First Published in EOS/ESD Technology
Monitoring Your Monitors
Your ground monitors may be fooled by their own
sensitivity and the "human-battery effect." The result
is that your wrist straps may be better than you think.
Edward H. Russell
Quality Assurance Specialist, Santa Barbara, CA
A vital part of an ESD-control program is grounding
workers via wrist, heel, or foot straps to electrical contact between
ground and the worker. With personnel grounders, voltages on workers
are reduced to levels safe for ESD-sensitive devices.
But simply put using grounders isn't enough. Workers must be taught
to use them properly, and grounder performance should be continually
monitored to assure that resistance to ground stays within acceptable
limits. Unfortunately, if improperly understood, monitoring can
lead to what seems like high rates of grounder failure. In reality,
some facilities are probably rejecting wrist straps needlessly.
During a recent study of wrist straps
from more than 30 electronics manufacturers, ESD consultant Roger
J. Pierce spotted a number of deficiencies. He properly emphasized
the importance of frequent monitoring to ensure the integrity of
wrist straps and grounding cords worn by the worker.
However, engineer Betty Smith found that some monitors were better
suited than other to given applications.
It is now clear that high failure rates, normally thought of as
due to defective or poorly fit wrist straps, can also be the result
of test-method anomalies. Effective wrist straps may be failing
because monitoring instruments are giving false alarms.
In our case, an excessive number of failures during wrist-strap
tests at General Dynamics (Pomona, CA) led to a study that uncovered
corrective actions that could prevent future rejection of acceptable
All failures occurred using testers that measured an overall
series resistance consisting of: the contact resistance between
the strap's cuff and the person's skin, the resistance of the grounder's
current-limiting series resistor, and the resistance of the ground
The testers were made by various manufacturers but operated on much
the same principle. Powered by a 9-V battery, one of three lights
on the tester indicated whether series resistance was less than
750 kOhms greater than 10MOhms , or between the two values. The
testers had been previously checked for accuracy and met manufacturers'
specifications. All failing wrist straps had an expandable fabric
cuff and were from the same manufacturer. The length of time the
wrist straps had been in service ranged from one week to six months.
The most common failure mode was indication of an overall series
resistance exceeding 10 MOhms . Replacing the failed wrist straps
with new ones resulted in a pass condition at some workstations.
The wrist-strap-and-cord combination was then removed from the worker
involved and its resistance measured using an ohmmeter. Measuring
from the fabric portion of the cuff to the end of the ground cord,
all of the grounders measured approximately 1 MOhm, that is, near
the value of the current-limiting resistor. This was acceptable
performance. At this point, the testers were rechecked and they
were found to be operating acceptably too. Why, then were the wrist
This isn't the first time such confusing
results have surfaced. In 1982, James R. Huntsman and Donald M.
Yenni, Jr. found similar false-resistance measurements when using
low-voltage ohmmeters. Huntsman and Yenni described a combination
of galvanic action and human biopotential effects that resulted
in a small flow of current from a person dependent on the wrist-strap
cuff material and which resulted in false-resistance readings.
Measurement errors due to this so called human-battery effect, according
to Huntsman and Yenni, could be reduced significantly by using ohmmeters
with 20- to 30-V power supplies. And the effect itself could be
reduced by assuring closer contact between a worker's skin and the
To determine whether the human battery effect was the cause of the
failures in our case, measurements were conducted on ten of the
failed wrist straps with a Triplett Model 630 ohmmeter which uses
a 30-V battery. The ohmmeter showed as low a 1 MOhm total series
resistance when worn by some people and as much as 6 MOhms on others.
In each case, the associated 9-V tester had indicated grounder failures.
Apparently the human-battery effect was at work.
Adjustable fabric-cuff straps were substituted and adjusted snugly;
causing the failure rate to drop slightly but remaining high. In
quest of lower contact resistance, an expandable metal cuff was
substituted; it measured acceptably on all ten people.
These tests results supported Huntsman and Yenni's findings about
the human-battery effect, and the investigation was carried further
in a special study.
