Insulation Resistance

TM 11-6625-298-14

of the resistance of an insulating material is

of surface conditions. A measurement taken with a

negative and numerically large. Therefore, even a

G U A R D connection and compared to a

small increase in temperature will cause a

measurement of the same equipment taken without

relatively large decrease in insulation resistance.

a GUARD connection will establish whether low

Always make measurements at the same tem-

insulation resistance readings are caused by poor

perature, if possible, because insulation resistance

insulation or bad surface conditions. If readings

drops at high temperatures. For example, the

taken with guarded connections are higher than

insulation resistance between the stator winding of

those without guarded connections, the surface of

a slow-speed generator and the frame is 100

the equipment has a low resistance.

megohms at 80° F., but falls to 10 megohms at

3-5. Insulation Resistance

140° F.

All tests shall be made with the hand generator

(2) Moisture and humidity. Chemically pure

crank rotated at approximately 160 revolutions per

water is a non conductor; however, the impurities

minute. Since the meter scale is calibrated in

that water absorbs from the atmosphere and other

megohms, the position of the pointer on the scale

sources cause it to be partially conductive.

indicates insulation resistance values directly in

Therefore, moisture will become a conductor in

megohms. Indications shall be read with the eyes

parallel with the insulation and will cause a

directly above the pointer and the scale to prevent

decided drop in insulation resistance. This is a

errors. Types of operation described in subsequent

temporary condition and may be remedied by

paragraphs are necessarily general to cover

drying the equipment.

equipment of all types and designs. Before testing

(3) Chemical fumes. Strong acid or alkali

any specific equipment, refer to the technical

fumes attack insulation and permanently lower its

manual covering the equipment for definite in-

insulation resistance.

sulation resistance standards.

(4) Surface conditions. Dirt, especially when

a. Unit of Measurement. Insulation resistance is

mixed with oil, or with a copper or carbon dust,

that property of an insulating material which

forms a low-resistance path across the insulation,

opposes the passage of electrical current. It is

lowering its resistance.

expressed in ohms or megohms. Insulation

c. Values of Insulation Resistance. Since in-

resistance increases as the thickness of the in-

sulation resistance is affected by varying con-

sulating material increases but decreases as the

ditions (b above), no constant rules can be

area of material under test increases. For example,

established to govern its value. Certain minimum

if an insulating material covering a given surface

values have, however, been established arbitrarily

and having a resistance of 100 megohms is ex-

for different types of cable and equipment (para 3-

tended to 10 times its area, the insulation

12). Actual values of insulation resistance are not

resistance will drop to 10 megohms. Thus, one mile

as important as changes in insulation resistance.

Of* spinal-four cable may measure 1,000 megohms

Periodic tests should be made and the results

but 10 miles will measure 100 megohms. If the

recorded together with information regarding

insulating material is doubled in thickness,

climatic conditions at the time tests are made.

however, the area having an insulation resistance

3-6. Insulation Test of Ac and Dc Rotating

of 100 megohms now will have a resistance of 200

Machines Rated 600 Volts or Less

megohms, and the area having an insulation

resistance of 10 megohms will have a resistance of

a. Connect a test lead from the LINE terminal

20 megohms.

of the ohmmeter to a lead, an exposed conductor,

or the commutator of the machine (fig. 3-1).

b. Conditions Affecting Insulation Resistance.

(1) Temperature. The temperature coefficient

3-2