ELECTRICAL TOOLS AND TEST EQUIPMENT
This section describes the following common tools used in performing electrical maintenance:
Combination cutting pliers are slip joint pliers that have a cutting notch at the back of each of their serrated jaws (see Figure 1).
Figure 1: Combination Cutting Pliers
Use combination cutting pliers to grip work and to cut small nails, cotter pins, and stiff wire. To use, open the jaws far enough for the cutting edges to line up. Insert the work through both cutting notches, then squeeze the handles together to cut the work. Sharpen the cutting edges with a file when necessary.
Lineman’s cutting pliers are cutting and gripping pliers which have serrated grippers at the tips of the jaws and knife-like cutting edges which extend from the rear of the grippers to the hinge on the pliers (Figure 2).
Figure 2: Lineman’s Cutting Pliers
To cut heavy wire and metal strips of medium thickness, set the work between the cutting edges, as far back in the jaws as possible, then squeeze the handles. The proper care of lineman’s cutting pliers is the same as for combination cutting pliers.
Diagonal cutting pliers are smaller than either combination or lineman’s cutting pliers. They have slightly curved jaws that allow the work to be cut at a variety of angles. Diagonal cutting pliers do not have any teeth or gripping surfaces. The jaws have cutting edges only. Refer to Figure 3.
Figure 3: Diagonal Cutting Pliers
Use diagonal cutting pliers to cut wire, cotter pins, small nails, and lightweight strips of metal. Place the work as far back in the jaws as possible and squeeze the handle to cut the work.
Wire strippers are cutting pliers that are slightly larger than diagonal cutting pliers. They have two sets of cutting notches in the cutting edges of their slightly curved jaws. Refer to Figure 4.
Figure 4: Wire Strippers
Select the stripping notch that is slightly larger than the diameter of the metal core of the wire. Center the wire in the notch and squeeze the handles of the pliers. Next, turn the wire in the notch to make a second cut. This ensures that the insulation is severed completely. Finally, push the wire strippers toward the end of the wire to strip the insulation off.
The procedure for stripping wire with a hand wire stripper is as follows (refer to Figure 5):
Figure 5: Stripping Wire with a Hand Stripper
A sharp knife may be used to strip the insulation from a conductor. The procedure is much the same as for sharpening a pencil. The knife should be held at approximately a 60 angle to the conductor. Use extreme care when cutting through the insulation to avoid nicking or cutting the conductor. This procedure produces a taper on the cut insulation as shown in Figure 6.
Figure 6: Taper Cut Insulation
When stripping wire with any of the tools mentioned, observe the following precautions:
After boxes are installed and conduit connected to them, the conductors (wire) can be pulled. On very short runs, small wires can be pushed through from one box to another. However, a fish tape is much faster. The tape is available in various lengths usually 50, 100, and 200 ft. of tempered flexible steel in several thickness and widths. Rigid plastic tapes and pull ropes are also suitable. The tape may have a hook or a ball on the end, Figure 7. If it has neither, you can fashion a hook with the aid of a pair of pliers. The tempered steel can be brittle, so make the bend carefully or reduce the temper by heating the end with a torch.
Figure 7: Fish Tape
Wires are difficult to push through conduit. The fish tape, being somewhat stiff, will snake through bends much easier and can be pushed considerable distances through several bends. Push the tape through the run and attach the wires being pulled, Figure 8. Be sure to wrap the wires securely so they do not become detached from the stress of pulling. It is better to work from the top down so the weight of the wires works with the pulling rather than against it. Wires being pulled should be kept straight. Twisted, crossed, or tangled wire will bind at bends, saddles, or offsets. Wires insulation can become abraded or torn. On long runs it may help to coat the wire with pulling lubricants such as those containing talc, soapstone, or liquid soap. Special wire pulling lubricants are manufactured for this purpose.
Figure 8: Methods of Attaching Wire Conductors to Fish Tape
This section describes the following types of standard electrical test equipment:
A voltage meter, usually called a voltmeter, will measure voltage by using the same meter movement as an ammeter. A simple voltmeter consists of the meter movement in series with the internal resistance of the voltmeter itself. For example, a meter with a 50 microampere meter movement and a 1,000-ohm internal resistance can be used to directly measure voltages up to 0.05 volts, as shown in Figure 9. When the meter is placed across the voltage source, a current determined by the internal resistance of the meter, flows through the meter movement.
