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Learning Goals:
• Learn basics of troubleshooting
• Learn troubleshooting techniques
• Understand various measuring devices and their functions
• Learn different testing methods.
1. Introduction
In the last two sections we have studied about symbols, measuring meters, reading control circuits, etc. This would help in building a planned and strategic approach for troubleshooting.
In a healthy state, any basic circuit that has a load and energy source has circuit paths, as shown in FIG. 1.
FIG. 1 A healthy electrical circuit.
Current flows in a closed circuit between two electrically unequal potential points.
Points A and B are any two points across which voltage is measured. The conductor offers resistance to this flow of electrons (i.e., current) depending upon the material. Generally electrical problems can be classified under two broad types.
1. A connection does not exist where it should. This is an open circuit fault and can be detected using a continuity tester (FIG. 2 illustrates this type of fault).
FIG. 2 A typical open circuit fault
2. A connection exists where none should. This is called a short-circuit fault and can lead to excessive current accompanied by mechanical forces and heating of circuit conductors. Such fault happen due to insulation failures and can be detected using insulation testing instruments.
The process of detecting these faults and rectifying the circuit to restore normal operating condition is called troubleshooting. We will discuss about the open circuit fault first.
Current has a tendency to flow between two points that are at an unequal potential (electrically), provided the path between the two points is electrically conductive.
• Resistance offered by the path is known as 'Resistance' ohm)
• Electrical potential is denoted as 'volts'
• Flow of electrons between two points is termed as 'Current' (amp). Therefore, while troubleshooting, the following points have to be checked:
-- Continuity of path (i.e., resistance)
-- Electric potential at two points of the path (i.e., voltage)
-- Flow of electrons through the path (i.e., current). An electrical circuit is made up of different paths and works on different voltages. Let us, therefore, identify the path that should be complete, also, when and how it is completed. Moreover, if it does not complete, let us identify the reason.
2. Basic principles in using a drawing and meter in troubleshooting circuits
To identify a faulty section, follow the guidelines given below, along with a drawing and a meter:
• Check the incoming supply voltages first
• Check for voltages at the specific test points in circuit (as per manufacturers test point data sheet)
• Do dead test of circuit for integrity of protection devices and others
• In dead test, check for continuity of circuits, as intended, and check for insulation resistance
• If it's not possible to perform a dead test, connect the supply to the circuit and do a live test of circuit.
Generally, any electrical circuit can be differentiated in two sections:
• Power circuit
• Control circuit.
It is always advisable to first check the power circuit. So, if the power circuit works, as it should, then troubleshoot the control circuit.
Power circuit check list:
-- Incoming power to circuit and its integrity
-- Check for correct functioning of protection devices
-- Check visual cable continuity
-- Check for any signs of flash or burning smell of devices.
Control circuit check list:
-- Control circuit power first
-- Check for proper functioning of relays, timers, and switches
-- Check visual cable continuity
-- Check for wire interconnections and terminal connections of circuit
-- Check logical operational sequence of contactor switching
-- Check for timer duration settings.
If the above criteria are checked and still the motor (final device) is not working, then test the motor (final device).
3. Checks for circuit continuity with disconnected supply
Dead circuit testing is testing performed with the power disconnected from the circuit.
The main benefit of disconnecting power supply while tests with an external energy source are performed is to eliminate hazardous risks to the environment or the person conducting the test. A continuity test, as well as, an insulation test can be performed in the dead circuit test.
(a) Continuity test
This is to be performed on a dead circuit for checking continuity.
Using an 'Audible Continuity Tester' can do it. This tester consists of a battery as a source of energy, an audible device, and two test leads. FIG. 3 shows an example of this test with an audible continuity tester.
By this test, the continuity of an electrical circuit is checked to ensure that the electrical path is complete. If the path is continuous, then an audio sound is emitted to confirm path continuity and the non-existence of an open circuit.
In some devices, along with the audio indication, an LED or some other visual indication is provided.
Similarly, an ohmmeter or multimeter can also be used to check continuity. An ohmmeter or multimeter consists of a battery as a source of energy, along with a meter to display the value of resistance. FIG. 4 shows an example of this test with an ohmmeter.
In an ohmmeter, the scale is calibrated from zero to an infinite range of resistance.
When the meter shows a zero reading, it indicates that the path between two test leads has zero resistance. This, in turn, indicates that the path is a continuous one. If the path or the conductor is open, then it will show resistance value as infinite.
FIG. 3 Continuity test with audio tester
FIG. 4 Continuity test with ohmmeter
In short, continuity testing is used to check the following purposes:
• Integrity of cables 9 Integrity of electrical circuit path 9 Integrity of the earthing system (i.e., electrical continuity and low- resistance value to earth)
- Accurate wiring of a control and power circuit to the correct terminals
- Differentiate active and neutral conductors before connecting them to a device
- Check for wrong wiring interconnections between different control and power circuits; thus indirectly, checking for short-circuit paths
- Integrity of switches, fuses, and other devices.
