Module 3 -- Introduction to Circuit Protection, Control, and Measurement
Pages i - ix
1−1 to 1−10
1−11 to 1−20
1−21 to 1−30
1−31 to 1−40
1−41 to 1−50
1−51 to 1−60
1−61 to 1−70
1−71 to 1−73
2−1 to 2−10
2−11 to 2−20
1−21 to 2−30
2−31 to 2−40
2−41 to 2−42
3−1 to 3−10
3−11 to 3−20
3−21 to 3−30
33−31 to 3−39
AI−1 to AI−3
AII−1 to AII−2
AIII−1 to AIII−10
Figure 1-30. - Reading a voltmeter at various ranges.
In figure 1-30(A) the meter is in the 1000-volt range. The pointer is barely above the 0 position. It is
not possible to accurately read this voltage. In figure 1-30(B) the meter is switched to the 250 volt range. From
the pointer position it is possible to approximate the voltage as 20 volts. Since this is well below the next
range, the meter is switched, as in figure 1-30(C). With the meter in the 50-volt range, it is possible to read
the voltage as 22 volts. Since this is more than the next range of the meter (10 volts), the meter would not be
switched to the next (lower) scale.
Q34. How is it possible to use a current sensitive meter movement
to measure voltage?
Q35. What is voltmeter sensitivity?
Q36. What method is used to
allow a voltmeter to have several ranges?
Q37. Why should you always use the highest range when
connecting a voltmeter to a circuit?
ELECTROSTATIC METER MOVEMENT
The final meter movement covered in this chapter is the ELECTROSTATIC METER MOVEMENT. The other meter
movements you have studied all react to current, the electrostatic meter movement reacts to voltage.
mechanism is based on the repulsion of like charges on the plates of a capacitor. The electrostatic meter movement
is actually a large variable capacitor in which one set of plates is allowed to
move. The movement of the plates is opposed by a spring attached to the plates. A pointer that
indicates the value of the voltage is attached to these movable plates. As the voltage increases, the plates
develop more torque. To develop sufficient torque, the plates must be large and closely spaced. A very high
voltage is necessary to provide movement, therefore, electrostatic voltmeters are used only for HIGH VOLTAGE
VOLTMETER SAFETY PRECAUTIONS
Just as with ammeters, voltmeters
require safety precautions to prevent injury to personnel and damage to the voltmeter or equipment. The following
is a list of the MINIMUM safety precautions for using a voltmeter.
· Always connect voltmeters in parallel.
· Always start with the highest range of a voltmeter.
· Deenergize and discharge the circuit completely before connecting or disconnecting the voltmeter.
· In dc voltmeters, observe the proper circuit polarity to prevent damage to the meter.
· Never use a dc voltmeter to measure ac voltage.
· Observe the general safety precautions of electrical and electronic devices.
Q38. What type of meter movement reacts to voltage rather than current?
Q39. What is the
only use for the voltage sensitive meter movement?
Q40. List the six safety precautions for the use
The two instruments most commonly used to check the
continuity (a complete circuit), or to measure the resistance of a circuit or circuit element, are the OHMMETER
and the MEGGER (megohm meter). The ohmmeter is widely used to measure resistance and check the continuity of
electrical circuits and devices. Its range usually extends to only a few megohms. The megger is widely used for
measuring insulation resistance, such as between a wire and the outer surface of the insulation, and insulation
resistance of cables and insulators. The range of a megger may extend to more than 1,000 megohms.
ohmmeter consists of a dc ammeter, with a few added features. The added features are:
1. A dc source of
potential (usually a 3-volt battery)
2. One or more resistors (one of which is variable)
A simple ohmmeter circuit is shown in figure 1-31.
The ohmmeter's pointer deflection is controlled by the
amount of battery current passing through the moving coil. Before measuring the resistance of an unknown resistor
or electrical circuit, the test leads of the ohmmeter are first shorted together, as shown in figure 1-31. With
the leads shorted, the meter is calibrated for proper operation on the selected range. While the leads are
shorted, meter current is maximum and the pointer deflects a maximum amount, somewhere near the zero position on
scale. Because of this current through the meter with the leads shorted, it is necessary to remove the
test leads when you are finished using the ohmmeter. If the leads were left connected, they could come in contact
with each other and discharge the ohmmeter battery. When the variable resistor (rheostat) is adjusted properly,
with the leads shorted, the pointer of the meter will come to rest exactly on the zero position. This indicates
ZERO RESISTANCE between the test leads, which, in fact, are shorted together. The zero reading of a series-type
ohmmeter is on the right-hand side of the scale, where as the
zero reading for an ammeter or a voltmeter is
generally to the left-hand side of the scale. (There is another type of ohmmeter which is discussed a little later
on in this chapter.) When the test leads of an ohmmeter are separated, the pointer of the meter will return to the
left side of the scale. The interruption of current and the spring tension act on the movable coil assembly,
moving the pointer to the left side (∞) of the scale.