Figure 9: Schematic Diagram for a Simple Voltmeter
To measure larger voltages a multiplier resistor is used. This increased series resistance serves to limit the current which can flow through the meter movement, thus extending the range of the meter. For example, using the same meter movement of 50A, the value of the multiplier resistor required to indicate a full-scale deflection of 1.0 volt can be determined. Using Ohm’s Law, the total resistance required is:
A single voltmeter can be made to measure several different ranges by including different multiplier resistors and a switch. Figure 10 illustrates how this is done. Table 1 lists the switch positions with the corresponding resistance.
Figure 10: Series Resistors
A voltmeter can also alter the circuit parameters. The example shown in Figure 11 illustrates this principle.
Figure 11: Effects of Placing a Voltmeter in a Circuit
In this circuit, it is the voltage drop across the 40,000 ohm resistor which is to be determined. In the first circuit, the current flowing through the two resistors is given by:
Connecting the voltmeter in parallel with the 40,000 ohm resistor, as shown, significantly changes the total resistance in the circuit. The parallel combination of 40,000 ohms and 200,000 ohms is:
Thus, with the voltmeter in the circuit, the current is:
This current causes a voltage drop across the combination of the meter and the 40,000 ohm resistor of:
The voltmeter will read 7.7 volts, whereas, the voltage drop across the resistor without the voltmeter is 8 volts.
In most cases, the current flowing through the voltmeter movement is negligible compared to the current flowing through the element whose voltage is being measured. When this is the case, the voltmeter has a negligible affect on the circuit.
The following precautions should be observed when using a voltmeter to avoid damage to the meter movement:
In most commercial voltmeters, the internal resistance is expressed by the "ohms-per-volt" of the meter. A typical value is 20,000 Ω/V for a voltmeter using a 50A movement. This quantity tells what the internal resistance of the meter is on any particular full-scale setting. In general, the meter internal resistance is the "ohms per volt" times the full-scale voltage. The higher the ohms per volt rating, the higher the internal resistance of the meter, and the smaller the affect of the meter on the circuit.
A low sensitivity meter may give a correct reading when measuring circuits having low resistance values, but may give very inaccurate indications when used to measure voltages in high resistance circuits such as electrical systems.
For example, if a voltmeter rated at 20,000 ohms/volt is set on 10 volts full scale, the total internal resistance will be:
If the meter were set to 0.5 volts full scale, the internal resistance would be 10,000 ohms and, if set to 50V full scale, it would be 1,000,000 ohms.
A voltage tester is a simple piece of electrical testing equipment used to determine the availability of voltage at the power source. The voltage tester is also used to ensure that the power source has been de-energized before work begins. A tester may indicate alternating current, direct current, or both. Some typical voltage testers are shown in Figure 12.
Figure 12: Typical Voltage Testers
The VOL-CON Model 61-076 voltage/continuity tester (Figure 13) can be used to test continuity, locate blown fuses, and find the grounded side of line (neutral). It is also used for testing the grounded side of a motor or appliance, testing for 25 to 60 Hertz frequency, checking the continuity of power cords, and locating excessive leakage to ground.
Figure 13: VOL-CON 61-076 Voltage/Continuity Tester
This device requires the use of four 1 -volt watch/calculator batteries
This unit uses a pair of LED indicators to indicate the presence of AC voltage. If the LED on the left does not come on when checked against a known good AC source, check the left side of the battery pack. If the LED on the right does not come on when checked against a known good AC source, check the right side of the battery pack.
A clamp-on ammeter (Figure 16), in many cases, is also a multi-function meter. It can measure voltage, current, and, sometimes, resistance. The main difference being how it measures current. With a volt-ohm meter (VOM), the circuit has to be de-energized, wires separated and, set up as an ammeter, connected between the two ends of the cable. The clamp-on ammeter needs only to be wrapped around one wire to determine the current flowing in the circuit (Figure 17).
