August, 1996
Ryne C. Allen,
NARTE certified ESD Control Engineer
ESD Systems, DII
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.
ESD Systems 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 240
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.
ESD Systems.com's Wrist Straps [and Foot Grounders] are Underwriter Laboratories
listed. UL cautions that thes 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.
ESD Systems.com's 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.)
|