1-39
The guard ring intercepts leakage current. Any leakage currents intercepted are shunted to the
negative side of the generator. They do not flow through coil a; therefore, they do not affect the meter
reading.
If the test leads are open-circuited, no current flows in coil a. However, current flows internally
through coil b, and deflects the pointer to infinity, which indicates a resistance too large to measure.
When a resistance such as Rx is connected between the test leads, current also flows in coil a, tending to
move the pointer clockwise. At the same time, coil b still tends to move the pointer counterclockwise.
Therefore, the moving element, composed of both coils and the pointer, comes to rest in a position at
which the two forces are balanced. This position depends upon the value of the external resistance, which
controls the relative amount of current in coil a. Because changes in voltage affect both coil a and coil b
in the same proportion, the position of the moving system is independent of the voltage. If the test leads
are short-circuited, the pointer rests at zero because the current in coil a is relatively large. The instrument
is not damaged under these circumstances because the current is limited by R3.
The external view of one type of megger is shown in figure 1-36(B).
Navy meggers are usually rated at 500 volts. To avoid excessive test voltages, most meggers are
equipped with friction clutches. When the generator is cranked faster than its rated speed, the clutch slips
and the generator speed and output voltage are not permitted to exceed their rated values. When
extremely high resistances-for example, 10,000 megohms or more-are to be measured, a high voltage is
needed to cause sufficient current flow to actuate the meter movement. For extended ranges, a 1,000-volt
generator is available.
When a megger is used, the generator voltage is present on the test leads. This voltage could be
hazardous to you or to the equipment you are checking. Therefore, NEVER TOUCH THE TEST
LEADS WHILE THE MEGGER IS BEING USED and isolate the item you are checking from the
equipment before using the megger.
Using the Megger
To use a megger to check wiring insulation, connect one test lead to the insulation and the other test
lead to the conductor, after isolating the wiring from the equipment. Turn the hand crank until the slip
clutch just begins to slip and note the meter reading. Normal insulations should read infinity. Any small
resistance reading indicates the insulation is breaking down.
Megger Safety Precautions
When you use a megger, you could be injured or damage equipment you are working on if the
following MINIMUM safety precautions are not observed.
Use meggers on high-resistance measurements only (such as insulation measurements or to
check two separate conductors on a cable).
Never touch the test leads while the handle is being cranked.
Deenergize and discharge the circuit completely before connecting a megger.
Disconnect the item being checked from other circuitry, if possible, before using a megger.
Q49. What is the primary use of a megger?
