Industrial Electronics Troubleshooting--Basic principles [part 2]

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6. Testing devices

To successfully troubleshoot in a short time, an understanding of the measurement meters that can be used and their various functions is mandatory. This is detailed in the following few sections.

6.1 Lamp indicators

A lamp indicator is the most basic tool used for troubleshooting by a practicing electrician. It is also known as a 'Voltage Tester'. It consists of two 240 V lamps connected in series.

Description As shown in FIG. 7, both lamps are connected in series along with the fuse and probe that form a testing set. The low-wattage lamps comprise of equal power rating, not greater than 25 W per lamp. Use of two lamps is advisable as the tester may at times be subjected to line voltage (380/400/480) during testing. Single lamp if used may fail and erroneously indicate that the circuit is not live. As a precaution, all voltage testers should be checked before and after the test using a known live source.


FIG. 7 The two 240 V lamps housed in the housing

For testing extra low voltages such as 12 or 32 V systems, one may use a single-lamp type.

Applications

Lamp indicator applications are listed below:

• For detecting the presence of a live potential

• For polarity of supply, i.e., the location of active points, neutral, and earth terminals or supply points

• For checking like or similar phases when 'phasing out' preparatory to paralleling two supplies

• Blown fuses

• Integrity of motor and three-phase supply system.

Testing of a motor

To check the earth condition, one lead of test lamps is connected to a live terminal of a single-phase supply and the second lead to a winding terminal. If the winding is earthed, the lamps will glow, else they will not glow.

Checking a three-phase supply voltage

To ensure there is no missing phase, connect both leads across the two phases. One of the following three instances will occur:

• If there is no supply then lamps will not glow

• If any one phase is missing then the lamps will glow at half brightness

• If both phases are connected then the lamps will glow at full brightness.

Present day voltage testers (duly approved types), when correctly handled, are safe live testing devices due to their impedance and rating that a user cannot even mistakenly cause a short-circuit.

One such voltage tester available in the market is known as the 'high-impedance tester', as shown in FIG. 8. This device gives an audible visual indication on detection of the presence of voltage. A common handy device that is used by technicians for detection of voltage is the 'Neon Tester'. This consists of a neon indicator with current-limiting resistor in series. When you put its conductive front portion over an active conductor, it will give an indication.

However, in most cases, this indication is faint and this confuses the user. In addition, it cannot be assumed that a lack of indication is the result of a lack of supply. In that case, a neon bulb may be inoperative.

A device used for the detection of electrical potential or for a polarity test is the 'neon test pencil'. It is manufactured in a variety of types and designs. The intensity of glow will increase if a finger is placed on the cap or if the cap is earthed.

Therefore, it is advisable to use a good-quality neon tester from a safety and reliability point of view. It should be noted that the neon tester indicates the voltage of a conductor with reference to earth (because it has only one test lead and the circuit gets completed through the body of the person using it and earth). This test is sometimes not conclusive because an open circuit may exist in the neutral and remain undetected by this test. Also, neon testers sometimes give a glow at very low voltages and results are thus erratic. A proper voltage measurement using a voltmeter or multimeter is always more reliable and conclusive.


FIG. 8 High-impedance tester

A comparison between analog and digital instruments is as follows:

• An analog meter shows reading by the movement of a pointer on a calibrated scale, whereas a digital meter shows a digital readout for the measurement directly.

• It is easier for a user to differentiate between readings in analog meters rather than in the digital one.

• In an analog meter, the reading is subjected to parallax error, while in a digital one there is no such possibility.

• For testing continuity, analog meters are a better choice. For example, for an open-circuit full-scale deflection of the needle of an ohmmeter will show infinite reading. While for a closed circuit, it will show zero-resistance reading, thereby misleading the user.

6.2 Voltmeters and ammeters

For the measurement of voltage (potential difference) between two points, a device known as a voltmeter is used. This is a device used in live testing of a circuit.

Voltage measurement is conducted by connecting the voltmeter across the test points in a circuit.