Figure 1-31. - A simple ohmmeter circuit.
USING THE OHMMETER
After the ohmmeter is adjusted for zero reading, it is ready
to be connected in a circuit to measure resistance. A typical circuit and ohmmeter arrangement is shown in figure
Figure 1-32. - Measuring circuit resistance with an ohmmeter.
The power switch of the circuit to be measured should always be in the OFF position. This prevents the
source voltage of the circuit from being applied across the meter, which could cause damage to the meter movement.
The test leads of the ohmmeter are connected in series with the circuit to be measured (fig. 1-32). This causes
the current produced by the 3-volt battery of the meter to flow through the circuit being tested. Assume that the
meter test leads are connected at points a and b of figure 1-32. The amount of current that flows through the
meter coil will depend on the total resistance of resistors R1 and R2, and the resistance of the meter. Since the
meter has been preadjusted (zeroed), the amount of coil movement now depends solely on the resistance of R1and R2.
The inclusion of R1 and R2 raises the total series resistance, decreasing the current, and thus decreasing the
pointer deflection. The pointer will now come to rest at a scale figure indicating the combined resistance of R1
and R2. If R1 or R2, or both, were replaced with a resistor(s) having a larger value, the current flow in the
moving coil of the meter would be decreased further. The deflection would also be further decreased, and the scale
indication would read a still higher circuit resistance. Movement of the moving coil is proportional to the amount
of current flow.
The amount of circuit resistance to be measured
may vary over a wide range. In some cases it may be only a few ohms, and in others it may be as great as 1,000,000
ohms (1 megohm). To enable the meter to indicate any value being measured, with the least error, scale
multiplication features are used in most ohmmeters. For example, a typical meter will have four test lead
jacks-COMMON, R x 1, R x 10, and R x 100. The jack marked COMMON is connected internally through the battery to
one side of the moving coil of the ohmmeter. The jacks marked R x 1, R x 10, and R x 100 are connected to three
different size resistors located within the ohmmeter. This is shown in figure 1-33.
Figure 1-33. - An ohmmeter with multiplication jacks.
Some ohmmeters are equipped with a selector switch for selecting the multiplication scale desired, so only
two test lead jacks are necessary. Other meters have a separate jack for each range, as shown in figure 1-33. The
range to be used in measuring any particular unknown resistance (Rx in figure 1-33) depends on the approximate
value of the unknown resistance. For instance, assume the ohmmeter in figure 1-33 is calibrated in divisions from
0 to 1,000. If Rx is greater than 1,000 ohms, and the R x 1 range is being used, the ohmmeter cannot measure it.
This occurs because the combined series resistance of resistor R x 1 and Rx is too great to allow sufficient
battery current to flow to deflect the pointer away from infinity (∞). (Infinity is a quantity larger than the
largest quantity you can measure.) The test lead would have to be plugged into the next range, R x 10. With this
done, assume the pointer deflects to indicate 375 ohms. This would indicate that Rx has 375 ohms x 10, or 3,750
ohms resistance. The change of range caused the deflection because resistor R x 10 has about 1/10 the resistance
of resistor R x 1. Thus, selecting the smaller series resistance permitted a battery current of sufficient amount
to cause a useful pointer deflection. If the R x 100 range were used to measure the same 3,750-ohm resistor, the
pointer would deflect still further, to the 37.5-ohm position. This increased deflection would occur because
resistor R x 100 has about 1/10 the resistance of resistor R x 10.
The foregoing circuit arrangement
allows the same amount of current to flow through the meter's moving coil whether the meter measures 10,000 ohms
on the R x 10 scale, or 100,000 ohms on the R x 100 scale.
It always takes the same amount of current 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, it is necessary to
ALWAYS "zero" adjust the meter for each multiplication fact or selected.
You should select the
multiplication factor (range) that will result in the pointer coming to rest as near as possible to the midpoint
of the scale. This enables you to read the resistance more accurately, because the scale readings are more easily
interpreted at or near midpoint.
Q41. What electrical quantity is measured by an ohmmeter?
Q42. What other measurement
can an ohmmeter make?
Q43. How is a series-type ohmmeter connected to the circuit being measured?
Q44. What is used to provide the ohmmeter with several ranges?
Q45. What area of an ohmmeter
scale should be used when measuring circuits?
The ohmmeter described to this point is known as a series
ohmmeter, because the resistance to be measured is in series with the internal resistors and the meter movement of
the ohmmeter. Another type of ohmmeter is the SHUNT OHMMETER. In the shunt ohmmeter, the resistance to be measured
shunts (is in parallel with) the meter movement of the ohmmeter. The most obvious way to tell the difference
between the series and shunt ohmmeters is by the scale of the meter. Figure 1-34 shows the scale of a series
ohmmeter and the scale of a shunt ohmmeter.
