TM 5-811-3/AFM 88-9, Chap. 3
e. Resistances to ground. Resistance to ground for
poses of this manual, static electricity charges should
dissipation of static electricity charges is not critical
be considered as being generated by three classifica-
in order to provide adequate leakage path to ground
tions of sources.
and to equalize static electricity charges as fast as
(1) Magnetic inductions.
they are generated. Resistance to ground for static
electricity dissipation may be as much as 1,000,000
(a) office equipment with moving parts as in
data processing systems, having integral electric
ohms. However, resistances to ground of less than
motor-driven parts assembled in a ferrous metal fire-
25,000 ohms should be avoided when used with the
proof enclosure where the motors are grounded into
usual g-rounded electrical distribution system in order
the building electrical distribution system.
to avoid increased electric shock hazard to personnel
which may result in using lower resistances to
(b) Portable, normally ungrounded, electric
motor-driven equipment having a ferrous metal en-
ground. Maintaining an average range of between
closure exposed to operating personnel. Induced
25,000 to 100,000 ohms resistances, to limit the cur-
charges from magnetic induction sources could be of
rent magnitude to ground, is complicated by ambient
continuous duration at utilization voltage of electric
wet or dry conditions, such as: atmospheres, building
motors.
materials, and foundations of concrete or earth, Re-
sistance to ground limitations will be established for
(2) Electrostatics as defined in NFPA No. 77.
corresponding applications herein.
(3) Lightning static results from accumulations
of extremely high voltage discharges, as discussed in
paragraph 2la. These magnitudes of potentials are
32. Applications
sufficient to break down the dielectric strength of air
a. Conditions. It is not the intent of this manual to
for distances upwards of 3,000 feet, It will suffice to
attempt to furnish a listing of all applications where
note here that lightning discharges can and do by
static electricity protection should be provided. The
their so-called side effects break down the dielectrics
electrical designer must analyze suspected potential
of many man-made condensers (ungrounded insulated
static electricity charges and decide what conductive
metals, for example) existing within most of our
paths will be available between them, particularly in
buildings, and thereby very rapidly generate hazard-
the following conditions:
ous and explosive accumulations of static electricity
(1) Hazardous locations as listed in the NFPA
energy in these condensers.
No. 70.
c. NFPA No. 77. This code suggests special stud-
(2) Locations containing hazardous materials
ies for determining the need to provide means of pre-
which will be handled or stored.
venting accumulation of static electricity in the hu-
(3) Movable and portable equipment having
man body. These studies include such means as:
static electricity generating capabilities which will be
conductive flooring, use of nonmetallic supports and
dangerous to personnel,
hardware for personnel assistance, and tie-down
b. Hospitals. Static electricity protection in inten-
rings for aircraft and hydrant refueling. These means
sive care, and surgical and obstetrical sections of hos-
of static control are included below, as appropriate.
pitals will conform to NFPA No. 56A.
d. Effects of static electricity discharges. There are
many reasons why concerns for protection against
other facilities will be in conformance with provisions
static electricity charges are important. Most of the
included below, unless otherwise requested on a
everyday, normal types of static charges find a quick
project-by-project basis by the using service. Where
natural means of dissipation without any hazardous
criteria of other Federal agencies conflict with crite-
effects. However, because static charges of instanta-
ria contained below, the most stringent criteria will
neous magnitudes greater than 10 kilovolts may be
govern.
encountered, it is mandatory that potential effects
from accumulations of these charges be considered.
This is particularly essential where personnel are in-
33. General. Building areas where static electric-
volved and where such static discharges may occur in
ity protection is required will be identified on the
hazardous areas with sufficient strength to produce
contract drawings in conformance with classifications
ignition. It is not the intent herein to provide a list-
contained in NFPA No. 70. A listing of hazardous
ing of effects of discharges of static electricity, as
materials, containers, and operating units will be in-
many are already well known. It is the intent, how-
cluded in the design, and fixed operating equipment
ever, to place every electrical designer on the alert to
locations indicated on the drawings. Portable and
use every reasonable precaution for including static
movable equipment requiring static electricity
electricity protection in each project specification
grounding will be distinctively identified by location
when such protection is required.
and with type of grounding locations required.
3-2
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