3M 725 Instructions Manual - Page 5

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- Model 725 Wrist Strap Monitor with dual conductor wrist strap assemblies

Fig. 1

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.

* 3M Dual Conductor Cord & Wrist Band are

required for use with the 725 Monitor,

ordered separately.

Model 725

Wrist Strap

Monitor

Dual Conductor

Metal Wrist Strap*

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%, -50%

Dual Conductor

Fabric Wrist Strap*