For the study, thirteen assemblers were selected
whose skins ranged from fair to dark complected, from dry to moist,
and who offered various amounts of hair on their wrists. The assembler's
body weight also ranged widely. The selection was eleven females
and two males.
Humidity ranged from 43 to 58% for the four-week testing period.
The group agreed to wear a specific wrist strap and ground cord
unless they became defective. All straps were of the expandable
fabric-cuff type, and were made by the manufacturer whose straps
had failed previously. For comparison, a new expandable fabric cuff,
a new expandable metal cuff, and a new adjustable fabric cuff were
later substituted and their resistance measured on each assembler.
The overall series resistance of each assemblers' skin, wrist strap,
and ground cord were measured and recorded weekly. Measurements
were made using two 9-V testers and the Triplett Model 630 ohmmeter.
The results are shown in Table 1.
When reviewing the data, it became obvious that the only real failure
was a broken ground cord; all the other wrist strap failures proved
to be false alarms when double checked with the Triplett 630 VOM.
The adjustable fabric cuff showed a small improvement because the
human-battery effect was reduced when slightly more intimate contact
was made with the skin than had been the case with the assembler's
original wrist strap. The majority of the assemblers' straps were
snug, and some were pushed up the arm to obtain a snug fit; however,
they still had "failed" despite these efforts to assure
Some of the fabric cuffs were adjusted so that the metal snap area
contacted the inner side of the wrist where there was less hair;
all measurements of these straps were acceptable. The expandable
metal cuff passed all tests.
The conclusion was that all wrist straps used in the study were
acceptable, but that the test method was flawed. To deal with the
ambiguity, immediate corrective action was replacement of all fabric
cuffs with expandable metal cuffs until a more reliable test method
could be decided upon.
The study's results also created a concern for the human-battery
effect using popular constant monitors. After analyzing several
different types of these monitors, the following conclusions were
1. Resistance-type monitors operate similarly to the 9-V testers-
because they are low-current devices, false alarms can occur.
2. Capacitance and impedance-type monitors are low-current devices
also, but are much less sensitive to the human-battery effect.
3. Capacitance and impedance-type monitors, however, have their
won problems. False alarms can be triggered by capacitance changes
such as those occurring when workers lift their feet off the floor
or move to a standing position form a sitting one. Some manufacturers
have already corrected this problem.
4. The upper alarm point is set at 3 to 5 MOhms on some monitors
and at 3 to 10 MOhms on others. Thus, false alarms can occur if
a small person with very dry skin is being monitored. Such a person's
skin resistance is typically near the alarm point and yet still
acceptable. These false alarms can be eliminated or reduced if the
upper alarm limit is adjusted to 6 to 10 MOhms.
So the question "How does your wrist strap measure up?"
is not a simple one. If you experience a high rate of grounder failures,
it could be caused by the test or monitoring method.
A high-quality fabric-cuff wrist strap should provide
many hours of service if cared for properly (e.g., by wearing the
strap at all times during working hours to reduce wear and unnecessary
If a fabric cuff is preferred, the adjustable type is recommended,
and to ensure intimate skin contact, adjust the wrist strap so that
its metal snap plate lines inside the wrist.
Creams designed to lower skin resistance when applied to the skin
will only improve contact for a short time and are thus not a long-term
or complete solution to false-alarm problem.
Expandable metal bands will provide longer service than fabric grounders,
will test accurately on low current testers, and minimize false
alarms during monitoring.
For More Information
For more information see the following:
J.R. Huntsman, D.M. Yenni, Jr., "Tests Methods for Static Control
Products," 3M Co. Presented at the 1982 EOS/ESD Symposium,
EOS/ESD Standard No. 1, Standard for Protection of Electrostatic
Discharge Susceptible Items: Personnel Grounding Wrist-Straps.
Betty Smith, "Constant Monitoring: Quality You Can't Afford
to be Without," General Dynamics, Pomona, CA. Presented at
the 1987 EOS/ESD Symposium, Orlando FL.