CHAPTER 7

Electrical Devices:
Troubleshooting and Safety

This chapter describes the process of locating the cause of malfunctions in electrical circuits associated with hydraulic-control systems. The information includes testing devices and types of grounding points. Also addressed in this chapter are the safety measures personnel should take when working on or around electrical circuits.

7-1. Hydraulics and Electricity. Hydraulics and electricity are often compared because the systems have similarities. A hydraulic circuit requires a power source (usually a pump), a load device (actuator), and conductors. The circuits differ mainly in the-

• • Types of devices used to control, direct, and regulate the hydraulic fluid flow.
  • Type and capacity of the actuators used to accomplish the work, which varies, depending on the application.

  An electrical circuit also requires a power source (battery, generator), a load device (light, bell, motor), and proper connections. An assortment of devices also controls, directs, and regulates the flow of electrical current.

  Hydraulic and electrical components are usually represented on diagrams by their own set of standardized symbols. Electrical diagrams are often called schematics. Figure 7-1 shows some of the more common symbols. Hydraulic and electrical systems and circuits have many differences. For example, electrical current is invisible, hydraulic fluid is not; electrical current flows through solid wires, hydraulic fluid flows through hollow lines. Figure 7-2 shows symbols for electrical and hydraulic components. Figure 7-3 compares a hydraulic circuit and an electrical circuit.

7-2. Troubleshooting Electrical Devices. Electrical troubleshooting is the process of locating the cause of malfunctions in electrical circuits. The following paragraphs contain some general troubleshooting information as well as specific tests for determining the status of some electrical devices. Skill in troubleshooting electrical equipment and circuits requires-

• • Knowledge of electrical principles to understand how a circuit or device should function.
  • Skill in reading and interpreting electrical schematics, diagrams, product data, and so forth.
  • Skill in operating test equipment and interpreting test measurements.
  • Ability to analyze problems in a logical manner.

  Following systematic steps that narrow down the problem to a smaller area of the equipment is much more efficient than trial-and-error methods. The troubleshooting procedure detailed below can be very useful in organizing the problem-solving effort and reducing equipment downtime:

  a. Procedure. The following troubleshooting procedure consists of five steps that you should perform in order. These steps represent the most reliable method of learning and applying a logical approach to problem solving and can be applied to any equipment, regardless of size.

  (1) Step One: Identify the Symptom. A symptom is an external indication that a circuit or device is not functioning correctly. You can identify a symptom by investigating the problem by sight, sound, smell, and touch. For example, visually inspecting the equipment may reveal that a circuit component has overheated and changed color or that an indictor lamp which should be on is not. A peculiar odor may lead you to discover melted insulation, or a chattering noise could indicate that a solenoid is about to fail. Moving controls or adjusting knobs may change the problem or have no effect at all. The fact that the equipment is not operating is a symptom.

  If someone else was operating the equipment when it failed, ask the person if he noticed anything unusual before it failed. Funny noises, things that do not look quite right, and improper operating sequences are symptoms that could lead to the cause of the problem. If you cannot find any immediately identifiable symptoms, try operating the equipment once you determine that it is safe to do so. Watch what works and what does not work. Note anything that does not seem right, no matter how small. Take the time to conduct a thorough investigation.

  (2) Step Two: Analyze the Symptom. In this step, you identify the functions where symptoms indicate a malfunction. Use the information you obtained during your identification, along with the schematic and functional block diagrams and knowledge of how the equipment is supposed to operate, to make logical technical deductions. For example, after careful examination, you find that a clamp in a plastic-injection molding machine will not pressurize. Further analysis, without using test equipment, narrows the problem to a clamp that is closed, clamp pressurization, or prefill shift, any of which might contain the faulty circuit.

  (3) Step Three: Isolate the Single Faulty Function. In this step, you use test equipment to decide which faulty function is actually causing the malfunction. When making these tests, use the following guidelines:

  • Make only those tests that are safe to make.
  • Make the least difficult tests first.
  • Test those functions first that will eliminate one or more of the other possible faulty functions.

  For example, if taking an ohmmeter reading can determine the fault, do not take a voltmeter reading as that requires power on the equipment. If you must disassemble half of the machine to reach a test point, perform a simpler test first. Test at a midway point in the circuitry, if possible. A good reading at the midway point eliminates the preceding functions and indicates that the problem is in the remaining circuits. A faulty signal at the midway point means that the problem is in the functions that process the signal before the midway point.