Figure 16: Clamp-On Ammeters
Figure 17: Using a Clamp-On Ammeter
Digital meters have revolutionized the test equipment world. Better accuracy is now easily attainable, more functions can be incorporated with one meter, and auto-ranging as well as automatic polarity indication is used. Technically, digital multimeters are classified as electronic multimeters. However, digital multimeters do not use a meter movement. Instead, a digital meter's input circuit converts a current into a digital signal, which is processed by electronic circuits and displayed numerically on the meter face.
A major limitation with analog meters that use electro-magnetic meter movements is that the scale reading must be estimated if the meter pointer falls between scale divisions. Digital multimeters eliminate the need to estimate these readings by displaying the reading as a numerical display.
With digital meters, personnel must revise the way the indications are viewed. If a technician were reading the AC voltage on a normal wall outlet with an analog voltmeter, any indication within the range of 120VAC would be considered acceptable. However, when read with a digital meter, you may think something was wrong if you got an indication of 114.53VAC. The thing to bear in mind is that the digital meter is very precise in its reading, sometimes more precise than is called for, or is usable. Also, be aware that the indicated parameter may change with the range used. This is primarily due to the change in accuracy and where the meter "rounds off."
There are many types of digital multimeters. Some are bench-type multimeters; some are designed to be hand-held. Most types of digital multimeters have input impedance of 10 megohms and above. They are very sensitive to small changes in current and are, therefore, more accurate.
Digital multimeters all operate on the same basic principles. This section discusses the features and uses of a typical Fluke model 80 series, which is a very common multimeter and with basic functions that can be applied to any digital multimeter. Figure 18 shows a Model 87 meter.
Figure 18: Fluke Model 80 Series Multimeters (Model 87)
Table 2 shows the meter's inputs.
Table 2: Fluke Model 80 Multimeter Inputs
Table 3 shows the meter's switch positions and functions.
Table 3: Fluke Model 80 Multimeter Switch Positions and Functions
If a test lead is plugged into the mA/:A or A terminal, but the rotary switch is not correctly set to the mA/:A or A position, the beeper warns you by making a chirping sound. This warning is intended to stop you from attempting to measure voltage, continuity, resistance, capacitance, or diode values when the leads are plugged into a current terminal. Placing the probes across (in parallel with) a powered circuit when a lead is plugged into a current terminal can damage the circuit you are testing and blow the meter’s fuse. This can happen because the resistance through the meter’s current terminals is very low, so the meter acts like a short circuit.
Holding a button down while turning the meter on activates a power-up option. These options are listed on the back of the meter.
The meter automatically turns off if you do not turn the rotary switch or press a button for 30 minutes. To disable automatic power-off, hold down the blue button while turning the meter on. Automatic power-off is always disabled in MIN MAX recording mode.
This section describes how to take measurements with a Fluke Model 80 meter.
Voltage is the difference in electrical potential between two points. The polarity of ac (alternating current) voltage varies over time, while the polarity of dc (direct current) voltage is constant over time. The meter presents ac voltage values as rms (root mean square) readings. The rms value is the equivalent dc voltage that would produce the same amount of heat in a resistance as the measured sinewave voltage. Models 85 and 87 feature true rms readings, which are accurate for other wave forms (with no dc offset) such as square waves, triangle waves, and staircase waves. The meterÃƒÂ¢Ã¢â€šÂ¬Ã¢â€žÂ¢s voltage ranges are 400 mV, 4 V, 40 V, 400 V, and 1,000 V. To select the 400 mV dc range, turn the rotary switch to mV. To measure ac or dc voltage, set up and connect the meter as shown in Figure 19.
Figure 19: Measuring AC and DC Voltage
The following are some tips for measuring voltage:
Press to turn the continuity beeper on or off.
The continuity function detects intermittent opens and shorts lasting as little as 1 millisecond (0.001 second). These brief contacts cause the meter to emit a short beep.
Figure 20: Testing for Continuity
''Resistance'' is an opposition to current flow. The unit of resistance is the ohm (∑). The meter measures resistance by sending a small current through the circuit. Because this current flows through all possible paths between the probes, the resistance reading represents the total resistance of all paths between the probes.
The meter’s resistance ranges are 400∑, 4 k∑, 40 k∑, 400 k∑, 4 M∑, and 40 M∑.