A voltmeter can be used to measure AC/DC voltages of different ranges. Therefore, AC voltages should be measured by selecting AC and vice versa.

A voltmeter is always connected in parallel or shunt with respect to test points.

While operating a voltmeter, ensure that proper range of voltage is selected before conducting the measurement, because an instrument is designed for a particular range.

Failure to maintain the above-mentioned precautions results in safety hazards to both the user and the instrument.

When high-voltage measurements are required, then the measuring range of a voltmeter can be extended further by the addition of a voltage transformer (step down) with the meter.

Accordingly, the measurement scale requires a multiplication factor.

Currently, digital multimeters (DMM) with voltage measurement have auto range facility. This enables the instrument to automatically get the correct range in spite of the user's incorrect selection of range.

A voltmeter is used for the following purposes:

• Test continuity of power in a electrical circuit

• Check integrity of single-three-phase power

• Check integrity of devices such as relays and timers

• Check integrity of earthing.

An ammeter is the other device used to measure current flowing through closed low voltage electrical circuits.

In addition, it is used in the live testing of an electrical circuit. Connecting the ammeter in series with the close electric circuit always does measurement of current. AC/DC currents of different ( LV) ranges can be measured using an ammeter.

When connected in series with load (motor, fan), an ammeter will indicate the current consumed by the load. The current shown depends upon the exact connection of the ammeter. In order to extend the range of measurement done by an ammeter, a CT is connected along with the meter. Accordingly, a multiplying factor will come into the picture.

FIG. 9 shows the connection of an ammeter and voltmeter used for troubleshooting a motor starting circuit. As explained earlier, an ammeter is connected in series with the path, while a voltmeter is connected across the test points.


FIG. 9 Simple circuit showing metering of current and voltage

For very-high voltage circuits, it is unfeasible to disturb the circuit or take physical connection risks with the meters.

To avoid physical connection of an ammeter with the circuit, another handy device known as the clip-on meter is available in the market (FIG. 10).


FIG. 10 Digital-type clip-on meter

A clip-on meter, as the name suggests, is a handheld device that requires to be clamped with an active current carrying conductor in a circuit (FIG. 11). The basic principle remains the same - a CT transfers high-rating current to a low-rating meter, which shows the reading on a calibrated scale.


FIG. 11 Using a clip-on meter.

This device works for AC/DC ranges with an option of various current ranges. It also has a hold facility, which helps in value storage after a reading is taken. Moreover, this device has the facility to be used as a voltmeter. It can be turned into a voltmeter by using the extra probes provided for testing. Therefore, it is a versatile measuring device. It can be used for rough and ready measurement of current flow in locations where individual phase leads are accessible. However, it cannot measure current in multi-core power cables where all three-phase conductors run bundled into a single cable. When using a clip-on ammeter care should be taken to see that the CT clamp is fully closed without any air gap because the readings can be quire erroneous in the event of improper closing of the clamp.

6.3 Multimeters and ohmmeters

To perform various tests to check AC/DC voltage, current, resistance, frequency, continuity of circuit, or device integrity, the multimeter is a very useful device (FIG. 12).


FIG. 12 Analog multimeter

Various companies have different models with different functions. A multimeter consists of an ammeter, voltmeter, and an ohmmeter combined, with a function switch to connect the appropriate function.

The ohmmeter is essentially a current-measuring device. However, the scale is calibrated in ohms, enabling resistance values to be read directly.

This combination of volt-ohm-milliammeter is a basic tool for troubleshooting. The proper use of this instrument increases its accuracy and life. The following precautions should be observed during its usage:

1. To prevent meter overloading and possible damage when checking voltage or current, start with the highest range of the instrument and move down the range successively.

2. For higher accuracy, the range selected should be such that the deflection falls in the upper half of the meter scale.

3. Verify the circuit polarity before making a test, particularly when measuring DC current or voltages.

4. When checking resistance in circuits, power supply to the circuit has to be switched off; otherwise, the voltage across the meter may damage the meter.

