Europe View
The current approach of the EN61340-5-1-2007 ESD standard is to consider
ESD safety only, there is no lower limit for Rg. It advises you to
consider electrical safety separately. All our electrical supplies on
the shopfloor go through RCDs/ELCBs (checked fully 3 monthly)reducing
the chance of electric shock. Equipment on benches is PAT'd yearly.
Rex
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gene Felder
Sent: 21 May 2009 16:16
To: [log in to unmask]
Subject: Re: [TN] ESD Heel Strap / Shoe Checker - low resistance
I don't think I disagree with you.
Since Foot Grounders are personnel grounding devices, we use the
information from Wrist Straps:
From Wrist Strap standard: "Current-Limiting Resistance, A resistance
value incorporated in series with the wrist strap's electrical path to
ground.
This resistance limits electrical current that could pass through the
ground cord in the event of inadvertent user contact with electrical
potential."
[ANSI/ESD S1.1 section 3.0 Definition of Terms] Construction Guidelines,
1. 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 volts alternating current (AC). The value of 1 megohm 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." [ANSl/ESD S1.1 ANNEX B
(Informational)]
The Foot Grounder standard practice document refers to resistance, which
could be the rubber, as opposed to a discrete resistor. Per ESD SP 9.2
Footwear - Foot Grounders Resistive Characterization (not to include
static control shoes) section 2.0 Definitions "Current Limiting
Resistance, A resistance incorporated in the electrical path to ground
of foot grounder system. This resistance is intended to limit the
electrical current that could pass through the foot grounder in the
event of inadvertent user contact with electrical potential."
and Standard use foot grounder:
"Type A (standard) foot grounder has a lower resistance limit of 8.0 x
10E5 ohms to limit current flow to <0.5 milliamp at 240 VAC."
Per the ESD Handbook ESD TR20.20 section 5.3.2.6 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. For personnel safety,
wrist straps should not be used in situations where there is an exposed
electrical circuit of 250 volts or higher.
Note: Several publications exist (beyond UL) that provide varying limits
of allowable current for personnel safety. For example, most power
supplies are current limited to 5 mA. For this reason, the user should
check with local safety personnel to determine requirements for their
particular area. Refer to the Personnel safety section of this handbook
for further guidance."
Gene Felder
[log in to unmask]
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