The circuit arrangement in view A of figure 3-14 allows the same amount of current to flow through the moving meter coil. The same amount is allowed to flow whether the meter measures 10,000 ohms on the R X 1 scale, 100,000 ohms on the R X 10 scale, or 1,000,000 ohms on the R X 100 scale.

The same amount of current must always be used to deflect the pointer to a certain position on the scale (midscale position, for example), regardless of the multiplication factor being used. Since the multiplier resistors are of different values, you must always "zero" the meter for each multiplication scale selected. When selecting a range on the ohmmeter, select the one that will result in the pointer coming to rest as close to the midpoint of the scale as possible. This will enable you to read the resistance more accurately because scale readings are more easily interpreted at or near midpoint.

The Megohmmeter

An ordinary ohmmeter cannot be used for measuring multimillion ohm values of resistances, such as those in conductor insulation. To test for such insulation breakdown, you need to use a much higher potential than that supplied by the battery of an ohmmeter. This potential is placed between the conductor and the outside of the insulation. A megger (megohmmeter) is used for these tests. The megger, shown in figure 3-15, is a portable instrument consisting of two main elements: (1) a hand-driven dc generator, which supplies the necessary voltage for making the measurement, and (2) the instrument portion, which indicates the value of the resistance you are measuring. The instrument portion is of the opposed-coil type, as shown in view A. Coils a and b are mounted on movable member c. A fixed angular relationship exists between coils, and they are free to turn as a unit in a magnetic field. Coil b tends to move the pointer counterclockwise, and coil a tends to move it clockwise.

Figure 3-15. - Megger internal circuit and external view.

Coil a is connected in series with R3 and unknown resistance R_x. The combination of coil a, R3, and R_x forms a direct series path between the + and - brushes of the dc generator. Coil b is connected in series with R2, and this combination is also connected across the generator. Notice that the movable member (pointer) of the instrument portion of the megger has no restoring springs. Therefore, when the generator is not being operated, the pointer will float freely and may come to rest at any position on the scale.

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 R _x, 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 R_x, 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.

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.

Q.27 What components in an electrodynamometer-type meter movement produce the magnetic field?

As shown in figure 3-16, the fixed coils are connected in series and positioned coaxially (in line) with a space between them. The two movable coils are also positioned coaxially and are connected in series. The two pairs of coils (fixed pair and movable pair) are also connected in series with each other. The movable coil is pivot-mounted between the fixed coils. The main shaft on which the movable coils are mounted is restrained by spiral springs that restore the pointer to zero when no current is flowing through the coil. These springs also act as conductors for delivering current to the movable coils. Since these conducting springs are very small, the meter cannot carry a high value of current.

Figure 3-16. - Internal construction of an electrodynamometer.

Q.28 What is the limiting factor as to the amount of current an electrodynamometer meter movement can handle?