Figure 1-34. - Series and shunt ohmmeter scales.
Figure 1-34(A) is the scale of a series ohmmeter. Notice "0" is on the right and "∞" is on the left.
Figure 1-34(B) is the scale of a shunt ohmmeter. In the shunt ohmmeter "∞" is on the right and "0" is on the left.
A shunt ohmmeter circuit is shown in figure 1-35.
In figure 1-35, R1 is a rheostat used to adjust the ∞
reading of the meter (full-scale deflection). R2, R3, and R4 are used to provide the R x 1, R x 10, and R x 100
ranges. Points A and B represent the meter leads. With no resistance connected between points A and B the meter
has full-scale current and indicates
∞. If a resistance is connected between points A and B, it shunts some of the current from the meter
movement and the meter movement reacts to this lower current. Since the scale of the meter is marked in ohms, the
resistance of the shunting resistor (between points A and B) is indicated. Notice that the switch has an OFF
position, as well as positions for R x 1, R x 10, and R x 100. This is provided to stop current flow and prevents
the battery from being discharged while the meter is not being used.
Figure 1-35. - A shunt ohmmeter with internal range resistors.
The shunt ohmmeter is connected to the circuit to be measured in the same way the series ohmmeter is
connected. The only difference is that on the shunt ohmmeter the ∞ reading is adjusted, while on the series
ohmmeter the 0 reading is adjusted. Shunt ohmmeters are not commonly used because they are limited generally to
measuring resistances from 5 ohms to 400 ohms. If you use a shunt ohmmeter, be certain to switch it to the OFF
position when you are finished using it.
Q46. What are the two types of ohmmeters?
What is the most obvious difference between the two types of ohmmeters?
Q48. List the four safety
precautions observed when using ohmmeters.
OHMMETER SAFETY PRECAUTIONS
The following safety precautions and operating procedures for ohmmeters are the MINIMUM necessary to prevent
injury and damage.
· Be certain the circuit is deenergized and discharged before connecting an ohmmeter.
· Do not apply power to a circuit while measuring resistance.
· When you are finished using an ohmmeter, switch it to the OFF position if one is provided and
remove the leads from the meter.
· Always adjust the ohmmeter for 0 (or ∞ in shunt ohmmeter) after you change ranges before making
the resistance measurement.
An ordinary ohmmeter cannot be used for measuring resistance of
multimillions of ohms, such as in conductor insulation. To adequately test for insulation break down, it is
necessary to use a much higher potential than is furnished by the battery of an ohmmeter. This potential is placed
between the conductor and the outside surface of the insulation.
An instrument called a MEGOHMMETER
(MEGGER) is used for these tests. The megger (fig. 1-36) is a portable instrument consisting of two primary
elements: (1) a hand-driven dc generator, G, which supplies the high voltage for making the measurement, and (2)
the instrument portion, which indicates the value of the resistance being measured. The instrument portion is of
the opposed-coil type, as shown in figure 1-36(A). Coils a and b are mounted on the movable member c with a fixed
relationship to each other, and 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 the pointer clockwise.
Figure 1-36. - A megger internal circuit.
Coil a is connected in series with R3 and the unknown resistance, Rx, to be measured. The combination of
coil, R3, and Rx forms a direct series path between the positive (+) and negative (-) brushes of the dc
generator. Coil b is connected in series with R2 and this combination is also connected across the generator.
There are no restraining springs on the movable member of the instrument portion of the megger. Therefore, when
the generator is not operated, the pointer floats freely and may come to rest at any position on the scale.
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).
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?
Q50. What is the procedure for using a megger to check the insulation of a conductor?
Q51. What is a normal indication on a megger when checking insulation?
Q52. List the four safety precautions observed when using a megger.
A MULTIMETER is the most common measuring device used in the Navy. The name multimeter comes from MULTIple
METER, and that is exactly what a multimeter is. It is a dc ammeter, a dc voltmeter, an ac voltmeter, and an
ohmmeter, all in one package. Figure 1-37 is a picture of a typical multimeter.
Figure 1-37. - A typical multimeter.
NEETS Table of Contents
- Introduction to Matter, Energy,
and Direct Current
- Introduction to Alternating Current and Transformers
- Introduction to Circuit Protection,
Control, and Measurement
- Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
- Introduction to Generators and Motors
- Introduction to Electronic Emission, Tubes,
and Power Supplies
- Introduction to Solid-State Devices and
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
- Introduction to Wave Propagation, Transmission
Lines, and Antennas
- Microwave Principles
- Modulation Principles
- Introduction to Number Systems and Logic Circuits
- Introduction to Microelectronics
- Principles of Synchros, Servos, and Gyros
- Introduction to Test Equipment
- Radio-Frequency Communications Principles
- Radar Principles
- The Technician's Handbook, Master Glossary
- Test Methods and Practices
- Introduction to Digital Computers
- Magnetic Recording
- Introduction to Fiber Optics