A few words of caution are warranted here. Checking for continuity in a control circuit can give erratic results due to the existence of parallel circuits. It is better to disconnect appropriate terminals to ensure correct results. Continuity test in power circuits can be tricky. Often, a circuit where there is an open circuit fault can register excellent continuity with a low power tester or ohmmeter. But when a voltage is applied, current may not flow. The reason for this is that the circuit may be partially continuous (Example: a partially burnt cable where one or two conductor strands may be making contact) but when feeding a heavy load it will behave as a high impedance. This type of fault will be detected by testing on load using voltage measurements (as illustrated later in this section).
(b) Insulation test
This is another test performed on a dead circuit only. The objective is to check for insulation of cables or a power circuit. The device used to check integrity of insulation is known as an 'Insulation-Resistance Tester'. Generally, this is used during the installation of high-voltage power cables and terminations.
In FIG. 5, a general motor circuit is shown with breaker, fuses, and overload relay.
To check insulation of the circuit (excluding motor), disconnect the power supply by opening the breaker.
FIG. 5 Insulation test with insulation-resistance tester.
Then, isolate the motor from the circuit through terminals T1, T2, and T3. First checking insulation resistance between earth and T1, then earth and T2, and finally earth and T3 checks insulation resistances of conductors, as well as other devices.
If the insulation resistance of any branch shows zero or a very low reading, then it can be concluded that there is an insulation failure. This test is also used in fault finding, to check for earthed motors or cables and for checking insulation failure of conductors. Individual phases of three-phase motor winding can be insulation-tested only if all six leads of the winding are brought out. The winding being tested should be connected to the tester's output with the other two windings connected together and to the earthed frame of the motor. Where only three leads are available, the insulation of the machine winding as a whole can only be tested with reference to the earthed frame of the motor.
These insulation testers are also called Meggers and have a built-in energy source (either DC generator or battery) to produce test voltages of rating 500 V DC or more.
This is required since the electrical circuit to be tested applies voltage of different ratings.
For example, when the insulation resistance of HV cables is checked, 1000 V minimal voltage is applied, whereas for a domestic circuit 500 V is sufficient for testing.
Testing on a live circuit requires extreme caution and should be restricted to LV circuits.
Precautions should be taken to prevent inadvertent contact of the technician with live parts. The probes and tools must be insulated with minimum exposure of conducting parts.
This will minimize inadvertent bridging of two terminals which are at different potentials which can cause a short-circuit and arcing leading to burn injuries to the technician.
4. Checks for circuit continuity with live supply
Generally, if possible, troubleshooting is done with a disconnected power supply, but in some circumstances, faultfinding is only possible if the circuit is live.
Therefore, the circuit under testing remains connected with the power supply. This uses the circuit power supply itself as a source of energy for testing.
This kind of testing should be done with extreme care following safety precautions.
As shown in FIG. 6, the integrity of a power supply or continuity of electric path can be checked by using test lamps. Test lamps are connected in between two phases. Thus, as with the dead circuit test, a continuity test can be performed. In addition, a lamp-type visual tester can be used for simple continuity testing. Alternatively, voltage indicators or multimeters can be used for checking voltage and the continuity of the conductors or electrical path.
While checking three-phase voltage, use two lamps connected in series and not a single lamp. Currently, most manufacturers give test voltage details for test points that helps to check the integrity of a particular section. Generally, equipments consisting of electronic cards follow this kind of practice.
While checking the voltage at these test points, measuring instruments must be accurate. Therefore, a comparison of voltages at these test points is sufficient to draw conclusions.
Diagnostics for a single-phase motor can be undertaken with this kind of visual indicators. This requires a sound knowledge of circuit and wiring arrangements - depending upon the test done, interpretation varies and so does an accurate fault diagnosis.
FIG. 6 Continuity test with series test lamps.
It is always advisable to check voltage between line-to-line than line-to-earth, since for the latter, the results may be misleading.
Testing on a live circuit requires extreme caution and should be restricted to LV circuits.
Precautions should be taken to prevent inadvertent contact of the technician with live parts.
The probes and tools must be insulated with minimum exposure of conducting parts.
This will minimize inadvertent bridging of two terminals which are at different potentials which can cause a short-circuit and arcing leading to burn injuries to the technician.
5. Tests and methods
Although most of the tests have been detailed above, there still remain certain tests that are common and critical to an electrical system. Parameters such as the resistance of leakage paths and resistance of a conductor have to be known for some applications.
The tests include the following:
-- Effectiveness of the power earth
-- Effectiveness of the electronic earth
-- Continuity of the earthing system and the required equipotentiality (so as to check the possibility of earth current loops)
-- Earth pit locations, resistance of earth conductor, material, and size
-- Location of protection devices, so that the path taken by fault current is minimum and insures the activation of a protection device under fault conditions
-- Ratings of fuse and other protection devices
-- Selection of suitable cable types with proper current ratings keeping in mind environmental conditions and length of run
-- Materials prone to environmental hazards which may be mechanical or chemical, which might be present, such as dampness, high temperature, explosive gases, vapors
-- Electrical equipments, to ensure that its operation will not cause overload conditions
-- Location of installation of electrical equipment and accessories.
If all the above points are considered during the installation of an electrical system, there will be a reduced need for troubleshooting. The quality of the system will also be good.