August, 1996
Ryne C. Allen,
NARTE certified ESD Control Engineer
Desco Industries Inc.
We would not recommend grounding personnel when working with voltages over 250 VAC or
500 VDC, as described in the Cenelec Electronic Components Committee standard section
4.1.1, CECC 00015/I. Both our foot grounders and wrist straps employ a 1/4 Watt carbon
composite resistor rated 250 Volts Alternating Current (VAC) and UL tested and listed for
voltages under 250 VAC.
UL Marking for Wrist Straps and Foot Grounders:
CAUTIONARY MARKING
This product is not recommended for use on equipment with operating voltage exceeding
250 volts.
There is always a safety concern when working around high voltage. All electrical
wiring and ground connections should adhere to the National Electrical Code (NEC) as
governed by OSHA.
If an operator came in contact with an exposed voltage of 250 VAC and wearing a ground
strap or other grounding mechanism with a 1 megohm resistor in line (soft ground), then
the current received through the operator to ground would be limited to 250 m A, well below the electrical
perception level of table VIII in DOD-HDBK-263, refer to Appendix A. In order for an
operator to have similar protection when working around high voltages of 20 kVAC, the
serial resistance in their ground path would need to be at least 80 megohms.
Appendix A
Listing of Relevant Documentation for ESD control and high voltages:
ESD Association ADV-2.0-1994
Safety rules for working with high voltages should dictate the design of the
workstation.
ESD Association ESD-S1.1 (Standard on Wrist Straps):
ESD-S1.1, Section 7.1, Construction Guidelines
Current-Limiting Resistance
A resistance of sufficient resistance to limit current to less than 0.0005 amps (0.5 mA),
at the highest voltage that may be encountered, should be incorporated into the wrist
strap.
Nominally, 800,000 ohms (800 Kilohms) are sufficient for voltages of up to 2401 VAC.
The value of 1 Megohms is specified because it is a standard value discrete resistor.
Special situations may dictate the use of values above or below the 1 Megohm value. Wrist
straps with nominal resistances other than 1 Megohm should be marked in accordance with
paragraph 5.9. Discrete current-limiting resistors should be located near the connection
between the ground cord and the cuff.
ESD Association, ANSI/ESD-S20.20
In the one of the most comprehensive ESD standards from the ESD Association
ANSI/ESD-S20.20, states in section 5. PERSONNEL SAFETY:
The Procedures and equipment described in this document may expose personnel to hazardous
electrical conditions. Users of this document are responsible for selecting equipment that
complies with applicable laws, regulatory codes and both external and internal policy.
Users are cautioned that this document cannot replace or supersede any requirements for
personnel safety. Ground fault circuit interrupters (GFCI) and other safety protection
should be considered wherever personnel might come into contact with electrical sources.
Electrical hazard reduction practices should be exercised and proper grounding
instructions for equipment must be followed.
According to the ESD S1.1-1998:
7.1 Construction Guidelines Current-Limiting Resistance
A resistance of sufficient resistance to limit current to less than 0.0005 amps
(O.5mA), at the highest voltage that may be encountered, should be incorporated into the
wrist strap.
Nominally, 800,000 ohms (800 Kilohms) are sufficient for voltages of up to 240VAC. The
value of 1 Megohms is specified because it is a standard value discrete resistor. Special
situations may dictate the use of values above or below the 1 megohm value. Wrist straps
with nominal resistances other than 1 megohm should be marked in accordance with paragraph
5.9. Discrete current-limiting resistors should be located near the connection between the
ground cord and the cuff.
According to the ADV-2.0:
Current Limiting
Most wrist straps have a current limiting resistor molded into the ground cord head on
the end that connects to the cuff. The resistor most commonly used is a one megohm, 1/4
watt with a working voltage rating of 250 volts. Resistors limit current as defined by
Ohm's Law, which states the current is equal to the voltage divided by the resistance.
