4
Figure 1 – 3M
TM
Wrist Strap Monitor Model 725 with 3M
TM
Dual Conductor Wrist Strap Assemblies.
3M
TM
Dual Conductor
Metal Wrist Strap*
3M
TM
Dual Conductor
Fabric Wrist Strap*
3M
TM
Wrist Strap
Monitor Model 725
SECTION 1
Theory of Operation
(Refer to Figure 1)
The 3M
™
Wrist Strap Monitor Model 725 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 Wrist Strap Monitor Model 725 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 Wrist Strap Monitor 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 Wrist Strap Monitor Model 725. 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.
*3M
™
Dual Conductor Cord & Wrist Band are required for use with
the 3M
™
Wrist Strap Monitor Model 725, ordered separately.
** Resistance values are ±15%
*** Resistance values are +15%, -50%
