PS-2119
Page 2 of 4
© 2007 DESCO INDUSTRIES INC.
Employee Owned
costs and rejected product. Periodic
testing detects wrist strap failures after
ESD susceptible products have been
manufactured. The costs of dealing with
the resulting catastrophic failures or
latent defects can be considerable. Dual
Operator Workstation Continuous
Monitors eliminate the need for users to
test wrist straps and log the results; by
their function, these monitors satisfy the
ISO and ANSI/ESD S20.20 test
logging. ANSI/ESD S20.20 Paragraph
6.2.2.2 Personnel Grounding Guidance
states "A log should be maintained
which verifies that personnel have
tested their personal grounding
devices." Per ESD-S1.1 Paragraph 6.1.3
"Daily (wrist strap system) testing may
be omitted if constant monitoring is
used."
WAVE DISTORTION DETECTION
TECHNOLOGY PROVIDES TRUE
100% CONTINUOUS MONITORING
From the technical alternatives
available, ESD Systems.com designs
and manufactures many of its
Continuous Monitor product offerings
using wave distortion technology. Wave
distortion circuitry monitors
current/voltage phase shifts and
provides true 100% continuous
monitoring. Electrical current will lead
voltage at various points due to the
combinations of resistance and
capacitive reactance. By monitoring
these "distortions" or phase shifts, the
wave distortion Workstation Continuous
Monitor will reliably determine if the
circuit is complete.
Wave distortion technology can be
referred to as "vector impedance
monitoring". This description is valid as
the wave distortion technology
measures the impedance at the
monitored banana jack and looks for
changes in either the capacitance or
resistance of the circuit which includes
the wrist strap and its wearer. It uses
filtering and time domain sampling to
filter out false signals caused by voltage
offsets, 60 Hz fields and other electro-
magnetic and electrostatic interference.
In normal factory environments, and
with persons whose capacitance with
respect to ground is within design limits
(5 feet tall 90 pound person to 6 foot 5
inch 250 pound person), the Dual
Operator Workstation Continuous
Monitor cannot be "fooled". It will
provide a reliable alarm only when the
wrist strap or work surface becomes
dysfunctional or unsafe according to
accepted industry standards. The Dual
Operator Workstation Continuous
Monitor is drift-free and designed to be
insensitive to the effects of squeezing or
stretching the coil cord.
ADVANTAGES OF WAVE
DISTORTION AND SINGLE-WIRE
TECHNOLOGY
The ESD Systems.com Dual Operator
Workstation Continuous Monitor allows
the use of any standard, single-wire
wrist strap and coil cord. The
monitor/wrist strap/cord system life-
cycle costs are by far lower than
alternative systems which require more
expensive & less durable dual-wire
cords and special wrist straps. Dual-wire
cords are expensive and are the weak
link of the system, the most likely
component to need replacement. Over a
five year period, this can make the dual-
wire system three times as expensive as
a system utilizing single-wire wrist
straps and cords.
The dictionary defines constant as
uniform and unchanging, and
continuous as uninterrupted.
Nonetheless, some dual-wire resistance
monitors utilize a pulsed test current
and do not really provide continuous
monitoring. For example, during each
2.2 second pulse cycle of a leading
"constant" resistive monitor, electrical
current is pulsed for only 0.2 seconds
followed by an unmonitored interval of
2 seconds. This leaves the user/wrist
strap unmonitored for over 90% of each
cycle. Damaging ESD events can easily
occur in the portion of the time in
between the pulses. The off period of 2
seconds equals 2 billion nanoseconds,
and "it takes only about 25 volts applied
for 100 nanoseconds to blow most
memories or microprocessors."* The
dual-wire system does not meet all
industry ESD S6.1 specifications, as the
cords do not meet the EOS/ESD
Association guidelines for the 1 to 5
pound "breakaway force" requirement
for ensuring a reliable path-to-ground
and preventing accidental disconnects.
By using the reliable wave distortion
technology to determine if the circuit is
complete, there are no false alarms.
There is no need to adjust or tune the
monitor to a specific user or installation.
The miniscule amount of electrical
current (less than 1 volt coil cord signal)
required to generate the waveform has
never caused reported skin irritation and
is extremely safe for use in voltage
sensitive applications such as disk drive
manufacturing.
Worksurface Compatibility
Work surface must have a conductive
layer such as Dual Layer Rubber or
Dissipative 3 Layer Vinyl or Micastat
®
Dissipative Laminate with conductive
buried layers. Desco Continuous
Monitors are not recommended for use
with homogenous matting. Upper limit
of ground point to monitor connection is
10 megohms (1 x 10E7 ohms) max.
Installation
Confirm that ground point to monitor
connection is 1 x 10E7 ohms or less.
Remove the monitor from its packaging
and inspect for any shipping damage.
Included with each Dual Operator and
Dual Workstation Continuous Monitor
should be:
1
120 VAC transformer
2 Work surface ground cord
2 Push and clinch snap (12102)
2 Satellite remote monitors
1 Monitor Control unit
2 7 Ft. phone cables for connection to
satellites
6 Mounting
screws
The Model 41222 monitor control unit
is normally installed under the bench
top toward the front edge of a worksta-
tion where the LEDs are easily visible.
The unit is designed to be mounted
using the mounting bracket provided.
See figure 2. To release monitor, push
up on the monitor release tab.
ESD Systems.com • 432 Northboro Road Central • Marlboro, MA 01752 • (508) 485-7390 • Fax (508) 480-0257 • Web Site: ESDSystems.com
*1981 article by Donald E. Frank - Electrical
Overstress Electronic Discharge Symposium
Proceedings
