Nace CP 1, Course Manual

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RP0177-2000
NACE International 11
tolerated by human beings depend on many factors.
The possibility of shock from lower voltages is the
most difficult to assess. The degree of shock hazard
depends on factors such as the voltage level and
duration of human exposure, human body and skin
conditions, and the path and magnitude of any
current conducted by the human body. The
magnitude of current conducted by the human body
is a function of the internal impedance of the voltage
source, the voltage impressed across the human
body, and the electrical resistance of the body path.
This resistance also depends on the contact
resistance (e.g., wet or dry skin, standing on dry land or in water) and on the current path through the body
(e.g., hand-to-foot, hand-to-hand, etc.).
5.2.1.1 The safe limits must be determined by
qualified personnel based on anticipated
exposure conditions. For the purpose of this
standard, 15 V AC (rms) open circuit or a source
current capacity of 5 mA or more are considered
to constitute an anticipated shock hazard.
Tables 3 and 4 indicate the probable human
resistance to electrical current and current
values affecting human beings.
TABLE 3: Human Resistance to Electrical Current
(A)
Dry skin 100,000 to 600,000 ohms
Wet skin 1,000 ohms
Internal body—hand to foot 400 to 600 ohms
Ear to ear (about) 100 ohms
(A)
Reprinted with permission from the National Safety Council. Accident Prevention
Manual for Business & Industry: Engineering & Technology , 10th ed. Itasca, IL:
National Safety Council, 1992.
TABLE 4: 60-Hz Alternating Current Values Affecting Human Beings
Current Effects
1 mA or less No sensation—Not felt.
1 to 8 mA Sensation of shock—Not painful; individual can let go at will; muscular control not
lost.
8 to 15 mA Painful shock—Individual can let go at will; muscular control not lost.
15 to 20 mA Painful shock—Muscular control lost; cannot let go.
20 to 50 mA Painful shock—Severe muscular contractions; breathing difficult.
50 to 100 mA
(possible) Ventricular fibrillation—Death will result if prompt cardiac massage not administered.
100 to 200
mA (certain) Defibrillator shock must be applied to restore normal heartbeat. Breathing probably
stopped.
200 mA and
over Severe burns—Severe muscular contractions; chest muscles clamp heart and stop it
during shock (ventricular fibrillation if prevented). Breathing stopped—heart may start
following shock, or cardiac massage may be required.
Source: Unknown
.
5.2.1.2 In areas (such as urban residential
zones or school zones) where a high probability
exists that children (who are more sensitive to
shock hazard than are adults) can come in
contact with a structure under the influence of
induced AC voltage, a lower voltage shall be
considered.
5.2.1.3 The beginning sensation of shock, which
may occur at 1 to 8 mA, may not be painful or
harmful to a human being but may lead to an
accident by causing rapid involuntary movement
of a person.
5.2.2 In areas of AC influence, any measured AC
voltages between a structure and ground (or some
other adjacent structure) shall be considered an
indication that further study is required.
5.2.3 When the voltage level on a structure presents
a shock hazard, the voltage level must be reduced to
safe levels by taking remedial measures. In those
cases in which the voltage level cannot be reduced to
a safe level on aboveground appurtenances, other
safety measures shall be practiced to prevent shock
to operating and maintenance personnel and to the
public (see Paragraph 4.3).
5.3 Construction
5.3.1 Severe hazards may exist during construction
of facilities adjacent to AC power systems. A
responsible person shall be in charge of electrical
safety. This person shall be fully aware of proper
grounding procedures and of the dangers associated
with inductive and capacitive couplings, fault current,
lightning, etc., on aboveground and underground