In a practical application, the maximum amount of current through a wrist strap ground
cord if it was placed across a 250 Volt source is 250 microamps or 0.25 milliamps. This
amount of current is well below the 0.7 milliamps that Underwriters Laboratories uses as
the peak current in a LIMITED CURRENT CIRCUIT. See document UL 1950.
ANSI/ESD S20.20 -1999 states:
5. PERSONNEL SAFETY
The Procedures and equipment described in this document may expose personnel to
hazardous electrical conditions. Users of this document are responsible for selecting
equipment that complies with applicable laws, regulatory codes and both external and
internal policy. Users are cautioned that this document cannot replace or supersede any
requirements for personnel safety. Ground fault circuit interrupters (GFCI) and other
safety protection should be considered wherever personnel might come into contact with
electrical sources. Electrical hazard reduction practices should be exercised and proper
grounding instructions for equipment must be followed."
ESD Ground cords serve one purpose, to connect a conducting surface to the same
potential as ground. There is always a safety concern when working around high voltage.
All electrical wiring and ground connections should adhere to the National Electrical Code
(NEC) as governed by OSHA. Per ESD Association ADV-2.0-1994, Safety rules for working with
high voltages should dictate the design of the workstation.
Charleswater Wrist Straps [and Foot Grounders] are Underwriter Laboratories listed. UL
cautions that these products are not recommended for use on equipment with operating
voltage exceeding 250 volts recommending that the electrical current that the operator be
exposed to be limited to 0.25 milliamp. At 250 volts, the 1 megohm [1,000,000 ohms]
resistor does this.
Charleswater Wrist Straps [and Foot Grounders] employ a 1 megohm ¼ Watt carbon
composite resistor rated 250 Volts Alternating Current (VAC) and UL tested and listed for
voltages under 250 VAC. If the Wrist Strap came in contact with an exposed voltage of 250
VAC, and the operator was wearing a Wrist Strap with a 1 megohm resistor, the current
received through the operator to ground would be limited to 250 microamps. This is below
the human electrical perception level.
When working with voltages over 250 VAC, ESD personnel grounding should not be used,
rather ionization may be required to neutralize electrostatic charges.
MIL-STD-454
All personnel ground straps should have sufficient resistance to ground to limit
current to the perception level as shown in MIL-STD-454, Requirement 1.
DOD-HDBK-263
Section 7.3.1.3, Table VIII, (Ref.: MIL-STD-454)
Effects Of Electrical Current
On Humans |
Current Values
(mA) |
|
AC |
DC |
Effects |
60 Hz |
|
|
0-1
|
0-4
|
Perception
|
1-4
|
4-15
|
Surprise
|
4-21
|
15-80
|
Reflex action
|
21-100
|
80-160
|
Muscular inhibition
|
40-100
|
160-300
|
Respiratory block
|
Over 100
|
Over 300
|
Usually fatal
|
CECC 00 015
Section 1.1. For areas with exposed conductors at potentials greater than 1.25 kVAC
or 2.5 kVDC, additional requirements specified in CECC 00 015: Part 4 shall apply.
Section 4.1.1, The EPA shall be constructed to ensure that the equipment used to
control static electricity does not create any additional risk of electric shock to
personnel, should energized conductors up to the level of 250 VAC (500 VDC) be exposed.
Section 4.1.2, Additional protection against exposed high voltages: In ESD
Protected Areas (EPA) where exposed energized conductors exist for each 250 VAC (500 VDC)
potential the minimum resistance of any working point to ground shall be 7.5 x 105
Ohms. Maximum resistance values in excess of those specified in Section 4 clauses shall
not be used.
Although this standard does not include requirements for personal safety, attention is
drawn to the need for all concerned to comply with relevant local statutory requirements
regarding the health and safety of all persons in all places of work including those
covered by this standard. (Attention is drawn to the fact that electrical potentials in
excess of 50 VAC or 120 VDC may be dangerous to personnel.)