5. Renew multimeter batteries frequently to ensure accuracy of the resistance scale.

6. Recalibrate the instrument at frequent intervals.

7. Protect the instrument from dust, moisture, fumes, and heat.

Digital multimeter

The features of a multimeter (FIG. 13) are listed as follows:

• Functionally easy

• Directly indicates the numeric value of measurement on an LCD display

• Measurement-variable -- voltage, current, resistance can be selected using the function button

• Auto range feature provides automatic adjustment of internal circuits to appropriate current, voltage, or resistance

• Hold feature allows storage of reading of quantity measurement in memory for future viewing

• Auto polarity feature automatically displays + or - sign on display to indicate polarity of DC measurements

• Some meters also provide Min/Max value indication for measurement

• Peak hold feature holds the peak value of measured quantity

• Quick check features such as diode test, transistor test, capacitor test, etc. are also available.


FIG. 13 Digital multimeter.

Operating DMM

To operate the DMM, please perform the following steps:

• Before connecting the test probe that leads to the circuit, ensure proper function has been selected as per measured quantity.

• Check the correct insertion of test probes in proper plugs - this will avoid possible damage to the multimeter due to an incorrect function selection or an incorrect probe insertion.

• If the multimeter does not have an auto range feature then check for the range selector switch position - now, the variable can be measured.

If the measuring position/location is awkward, data can be stored using the hold function for later viewing. The data can be viewed even after the probes are disconnected from the circuit.

6.4 CRO (cathode ray oscilloscope)

The CRO measuring instrument may sound very familiar, as it is a very useful device. It is used for measurement of voltages (AC/DC) and display of waveforms by providing information on time duration, frequency, and their shapes.

In the following section, we will discuss the features and mode of operation of the CRO.

Features of CRO

Below are listed the various features of the CRO:

• It allows voltage (AC/DC) amplitude measurement and time period measurement from the waveform displayed on screen.

• Dual trace CRO allows the user to see two traces at a time on two different channels for comparison.

• Two sets of controls provide the facility to show time period differences, amplitude differences, and shape/distortion comparison.

• Storage oscilloscopes allow storage of waveforms for later analysis.

• Storage facility is very useful since it provides a cursor function, which shows the value of a measured variable at a particular instance.

Operating CRO

To operate the CRO, perform the following procedure:

• The power on switch is provided for on/off control.

• The measurement probe provided consists of two leads - one connected to the signal and the other, ground probe, connected to the ground of circuit.

• Turn on the CRO.

• Check the integrity of leads and the CRO by connecting the I/P probe to a test socket of 5V square wave signal.

• While checking non-isolated signals (that are earthed) do not connect ground/earth to the CRO, else it may create a short-circuit at the input signal.

• Adjust both channels' vertical axis by placing AC\DC\GND signal in GND position.

• Place the function switch in the suitable signal function as required (AC\DC).

• Check the test probe selection, i.e., divide by 1 or 10 that allows signal attenuation.

• The intensity knob is used to vary the brightness of the trace.

• The focus knob is used to change the sharpness of the trace displayed.

• The Y shift allows you to shift the waveform displayed vertically (up/down).

• The X shift allows you to shift the waveform displayed horizontally left or right.

• The Volts/Div switch is used to vary the magnitude of the voltage variable displayed on the screen. It is calibrated in Volts/Div of the vertical scale. A control knob is provided in the center to adjust amplitude between calibrated settings.

• To find the amplitude of a signal multiply the Y-axis reading with the Volts/Div setting.

• The Time/Div is used to control the span of the X-axis.

• Physical markings between two points can be used to calculate the time span.

The same time span can be used to measure the frequency of the waveform displayed.

• A control knob is provided at the center for the same purpose as in Volts/Div.

• To find the time duration of a waveform, measure the signal span reading difference. When this is multiplied by Times/Div it will give the time duration of the signal.

