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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