difference between
equipments connected to the earth electrode subsystem when fault currents flow. Thus personnel who come in
contact with two or more pieces of equipment at one time are protected.
Ideally, the earth connection should exhibit zero resistance between the earth and the equipment and facilities
connected to it. Any physically realizable connection, however, will exhibit a finite resistance to earth. The
economics of the design of the earth electrode subsystem involves a trade-off between the expense necessary
to achieve a low resistance and the satisfaction of minimum subsystem requirements. The 10 ohm design
objective of MIL-STD-188-124A is considered such a trade-off.
4.3 AC POWER LINE GROUND.
The grounding conductor (green wire) in a single-phase 115/230 volt ac power distribution system in a facility is
one of four leads, the other three being the two phase or "hot" leads (black/red) and the neutral lead (white
wire). The green wire is a safety conductor designed to carry current only in the event of a fault. The "hot"
leads are connected from the first service disconnect to the high sides of the secondary of the distribution
transformer and the neutral is connected to the center tap which is grounded to a ground terminal at the
transformer. When a single transformer supplies power to only one communications building, for fault