6.5 Safety standards for measuring instruments

While a person is conducting tests with an instrument on live line supply, there is a possibility of a sharp rise in voltage for a short duration. This may result in an arc or flash between measurement terminals of the testing device. In addition, if a heavy flash occurs, it may critically injure a person handling the instrument. To safeguard the person using the measuring instrument and to classify the various instruments as per the application they are used in, the IEC has classified instruments in the following categories:

• Category IV: Distribution systems, service connections, and primary over current protection for larger installations

• Category III: Three-phase and single-phase distribution within a premises

• Category II: Appliances, lighting points, socket-outlets

• Category I: Transient-protected electronic equipment.

The IEC Standard 61010 provides guidelines for manufacturers to follow safety norms for testing devices. It is to be noted that irrespective of their maximum voltage rating, a Category IV device provides a greater degree of transient protection than Category III, etc. The Category III device is suitable for most of the testing undertaken by electricians.

6.6 Insulation-resistance testers or meggers

Another common method of measuring resistances ranging 0-1000 M-ohm is by using meggers or insulation-resistance testers. This is the usual ohmmeter with a battery used for voltage source.

This instrument is used to measure very high resistances, such as those found in cable insulations, between motor windings, in transformer windings, etc.

Normal multimeters do not provide accurate indications above 10 M-ohm because of the low voltage used in the ohmmeter circuit. Meggers can apply a high voltage to a circuit under test and this voltage causes a current if any electrical leakage exists. This makes it useful as an insulation tester.

Some laboratory test meters have a built-in high-voltage source. The high voltage permits accurate high-resistance measurement, but such meters are usually not portable.

The megger is essentially a portable ohmmeter with a built-in high-voltage source.

The built-in high-voltage source may be derived from a magnet-type DC generator or battery.

In a DC generator-type megger, a hand crank is used to turn the armature to produce voltages up to 500, 1000, and 2500 (depending on the model used). An electronic battery-operated type of instrument is popular because it is light, compact, and can be held and operated in one hand, i.e., there is no generator to turn.

High-testing voltage is produced by an electronic circuit, which uses an internal battery as an energy source. The instrument in FIG. 13 has a range of 0-100 M-ohm and infinity at a testing voltage of 500 V. It has the same voltage-measuring facility as the hand-driven insulation tester.

Resistance is directly displayed on the front panel digital display. A range of voltages can be selected while testing.

6.7 Testing accessories

Depending upon specific applications, special testing devices are used specifically to improve the accuracy and efficiency of testing an installation. Some of these devices are shown in the figures given here. FIG. 14 shows a hand operated insulation tester (commonly called as Meg. Ohm Meter or 'Megger').


FIG. 14 Hand-driven insulation and continuity tester

FIG. 15 shows an electronic version of such a device, a multi-voltage digital insulation tester with the test voltage being generated from a battery source. FIG. 16 shows an analog version of a similar instrument.


FIG. 15 Multi-voltage digital insulation-resistance tester


FIG. 16 Battery-operated insulation-resistance tester

FIG. 17 shows various accessories forming part of the testing instruments. These instruments help electrical installation contractors to test their installation in order to ensure that the installation is safe to connect to the supply.


FIG. 17 Installation tester

FIG. 18 shows an instrument designed for the dead test of final circuits of an electrical installation.


FIG. 18 Installation tester.

These devices are intended to help improve the safety and efficiency with which tests are carried out. The testing principles remain the same and these must be understood before the benefits of using such devices can be fully realized.

7. Circuits

The previous section has listed the symbols representing electrical and electronic components. Symbols make a drawing or circuit more readable.

Generally, a schematic consists of a control and a power circuit. The power circuit provides power to the motors through contactors, whereas a control circuit controls these contactors through safety interlocks.

7.1 Reading a circuit

• All control devices such as switches and relay contacts are either NO or NC contacts.

• The switch position normally shown in any circuit diagram is the default de energized condition state.

• For denoting sensor contacts notations such as LS, PS, TS are used.