  In the injection molding example, test the clamp's pressurization circuits where the clamp's fully closed signal input either eliminates that function or confirms that the cause of the problem is a clamp that is not fully closed and, therefore, cannot be pressurized. Continue testing inputs and outputs of the suspect functions until you identify and confirm the single faulty function.

  (4) Step Four: Isolate the Faulty Circuit. In this step, you locate the single malfunctioning circuit within a functional group of circuits. Use the accumulated symptom and test data to close in on the single faulty circuit. Follow the guidelines from step three, but apply them to the circuits related to the faulty function. Use schematic and block diagrams to locate test points.

  In the injection-molding-machine example, assume that the clamp's fully closed signal is not present at the input to the clamp's pressurization circuits. Test within the clamp's closed circuits until you identify a single faulty circuit. The first test may reveal that the output of the clamp's fully closed circuit is bad. A check of the inputs to this circuit may indicate that the input from a clamp's closed-limit switch is bad but that all others are good. You can now identify the problem as being associated with one of the relatively few parts contained in a single circuit.

  (5) Step Five: Locate/Verify the Cause of the Malfunction. The tests you make in this step identify the failing part within the faulty circuit. Test the circuit until you find the cause of the malfunction. Examine and test the faulty part to verify that it has caused the problem and produced the observed symptoms.

  In checking out the clamp's fully closed circuit, for example, remove the suspected limit switch from the circuit and test it with an ohmmeter to determine if the switch's contacts are closing correctly to complete the circuit. Connect the ohmmeter across the contacts of the switch and actuate the switch's arm several times while checking the meter reading. If the contacts close properly, the meter should read zero ohms when the arm is in one position and infinity when the arm is in the other position.

  If the meter pointer does not move when the switch arm is actuated, disassemble and examine the switch. If this last examination reveals that the mechanical linkage connecting the switch's arm to the contacts is broken, then you have found the cause of the malfunction. A final analysis should show that this cause explains the observed symptoms. However, the procedure is not complete until you verify the findings. In this example, you would install a new limit switch in the circuit and operate the equipment to confirm that you have fixed the problem.

  b. Testing Devices. The following paragraphs outline some basic electrical tests that you can conduct on specific pieces of equipment that were discussed earlier. As part of a troubleshooting test, you should mechanically inspect these devices. Also, if spare parts are available, substitute a good part for a suspect part as a quick method of returning the equipment to operation. Test the suspect part and either repair it or discard it.

  (1) Potentiometer. Since a potentiometer is a variable-resistance device, it should be disconnected from its circuit and tested with an ohmmeter, if it is suspect. Only two of the three leads need to be disconnected for this test. Be very careful when adjusting small potentiometers on printed circuit boards. They are quite fragile and can easily be broken if rotated beyond the end stops. Test a potentiometer as follows:

  • Determine the expected resistance value from a schematic diagram for the circuit. The value may also be printed on the case of the device.
  • Connect the ohmmeter across the ends of the potentiometer and confirm that the reading matches the expected value.
  • Remove a test lead from one end and move it to the middle terminal.
  • Rotate the shaft or turn the screw that varies the resistance of the device. The ohmmeter reading should indicate zero ohms at one end of the shaft rotation and the full expected resistance value of the potentiometer at the other end. It should also show a smooth change in resistance as the shaft is turned.
  • Move the lead that is still connected to an end terminal over to the other end.
  • Rotate the shaft again while looking for the same smooth transition from zero to maximum resistance.

  (2) Solenoid Coil. If a solenoid is thought to be faulty, do the following:

  • Remove it from the machine (plug the opened ports on the valves if necessary).
  • Disassemble and examine the solenoid for signs of overheating or mechanical problems.
  • Test the solenoid coil by attaching an ohmmeter (set to a low resistance range) across the coil terminals. If the coil is good, the meter will show a relatively low reading (a few thousand ohms or less). A zero reading would indicate that the coil windings are shorted to each other, probably as a result of melted insulation. An infinity reading on the ohmmeter means that the coil has opened up and is defective.

  (3) Relay. Test a suspect relay as follows:

  • Actuate the relay armature, manually.
  • Remove the relay from the equipment.
  • Examine the relay carefully for signs of mechanical problems.
  • Check the relay coil in the same way as a solenoid coil, if you do not find any mechanical problems. Test the electrical contacts with an ohmmeter as you do the switch contacts. The meter should read zero when the contacts are closed and infinity when they are open.
  • Test the normally open and the normally closed circuits.