To measure resistance, set up the meter as shown in '''Figure 21'''.
'''Figure 21: Measuring Resistance'''
The following are some tips for measuring resistance:
Conductance, the inverse of resistance, is "the ability of a circuit to pass current." High values of conductance correspond to low values of resistance.
The unit of conductance is the Siemen (S). The meter’s 40 nS range measures conductance in nanosiemens (1 nS = 0.000000001 Siemens). Because such small amounts of conductance correspond to extremely high resistance, the nS range lets you determine the resistance of components up to 100,000 M∑, or 100,000,000,000 ∑ (1/1 nS = 1,000 M∑).
To measure conductance, set up the meter as shown for measuring resistance (Figure 21); then press ∑ until the nS indicator appears on the display.
The following are some tips for measuring conductance:
A digital multimeter can be used for testing electronic components as discussed below.
The first step is to disconnect the diode from the circuit. Then, insert the meter leads into the proper jacks on the front panel and press the appropriate function and range buttons. The function and range buttons chosen for a diode or transistor check depend on the meter that is used. While a high resistance range should be selected to protect the diodes from high meter current, the range should still be low enough to yield a usable resistance reading.
Checks are made on diodes and transistors with the meter set for resistance. The meter’s internal battery can then forward bias or reverse bias the junctions of the diode or transistor. Some digital multimeters have symbols that indicate what resistance range should be used. Most diodes are checked with the KΩ function button and the 2 range button pressed.
To check a diode, connect the meter’s positive lead to the anode of the diode and the negative lead to the cathode. With this arrangement, the meter forward biases the diode; the resistance reading indicated by the meter should be relatively low value. Next, reverse the meter leads so that the meter reverse-biases the diode; the resistance reading should be relatively high. As a rule, a diode is considered good if the reverse reading is at least 10 times the forward reading.
A diode is usually open if an infinite resistance reading is obtained for both measurements. A zero resistance reading for both measurements means that the diode is probably shorted. If the resistance readings are low when the diode is forward biased and when it is reverse biased, the meter may be indicating the proper resistance value when the diode is forward biased, but the meter’s internal battery may be causing the diode’s junction to break down when the diode is reverse biased. In many of these cases, the diode should probably be replaced.
Since the reverse resistance reading of a good diode is expected to be 10 times the forward reading, if the forward resistance reading multiplied by ten exceeds the selected range, go to the next highest range and start again. For example, if you obtain a .22 kilohm reading as the forward resistance reading, and you have selected the 2 kilohm range, you should use the next highest range for the meter; .22 kilohms times 10 is greater than 2 kilohms.
A digital multimeter can also be used to check a transistor for shorted or open junctions. The meter’s internal battery is used to bias each of the transistor junctions in the same manner as in a diode check. After the transistor to be checked is disconnected from its circuit, the meter is set up as described for the diode check.
The emitter-base junction of the transistor is checked first, with the junction forward biased and then reverse biased. If the junction is good, the reverse reading will be at least 10 times the forward reading. Then, the same procedure is repeated for the collector-base junction.
When testing the emitter-to-collector junction, both readings should indicate very high resistance. Since current is not supposed to flow from the collector to the emitter, a low resistance reading would indicate that the transistor might be shorted.
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 (Figure 23) 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 23: 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 (+) 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.-) and negative (
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 Rxis 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 counter-clockwise. 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 163(B).
Most 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.
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.
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.
Have you ever wasted frustrating minutes trying to find the right fuse or breaker With a handy new gadget called a circuit tracer, you can pinpoint the right circuit in just a few seconds.
The tracer works by sending signals through the electrical wires in your home. Just plug the small transmitter into the outlet you’re working on. Then take the receiver unit to the service panel and run it slowly up and down the row of fuses or breakers (see Figure 24).
Figure 24: Using a Circuit Tracer
When the receiver nears the correct circuit, you’ll hear a beeping noise – the louder it gets, the closer you are. When you locate the circuit, remove the fuse or flip the breaker and try the receiver again. If you have located the right one, the beeping will stop. Before starting on any electrical project, be sure to test the outlet or switch with a circuit tester first.