• A relay coil is denoted with a symbol inside a circle and contacts of relay used in the circuit are represented with the same tag as the coil. If a relay coil has multiple NO and NC contacts then the contact identification numbers, as shown on relays, are mentioned in the drawing.

• In between the control supply lines L1 and L2 you will find either the relay coil or a solenoid coil or the lamp load.

• If several devices are to be turned 'On' at the same condition then you will find them connected in parallel between L1 and L2.

• If wires are common for two devices then in the diagram they are shown with the same identification number.

• Generally, power circuit conductors are shown with thick lines while thin lines are used for control circuits.

• A broken line indicates a mechanical function. Generally, it is used to show linkage between two different contacts of the same push button.

7.2 Different wiring diagrams

In order to troubleshoot electrical equipments two things are required. One is the location of the equipment to be tested and the other is the interconnection between all the devices (contactors, timers, relay). The wiring diagram of electrical equipment gives information as stated above. In addition, it shows the identification tags of wires, connectors, relays, etc.

In FIG. 19, the wiring diagram of a DOL starter is shown along with the physical location of the devices. The terminal numbers of overload relays are also shown in the wiring diagram. This enables accurate device wiring and wire tracing during troubleshooting. Generally, this kind of wiring diagram is given inside an electrical equipment panel cover.

This diagram shows the actual position of different devices as closely as possible. The bold line indicates the heavy current carrying conductors, while the thin line indicates the control circuit.


FIG. 19 Simple motor circuit

8. Accurate wiring of circuits and connections

While troubleshooting electrical equipments, continuity tests of circuits and wiring are done by performing the following procedure:

• Checking correct polarity and ensuring that supply polarity follows the correct circuit route.

• Ensuring that there are no short-circuits in supply due to a wrong connection or termination of wires.

• Identifying different conductors before making connections to a device to ensure correctness of circuits and connections.

• Ensuring that there are no interconnections between two different circuits.

• Correctly identifying circuit loads such as contactors, relays, and their contacts.

• Identifying active conductors and their corresponding neutral conductors to check the integrity of the circuit. A continuity test is particularly useful to help detect a short-circuit condition, which is a result of cross-linking of wires between two different circuits. An interconnection between circuits is likely to be due to the following reasons:

• Incorrect termination of wires

• Result of insulation breakdown

• Incorrect connection at field junction box.

FIG. 20 is an example of an electrical appliance connected with a supply system. If, due to any reason, a fault occurs within the appliance causing current flow in its body, the fault current flows back to the mains supply. The circuit shown here is a TN-C-S type of supply where the earth is derived from the supply neutral at the service entrance. In the case of other supply systems the earth lead may not be interconnected at service entrance but go right back to the source (TN-S). However the general principles are still valid.


FIG. 20 Earth fault within an installation

It is required to conduct tests between neutral conductors of all other circuits and the active conductor of the same circuit at the mains supply distribution to reveal any interconnection faults. Before conduction of tests, perform the following steps:

• Disconnect neutral link from circuit

• Keep circuit protective

• Close all contactors or switches.

Check all direct interconnections with the low-range ohmmeter. If resistance shown in the ohmmeter is very low then it indicates a short-circuit condition. Suppose, the load is connected with an active phase and is neutral from different circuits, then it can be detected only with connected loads. If these steps are performed prior to the start of the test, then check the resistance between the neutral and the active conductors.

To check for insulation resistance of cables, take insulation resistance with megger or insulation-resistance tester, especially if insulation breakdown is suspected. If the resistance shown is less than 1 M-ohm then it can be said that the wiring or device terminal has an insulation problem.

To identify each electrical circuit and its active and neutral conductors, calculate load resistance with the ohmmeter and accordingly, identify each active and neutral conductor.

Conducting an insulation-resistance test: To conduct an insulation-resistance test, perform the procedure listed below:

1. Check the insulation tester by shorting its test leads. It should show zero resistance. If test leads are kept open, it should show infinite resistance.

