It is considered good practice to insist on 1 megohm or more before applying insulation tests to any
electrical machinery. For diesel-electric equipment, the megger reading should be taken over a 30-
second period or longer. If the reading is constant or rises steadily with continual application of the
megger voltage, the insulation is considered sound and can be safely tested. If the reading is unsteady
and rises and falls excessively with steady application of megger voltage, it is likely that leakage paths
may be present, caused by dirt or moisture, and further cleaning and drying will be necessary.
(2) The test leads be securely connected to the circuit before power is applied to the test box,
to avoid surges from poor connections.
(3) The voltage from the test box be varied smoothly, both when increasing and decreasing
the setting, to avoid surges.
c. Test boxes. A test box measures the voltage or amperage of any unit connected to it. When
making tests, guard against damaging the test box. Sudden application or removal of test potentials may
result in surges as much as 100 percent more than the intended test voltage. Excessively burned contacts
or contact tips on the test box may result in undesirable fluctuations in test voltage. Test boxes that have
fixed, definite secondary steps of voltage are certain to produce surging if the primary voltage cannot be
smoothly controlled from zero to maximum. The most effective and inexpensive modification for test
boxes of 1/2 to 5 kilovolt-ampere (kv-a) capacity can be made by adding a Variac (trade name for
variable auto-transformer) to the primary winding. A Variac has the capability of varying the voltage
smoothly without steps, resulting in practically no surging of voltage. To use a Variac, connect the test
leads to the machine or circuit being tested before applying power to the test box, then raise the voltage
to the test value with the Variac, hold for the prescribed time, and then lower the voltage to zero with the
Variac before removing power and disconnecting the test leads.
To operate properly, the generator's armature shaft and frame must be in line with the engine
crankshaft, and its airgap space must be equal under each main pole, under each commutating pole, and
from front to rear. The generator requires the same maintenance as the motor. Among the things to be
inspected in the generator or the motor are the engine starting contactor, field windings and leads,
armature, brushes and brush holders, commutator, insulation, and bearing assemblies.