3-22
The guard ring, shown in view A of figure 3-15, shunts any leakage currents to the negative side of
the generator. This prevents such current from flowing through coil a and affecting the meter reading.
Q-24. What is the purpose of the guard ring in a megohmmeter?
If the test leads are open, no current will flow in coil a. However, current will flow internally through
coil b and deflect the pointer to infinity. This reading 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; the pointer tends to
move 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 at a position in which the two
forces are balanced. This position depends upon the value of Rx, which controls the amount of the current
in coil a. Because changes in voltage affect both coils in the same proportion, the position of the moving
element is independent of the voltage. If you short the test leads together, the pointer will come to rest at
zero because the current in coil a is relatively large. Since R3 limits the current, the instrument will not be
damaged under these circumstances. The external appearance of one type of megger is shown in view B
of figure 3-15.
Most meggers you will use are rated at 500 volts; however, there are other types. Meggers are
usually equipped with friction clutches, which are designed to slip if the generator is cranked faster than
its rated speed. This prevents the generator speed and output voltage from exceeding rated values. A
1,000-volt generator is available for extended ranges. When an extremely high resistance, such as 10,000
megohms or more, is to be measured, a high voltage is needed to cause enough current flow to actuate the
meter movement.
CAUTION
When using a megger, you can easily be injured or damage equipment if you do
not observe the following MINIMUM safety precautions:
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.
De-energize and discharge the circuit completely before connecting a megger.
Whenever possible, disconnect the component being checked from other circuitry before using a
megger
Q-25. Most meggers you will use are rated at what voltage?
Q-26. The development of excessive test voltages is avoided by the use of meggers equipped with what
device?
ELECTRODYNAMOMETER-TYPE METERS
The electrodynamometer-type meter differs from the galvanometer types we have just studied in that
two fixed coils are used to produce the magnetic field instead of a permanent magnet. Two movable coils
are also used in the electrodynamometer meter. The electrodynamometer meter is most commonly found
in various types of power meters.
