4
Section 1.
Theory of Operation
(Refer to Fig. 1)
The 3M™ Model 725 Wrist Strap
Monitor is designed to monitor the
operation of the wrist strap grounding
system of a single operator. The
system features special wrist bands
and ground cords that contain two
independent conductors.*
The Model 725 Monitor performs a
resistance measurement by applying an
electrical current of less than 1 µA.
The path for the current is through one
conductor of the wrist strap ground
cord that contains a current-limiting
resistor, through one side of the wrist
band, through the skin of the wearer
under the band, through the second
side of the wrist band, through the
second conductor of the wrist strap
ground cord that contains a current-
limiting resistor, and finally back to
the monitor.
If the resistance of the wrist strap loop
is less than 35 megohms**, the wrist
strap ground cord, wrist band, and the
interface of the wrist band to the arm
of the wearer are considered to be
functioning correctly. If the measured
resistance is over 35 megohms**, a
red lamp flashes along with a chirping
audible alarm.
In addition, the Model 725 monitors
the ground clip connection to a
predetermined electrical ground.
This is accomplished by measuring
the resistance from the monitor,
through one conductor of the
monitor’s ground cord and metal clip,
through the ground point conducting
medium, through the other metal clip
and conductor of the monitor’s ground
cord, and finally back to the Model
725 Monitor. If the resistance of this
loop exceeds 10 megohms***, a red
lamp will illuminate with a continuous
audible alarm. This indicates that
there is a problem with the monitor’s
ground cord or the ground point
connection.
Fig. 1
-
Model 725 Wrist Strap Monitor with dual conductor wrist strap assemblies
Model 725
Wrist Strap
Monitor
Dual Conductor
Metal Wrist Strap*
Dual Conductor
Fabric Wrist Strap*
* 3M Dual Conductor Cord & Wrist Band are
required for use with the 725 Monitor,
ordered separately.
** Resistance values are ±15%
*** Resistance values are +15%, -50%
