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From:
"Helena Pasquito (Cobham DES SSA)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Helena Pasquito (Cobham DES SSA)
Date:
Fri, 19 Mar 2010 10:06:04 -0400
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Hello All,

Here is some of the information we use to educate employees regarding ESD around high voltage.

Almost all wrist and heel straps protect you with a 1 megohm resistor up to about 250 Volts. However be aware that there are some wrist and heels straps that do not have a 1 megohm resistor. It is not recommended to use wrist or heel strap without the 1 megohm resistor even if you do not work on high voltage as it may be used by mistake at some point on a high voltage area. So please throw them away if you have one. Also wrist straps with the 1 megohm resistors are typically only rated to ¼ watt. The information below is more informative about high voltage relative to using wrist and heel straps with the 1 megohm resistors and safety regulations.


ESD Around High Voltage
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  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.


Regards,
Helena Pasquito
Training Specialist - Certified IPC Trainer 
Cobham Sensor Systems
1001 Pawtucket Blvd., M/S 160
Lowell, MA 01853
978-442-5024
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Friday, March 19, 2010 9:34 AM
To: [log in to unmask]
Subject: [TN] ESD protection / high voltage safety question

Hello TechNet
A question for you guys concerning ESD protection and it's interaction
(conflict) with safety.

We use the standard wrist strap and cord with the built in resistor, and
practice that any time an ESD sensitive assembly is being worked on we
have to wear the wrist strap.  This is fine in the production
environment, but in test we can be dealing with some scary voltages, and
being nicely grounded is not necessarily a good thing.  Say for example,
when working with something that has up to 20,000 v DC running through
it.

What are other folks doing in cases like this?

My first instinct is that our rule should be modified to say that the
wrist strap is not to be used if voltages in excess of 250v are present.

regards, 
  
Graham Collins 
Halifax Production Engineering 
L-3 communications Electronic Systems 
(902) 873-2000 ext. 6215 


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