Model 725 | Dual Conductor | Dual Conductor | |
Wrist Strap | |||
Metal Wrist Strap* | Fabric Wrist Strap* | ||
Monitor | |||
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Fig. 1 - Model 725 Wrist Strap Monitor with dual conductor wrist strap assemblies
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
*3M Dual Conductor Cord & Wrist Band are required for use with the 725 Monitor, ordered separately.
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.
**Resistance values are ±15%
***Resistance values are +15%,
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