2. Isolate the section to be tested from the power supply.

3. Disconnect all lamps or electronic devices from the circuit to be tested.

4. Select the proper operating voltage for conducting the test, depending upon the rating of the system.

5. Check for connections while conducting tests so that only the section to be tested is included in the test.

6. There should not be any stray parallel leakage paths.

7. Check the instrument for pointer index or any other pre-adjustment necessary.

8. Test leads to be used should have good-quality insulation.

9. Before starting the test, insure that all the capacitors in the circuit are discharged by shorting their two leads together. Similarly, after the test ensure that they are in discharge condition. If this is not done they may give false readings.

10. Before touching cable ends after testing, discharge any energy that might have been stored in the cables during the test. This is most likely to occur in long runs of larger cables due to their capacitance.

11. Checking continuity of an earthing system requires the use of low-reading ohmmeters, which should be zero-adjusted before each test and calibrated on regular intervals.

12. Where the testing of the earth electrode resistance is required (i.e., the resistance between the electrode and the general mass of earth), one of the special types of earth-resistance testers must be used.

8.1 Optional tests

There still remain a few useful tests using the measuring devices. These tests are used for checking single- or three-phase systems and other electrical devices. Some of the tests we have discussed in the next few sections. These can be used to strengthen troubleshooting techniques.

(a) Megger testing cables and auxiliary devices of a single-phase system Disconnect P and N from the supply side, as well as from the other end.

Now we have isolated our test circuit, making it dead. Short P and N with a temporary short link. Close switch and protection devices.

As shown in FIG. 21, open motor terminals, so that the motor remains isolated from the test circuit. Check resistance with the insulation tester between the neutral link and earth. If the value shown in the meter is less than 1 M-ohm, then there is a fault with either the cable insulation or device terminals.

(b) Megger testing cables and auxiliaries of a three-phase system Disconnect L1, L2, and L3 from the supply side, as well as from the other end. This makes it a dead circuit. Short L1, L2, and L3 terminals with a temporary link. Close the breaker device and protection devices. As shown in FIG. 22, open motor terminals T1, T2, and T3, so that the motor remains isolated from the test circuit.

Check resistance with insulation tester between each conductor and earth.

If the meter shows a low value less than 1 M-ohm, there is a fault in either cable insulation or device terminals.

(c) Megger testing of motor

A pre-condition for megger testing of a motor is to isolate the motor from the supply totally. Take the megger value of a motor between each conductor and earth, as shown in FIG. 23, to check the earthing of the stator winding. This will help us to conclude on earthing status of the stator winding. Similarly, check for shorting between two windings by checking the megger value between two stator-winding terminals, as shown in FIG. 24.

Thus, a low reading can identify insulation failure of any winding inside the motor.


FIG. 21 Megger of a single-phase system


FIG. 22 Megger of a three-phase system


FIG. 23 Megger testing for an earthed winding condition


FIG. 24 Megger testing for winding-to-winding short condition

Fault finding on an underground cable

Generally, underground cables are prone to insulation failure, although due care is taken.

Since they are buried underground, it is difficult to exactly pinpoint the fault location. Resistance values between earth and a conductor from the two ends can be checked. If the value reduces drastically, then the faulty location can be isolated.

9. Tests for installation and troubleshooting

The following are a few tests used during commissioning and troubleshooting:

1. Insulation test: This is the most important test for troubleshooting of any electrical equipment. Depending on the system, a suitable test voltage is applied to check the insulation resistance between the live conductors and earth.

2. Earth continuity test: For electrical equipment, continuity between the exposed portion (metallic) of earthed equipment and the earth terminal is checked. Resistance value should be low. If resistance value is high, then it is indicative of poor earthing.

3. Flash test: To check the insulation strength of cables, a high voltage (as specified by the cable manufacture) is applied in the same way as for the insulation test. This determines the withstand capability of the cable insulation.

4. Electronic earth test: Generally, for microprocessor-based or electronic based sensitive devices, separate earthing is provided. This is called electronic earth. The voltage between the electronic earth and the power earth should be lower than 2V.

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