Static Electricity 2000 Edition

6.6.3* Personnel Grounding Devices.
6.6.3.1
Where ESD footwear will not provide adequate per-
sonnel grounding, supplementary devices should be used.
Such devices include wrist straps, heel/toe grounders, and
conductive overshoes. They should be selected so that accu-
mulation of hazardous static electric charge is prevented,
while the risk of electrocution is not increased. In most practi-
cal situations, grounding of personnel is achieved by ensuring
that the resistance from the skin to ground is approximately
10
8
ohms or less. The need to protect against electrocution via
a grounding device imposes a minimum resistance from skin
to ground of 10
6
ohms. Based on skin contact and contact with
the floor, especially during activities where the entire sole of
the footwear is not in contact with the floor (e.g., kneeling),
effectiveness can be compromised. Grounding devices should
have a minimum resistance of 10
6
ohms for shock protection.
6.6.3.2
The simplest type of commercial device is a grounding
bracelet with a built-in resistor, typically giving a resistance to
ground of about 10
6
ohms for shock protection. Wrist straps of
this type have the greatest utility at ventilation hoods and at
other locations where limitation on the operator’s mobility
can be tolerated. Breakaway wrist tether systems could be nec-
essary where emergency egress is needed. A hood can be
equipped with two external coiled grounding cords with
removable cuff attachments, the latter being kept by individ-
ual users.
6.6.3.3
Ground continuity should be checked periodically to
the manufacturer’s specified limits using a voltmeter or volt
ohmmeter or a commercial tester.
6.6.4 Antistatic or Conductive Clothing.
6.6.4.1
Although silk and most synthetic fibers are excellent
insulators, and undergarments made from them exhibit static
phenomena, no conclusive evidence exists to indicate that
wearing such undergarments constitutes a hazard. However,
removal of outer garments is particularly hazardous in work
areas, such as in hospital operating rooms, explosives manu-
facturing facilities, and similar occupancies and where cloth-
ing is contaminated by flammable liquids. Outer garments
used in these areas should be suitable for the work area and
should be antistatic. NFPA 99, Standard for Health Care Facilities,
provides information on test methods for evaluating the anti-
static performance of wearing apparel.
6.6.4.2*
Although usually a very small likelihood of ignition
by a grounded person due to any type of clothing exists, the
charging of personnel (which can occur, for example, when
personnel are getting out of a forklift truck) is greatly
increased by clothing having high resistivity.
6.6.4.3*
In oxygen-enriched atmospheres, such as could be
present in liquid oxygen filling plants, vapor from the cooled
gas can permeate the employee’s clothing, rendering it more
combustible. A static electric charge that accumulates on the
person and then suddenly discharges can ignite the clothing.
6.6.5 Gloves.
Gloves should be antistatic or conductive with
the same resistivity as prescribed for footwear. Gloves should
be tested in conjunction with the footwear.