MIL-HDBK-1025/10
electrical system to meet safety and design requirements. A ground system consists of a
grounding connection, a grounding conductor, a grounding electrode, and the earth (soil)
that surrounds the electrode or some conductive body which serves instead of the earth
(an airplane's frame). A jumper connects conductors so that continuity is maintained.
Bonding is the joining of metallic parts to form a conductive path. Temporary grounds are
used so that work may be safely done on parts of the system that are temporarily isolated
and cleared (de-energized). To be safe when working on a de-energized circuit it should
always be de-energized, tested dead; isolated, tested dead; locked out and tagged; and
grounded.
5.3.1 Why Temporary Grounds Are Necessary. Energized lines over 50
volts which have been opened and checked as showing no voltage should be considered as
hot if they have not been grounded. Potential differences can occur on de-energized lines
from any of the factors described in Table 20. Temporary grounding is essential for
safety.
Table 20
Causes of Hazardous Induced Potential Differences
I.
Potential differences caused by various line effects (such as induced voltages from adjacent
energized lines and electrostatic build-up from wind action).
II. Lightning strikes anywhere in the circuit.
III. Fault-current feed-over from adjacent energized lines.
IV. Connection to an energized source through switching equipment, either by equipment
malfunction or human error.
V. Accidental contact of the de-energized line with adjacent energized lines.
VI. Residual charge from power-factor correction capacitors or surge arresters.
5.3.2 Equipotential (Single Point) Grounding. Whenever possible install
temporary grounding to provide an equipotential zone at the work site. An equipotential
zone provides a zero ground potential gradient across a worker's body, thus preventing a
harmful electrical current through the worker. Figure 1 shows the voltage gradient around
a grounded energized object when a ground fault occurs. Figure 2 shows the current path
across the worker's body that flows when there is a potential difference between two
different points or an individual's contact with the ground or a grounded structure. Table
21 indicates where grounds are provided in the sequence of de-energized
lockout/tagout/tryout steps.
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