  (4) Transformer. When you determine, by voltage readings or symptom information, that a transformer may be the cause of a malfunction, check the primary and the secondary coil resistance with an ohmmeter. Disconnect one end of the primary winding and one end of the secondary winding from the rest of the circuit before testing. If the failure is the result of an open winding, the ohmmeter will read infinity when connected across the defective winding. If the failure is caused by shorted turns within a winding, the problem is more difficult to diagnose because the ohmmeter will indicate a very low resistance. Since a winding consists of a length of conductor wound into a coil, the resistance readings are normally quite low anyway. If you suspect shorted turns-

  • Use the expected primary and secondary operating voltages to determine the approximate turns ratio. Divide the secondary voltage into the primary voltage to get the ratio. For example, 120 volts divided by 24 volts equals a ratio of 5:1.
  • Use this ratio to compare the measured primary resistance to the measured secondary resistance. In the example, if the primary resistance is 20 ohms, then the secondary resistance should be about 4 ohms (20/5).

  Be sure to adjust the zero-ohms control before making the measurement; hold the test probes by the insulated portion only. You may have difficulty determining if the reading is accurate since the measurement is so close to the low end of the ohms scale. Compare the readings to a replacement transformer's, if one is available. To positively verify that the transformer is faulty, you may have to substitute a good transformer for the suspect one.

  (5) Diode. You can use a simple resistance check with an ohmmeter to test a diode's ability to pass current in one direction only. To test a suspect diode-

  • Remove one end of the diode from the circuit.
  • Connect the positive ohmmeter lead to the anode and the negative lead to the cathode. When the ohmmeter is connected this way, the diode is forward biased, and the measured reading should be very low. Set the ohmmeter for the appropriate diode test range.
  • Reverse the ohmmeter connections. When the negative ohmmeter lead is attached to the anode and the positive lead is attached to the cathode, the diode is reverse biased, and the meter should read a high resistance.

  A good diode should have real low resistance when forward biased and high resistance when reverse biased. If the diode reads a high resistance in both directions, it is probably open. If the readings are low in both directions, the diode is shorted. A defective diode could show a difference in forward and backward resistance. In this case, the ratio of forward to backward resistance is the important factor. The actual ratio depends on the type of diode. As a rule of thumb, a small signal diode should have a ratio of several hundred to one. A power rectifier can operate with a ratio as low as ten to one.

7-3. Ground. Every electrical circuit has a point of reference to which all circuit voltages are compared. This reference point is called ground, and circuit voltages are either positive or negative with respect to ground. Connections to ground that are made for safety reasons refer to earth ground. When voltage measurements are taken, the difference of potential between a point in the circuit and a ground point is measured by the voltmeter. This type of ground is referred to as chassis or common ground.

a. Earth Ground. Initially, ground referred to the earth itself and since has represented a point of zero potential or zero volts. A short circuit within a device that connects live voltage to the frame could cause a serious shock to anyone touching it. However, if the frame is connected to earth ground, it is held at the safe potential of zero volts, as the earth itself absorbs the voltage.

Today, a third prong on grounded power plugs connects most stationary equipment to earth ground through the electrical wiring system. Some equipment is connected to earth ground by a conductor that goes from the metal frame of the equipment to a long copper rod that is driven into the earth. Some appliances are often grounded by connecting the conductor to a water pipe running into the ground. In any case, the frames of all equipment connected to the earth are at the same zero volt potential. This prevents shocks that might occur should a person touch two pieces of ungrounded equipment at the same time.

b. Chassis or Common Ground. In some cases, electrical circuits used today are not connected directly to earth ground; however, they still require a point of reference or a common point to which elements of each circuit are connected. For example, a portable battery-operated transistor radio does not have a ground conductor connecting it with the earth. A strip of conducting foil on the internal circuit board is used as the common point. In an automobile battery, the negative terminal is generally connected to the engine block or chassis frame by a heavy cable. The connecting point, as well as every other point on the metal frame, is considered to be a ground for the electrical circuits of the vehicle. The rubber tires insulate the vehicle from the earth ground. In these examples, ground is simply a zero reference point in an electrical circuit and is referred to as chassis ground. All voltages in the circuit are measured with respect to this common point.

