To detect any weakness which may cause a breakdown in service, insulation should be tested
periodically at voltages in excess of the normal operating voltage. Any defects must be remedied before
failure occurs en route. Insulation tests are either the resistance test or the dielectric test.
a. Resistance test. Resistance tests point up the presence of moisture, grease, carbon, and dirt
that would cause grounds, false indications, or miscellaneous failures not due to a breakdown of the
insulation. They are adequate for the low voltage control circuits which operate at about 75 volts.
Control circuits should be kept free from grounds. Although high insulation resistance does not
necessarily indicate high dielectric (nonconducting) strength, low insulation resistance does indicate low
dielectric strength. Control circuits of new locomotives should have a resistance of 1 megohm, and
locomotives in service should not fall to less than one-half of this value. An important feature of these
tests is the comparison of readings taken under similar conditions at various timed. If wide variations
appear in successive readings made under approximately the same conditions of temperature and
dryness, the cause should be determined.
The recommended minimum megger reading for the power circuit of either a main
generator armature plus commutation field or a traction motor is 1 megohm at 77 F. If the windings or
parts of the windings are oil or water soaked, the damaged parts should be removed regardless of the
megger reading. Insulation resistance of the windings varies greatly with humidity and temperature. As
an approximation, the insulation resistance for dc motors halves for very 22 F. rise in temperature.
Because temperatures of the winding cannot be measured accurately in shops, it is recommended that the
apparatus be permitted to reach room temperature before insulation resistance is measured.
b. Dielectric test. Once a year, or whenever the insulation of apparatus has been repaired,
dielectric tests of not less than 1-minute duration should be made on the main power circuits. It is
(1) The equipment be clean and dry, and checked with a megger before making the test.
High voltage should not be applied unless the insulation resistance measures at least 1 megohm when the
motors and generators are clean, dry, and cool--about 77 F.