Figure 1-2. Ohmmeter ZM-21(*)/U, exterior view.

TM 11-6625-298-14

Figure 1-2. Ohmmeter ZM-21(*)/U, exterior view.

1-7. Purpose and Use

1-8. Tabulated Data

Resistance range . . . . . . . . 0 through 1,000 megohms.

a. Ohmmeter (fig. 1-2) is used to determine the

Test potential..................... 500 volts dc 5 percent (when

insulation condition of telephone cables, of trans-

measuring values below 2

formers, between windings and ground of rotating

megohms, the voltage drops

equipment, and of all other types of electrical

materially).

equipment for which insulation resistance is an

Accuracy . . . . . . . . .. Within 1 percent of any car-

dinal calibration point.

important factor.

NOTE

b. The ohmmeter applies a high potential to

Insulation resistance above 1,000 megohms

equipment under test in order to detect low in-

sulation resistance which may not indicate

is indicated by a point marked infinity

satisfactorily on a low potential ohmmeter.

on the meter scale.

c. Regular use of the ohmmeter can minimize

1-9. Items Comprising an Operable Ohmmeter ZM-

failure in circuits or equipment caused by faulty

21(*)/U

insulation. A sudden lowering of insulation

Refer to table 1-1 for items comprising an operable

resistance indicates a fault that should be in-

Ohmmeter ZM-21(*)/U.

vestigated immediately.

1-2