c. Zero Reference Point. Without a zero reference point, voltage could not be expressed as a positive or negative value. The schematic diagrams in Figure 7-4 illustrate this point:

• Diagram A shows a voltmeter connected to the two terminals of a 6-volt, dry-cell battery. Without a ground in the circuit, the measured voltage is 6 volts between the two terminals. It is neither positive nor negative.
• Diagram B shows that the negative battery terminal is connected to ground. The voltmeter measures the difference of potential between the positive terminal and the ground point. The measured voltage is +6 volts because the ungrounded terminal is 6 volts more positive than the ground or zero reference point.
• Diagram C shows that the voltmeter measure -6 volts when the positive terminal of the battery is connected to the zero reference point. The ungrounded battery terminal is now 6 volts more negative than the reference point.
• Diagram D shows two 6-volt batteries that are connected in series. The voltage between points A and C is 12 volts. When a ground is placed at point B, which is between the two batteries, + 6 volts are available between points A and B, and -6 volts are available between points C and B. (Many modern electronic circuits require both positive and negative voltage for proper operation. This would be impossible without a zero reference point in the circuit.)

d. Isolation Between Earth and Chassis Ground. Industrial equipment often requires an earth and a separate chassis ground for proper operation. The earth ground represents an actual potential of zero volts, while the chassis ground is used only as a reference point and may be at some potential above or below the earth ground. In these cases, the earth ground and the chassis ground are not connected together at any point in the equipment. However, during installation or repairs, the chassis ground may be inadvertently connected to the earth ground. To check for this condition, use a 1.5-volt, D-cell battery and holder, connecting wires, and a voltmeter. Make sure that the equipment is OFF before making the test.

In Figure 7-5, the battery is installed between the chassis ground and the earth ground. The voltmeter, set to measure 1.5 volts direct current (DC), is connected across the battery. If a connection exists between the chassis and the earth ground, it will place a short circuit across the battery, and the voltmeter will indicate zero volts. If this is the case, temporarily disconnect one end of the battery to keep it from discharging while looking for the improper connection between the grounds. When you find the connection, remove it and reconnect the battery and the meter. The voltmeter should read the battery potential of 1.5 volts. If the voltmeter reading is still zero volts, an improper connection still exists in the equipment. Repeat the test until the voltmeter reads the battery voltage. Remember to disconnect the battery after completing the test.

7-4. Safety. Effective safety measures are a blend of common sense and the knowledge of basic electrical and hydraulic principles and of how a system or circuit operates, including any dangers associated with that operation. General safety information and safety practices are listed below. The list is not all inclusive, is not intended to alter or replace currently established safety practices, and does not include safety practices for hydraulic equipment.

a. Information. When working with electrical equipment, consider the following information regarding safety:

• Injuries associated with electrical work may include electrical shocks; burns; and puncture, laceration, or abrasion wounds.
• Current flowing through the body can be fatal. As little as 0.01 amp produces muscle paralysis and extreme breathing difficulty in the average person; permanent physical damage and death can result from 0.1 amp flowing through the heart.
• The amount of current received from an electrical shock depends on the voltage applied and the resistance of that part of the body through which the current flows. About 30 volts can produce 0.1 amp, so use extreme caution when working with circuits that include voltages higher than 30 amps.
• Most electrical shocks are unexpected. Even ones that are not particularly dangerous could cause you to jerk your hand into heavier currents or hit some sharp object. Always check to see that the power is turned off before placing your hand in a circuit.

b. Practices. When working with electrical equipment, consider the following safety rules:

• Never put both hands in a live circuit as this provides a path for a current flow through the heart. Keep one hand behind you or in your pocket when taking measurements with a meter.
• Never work on live circuits when wet, as this lowers the body's resistance and increases the chance for a fatal shock.
• Never work alone on electrical equipment. Shocks above 0.01 amp can paralyze your muscles and leave you unable to remove yourself from the source of the current flow. Always be sure someone else is around to help in an emergency.
• Use the proper equipment for circuit testing. Check for correct junction settings, range switches, proper insulation on test probes, and so forth.
• Remove all watches, rings, chains, and any other metal jewelry that may come in contact with an electrical potential or get caught in moving mechanical parts. Do this before you work on any electrical equipment, circuit, or battery.
• Have a good understanding about the circuit you are working on. Think about what you need to do before working on the circuit. Ask for help if you do not know enough about the task you are to perform.

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