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]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Joseph E (US SSA)
Sent: Thursday, May 21, 2009 7:35 AM
To: [log in to unmask]
Subject: Re: [TN] ESD Heel Strap / Shoe Checker - low resistance
And yet Desco still puts one megohm resistors in all
of their straps, as do your competitors. Why do this
if not for safety reasons?
This can't be to increase discharge time, because
equipment is hard grounded, while only our people are
grounded through 1 megohm.
Surely there are lots of ways that electricity can
hurt us, and it makes sense for UL to caution people not
to do anything stupid. But putting some resistance in
the path to ground seems a reasonable precaution.
And UL and ESD association specs still require a minimum
resistance to ground. Probably OSHA too (I've seen OSHA
compliance asserted in data sheets, but there's no specific
documents referenced).
-Joe
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gene Felder
Sent: Wednesday, May 20, 2009 6:07 PM
To: [log in to unmask]
Subject: Re: [TN] ESD Heel Strap / Shoe Checker - low resistance
UL agrees with Rex. See below "It will not reduce or increase your risk
of receiving electric shock".
Wrist Straps and Foot Grounders covered by our listing are to note this
UL Underwriters Laboratories listing caution warning:
"Note: This product is not recommended for use on equipment with
operating voltage exceeding 250 VAC.
CAUTION: The ESD Series is for electrostatic control. It will not reduce
or increase your risk of receiving electric shock when using or working
on electrical equipment. Follow the same precautions you would use
without wrist straps or foot grounders, including:
. Make certain that equipment having a grounding type plug is properly
grounded.
. Make certain that you are not in contact with grounded objects other
than through the ESD Series."
Gene Felder
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rex Waygood
Sent: Wednesday, May 20, 2009 2:53 PM
To: [log in to unmask]
Subject: Re: [TN] ESD Heel Strap / Shoe Checker - low resistance
Your work environment should not present an electric shock risk, if it
does then fix it. If that risk exists then there are as many ways for
electrocution to occur bypassing the resitance of the wrist strap or
heel grounder.
The first esd standard was written to have no risk of electrocution.
Unfortunately it was unworkable.
Rex
Sent from my iPhone
On 20 May 2009, at 21:30, "Kane, Joseph E (US SSA)"
<[log in to unmask]
> wrote:
> As has been mentioned here, LOW resistance indicates a safety problem.
> If the operator has resistance of less than 1 megohm (nominal) to
> ground, then they risk electrocution if they touch 110V hot. Without
> sufficient current limiting resistance, it's like having your guy
> standing around in a puddle of water.
>
> With one megohm resistance in series, 110 volts will give him a good
> shock, but the current is limited to less than 1 milliamp (I=V/R),
> which is not enough to stop his heart.
>
> Lotion and more sweat will lower the resistance, not increase it. You
> could have him wear insulating socks inside those shoes, but that
> won't be very comfortable and probably won't last for long, as sweat
> builds up. In fact, some ESD-protective shoes are deliberately built
> so that they don't "breathe" very well, facilitating buildup of a
> sweat layer.
>
> We've had issues with conductive shoes failing low, because the shoes
> were too conductive for some people. We replaced those shoes with
> ones that meet spec.
>
> If heel straps are failing as well, that's a puzzle, because something
> is circumventing the 1 megohm resistor. The only way this can happen
> is if there's another parallel path to ground. Are you testing with
> both feet on the conductive surface at once? The spec says to test
> each foot individually, because a person keeps only one foot in
> contact with the floor at a time when walking. If your footwear
> tester doesn't have independent footplates, have this person lift one
> foot at a time while pushing the little button.
>
> As for the wrist straps, that's another head-scratcher. If this
> person is wearing conductive footwear on a conductive surface while
> he's testing the wrist strap, there's your other path to ground. Try
> having him stand on an insulating pad.
>
> Joe Kane
> BAE Systems
> Johnson City, NY
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
> Sent: Wednesday, May 20, 2009 3:59 PM
> To: [log in to unmask]
> Subject: Re: [TN] ESD Heel Strap / Shoe Checker - low resistance
>
> Most shoe testers are checking for a range of 1 meg to 10 meg.
> Anything
> in between is a pass. Over 10 meg is too much resistance in series
> between the "touch button" and the shoe/strap floor plate, which is
> the operator, the sock, and the shoe or heelstrap. Under 1 meg is too
> little resistance between same. Most people fail high due to dry feet,
> loss of conductivity in the shoe, or their hands are too dry. People
> that fail low are unusual.
>
> If your warehouse worker is working in a warehouse that is not air
> conditioned and he sweats heavily his socks and his shoes can actually
> become saturated to the point where the reading is "too conductive"
> and
> he will fail low.
> Applying lotion to the strap will exacerbate the situation. If he put
> on someone else's shoes and walked around in them with damp socks plus
> the lotion is still on his socks and skin he could possibly saturate
> someone else's shoes, but I doubt it.
> If his shoes are saturated with any oils or chemicals or metal
> filings/nails from the warehouse or loading dock area they would also
> fail low.
>
> A portable ESD checker can be used to measure the actual resistance
> between the bottom of his shoe and his finger. If it is below 1 meg
> you may wish to have him wear thicker socks and make sure the heel
> strap is worn on the inside of his shoe, but not on the inside of his
> sock.
>
> I suspect that he is really failing high, in which case he may need a
> good foot lotion, non-acrylic (thin wool or cotton) socks, and
> possibly even a hand lotion. Be sure the lotion is something like I.C.
> lotion from R&R products or the equivalent and does not contain any
> silicone or lanolin.
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Helena Pasquito
> (Cobham DES SSA)
> Sent: Wednesday, May 20, 2009 1:53 PM
> To: [log in to unmask]
> Subject: [TN] ESD Heel Strap / Shoe Checker - low resistance
>
> Hello Tech Net,
> Assemblers and techs check their shoes on an ESD Shoe checker, each
> day before the start of their shift. We have one person that works in
> the warehouse that always fails low (resistance). We have tried new
> heel straps, walking for an hour or two in regular ESD shoes, even
> wearing someone elses shoes that had just passed. Nothing seems to
> work. He has also tried to apply lotion to the strap that sits inside
> his sock. Do you know of any tricks we can try to employ? Have you
> seen issues like this before? I think some of the steps we took may
> have helped for a high resist fail but not a low resist. Even the
> wrist strap test fails for him (multiple wrist straps). Any
> suggestions would be much appreciated.
> Helena Pasquito
> Training Specialist
> Certified IPC Trainer
> Cobham Electronic Systems
> 1001 Pawtucket Blvd., M/S 160
> Lowell, MA 01853
> 978-442-5024
> [log in to unmask]
>
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