Electrical General Test Equipment.pdf

Electrical Measuring Instruments Introduction to Electrical Measuring Instruments Measurement of electrical quantities is essential to the maintenance of any modern device. As a technician, you must be able to measure each of the four electrical variables: current, voltage, resistance...

Electrical Measuring Instruments Introduction to Electrical Measuring Instruments Measurement of electrical quantities is essential to the maintenance of any modern device. As a technician, you must be able to measure each of the four electrical variables: current, voltage, resistance and power. A number of principles are used for these measurements, but by far the most common is electromagnetism. It is based on two fundamental assumptions: The strength of an electromagnetic field is proportional to the amount of current that flows in the coil. Voltage, resistance and power all relate to a flow of current, and if the amount of current is known, the other values may be found. The most common electrical measuring instruments are the multimeter, megger and ohmmeter. Digital meters are certainly the most common meters used today but occasionally analogue meters are required. There are two advantages to using an analogue meter: 1. It is easier to identify trends and rates of change within a circuit, by looking at a sweeping needle instead of a number on a screen changing value. Examples of this include a voltage rising and falling in a low frequency AC circuit, or a capacitor charging. 2. Analogue meters do not read ‘ghost’ voltages – interference induced into a wire by neighbouring circuits. This is because analogue meters connect the circuit being tested to a load. As ghost voltages are voltage only (with zero/ negligible current) they are effectively cancelled out. 2023-02-06 B-07c Maintenance Practices Page 11 of 255 CASA Part 66 - Training Materials Only  Analogue and digital multimeters 2023-02-06 B-07c Maintenance Practices Page 12 of 255 CASA Part 66 - Training Materials Only  The d’Arsonval Meter Movement Analogue meters often operate on the d’Arsonval meter movement principle. This basic DC type of meter movement – first employed by the French scientist d’Arsonval in making electrical measurement – is a current-measuring device used in the ammeter, voltmeter and ohmmeter. The pointer is deflected in proportion to the amount of current through the coil. A reference magnetic field is created by a horseshoe-shaped permanent magnet, and its field is concentrated by a cylindrical keeper in the centre of the open end. The current being measured flows through the coil and creates a magnetic field whose polarity is the same as that of the permanent magnet. The two fields thus oppose each other and cause the coil to rotate on its low-friction bearings until the force of a calibrated hairspring exactly balances the force caused by the magnetic fields. Usually there is a slotted screw on the front of the case for ‘zero adjustment’ of the pointer. © Aviation Australia d’Arsonval meter movement 2023-02-06 B-07c Maintenance Practices Page 13 of 255 CASA Part 66 - Training Materials Only  Permanent-Magnet Moving-Coil Meter The permanent-magnet moving-coil (PMMC) meter is designed based on the d’Arsonval meter movement. A coil of wire is wound on an aluminium frame, or bobbin, and the bobbin is supported by jewelled bearings which allow it to move freely. Aviation Australia Permanent-magnet moving-coil meter (PMMC) To use this permanent-magnet moving-coil device as a meter, two problems must be solved. First, a way must be found to return the coil to its original position when there is no current through the coil. Second, a method is needed to indicate the amount of coil movement. The first problem is solved by the use of hairsprings attached to each end of the coil as shown in the illustration. These hairsprings can also be used to make the electrical connections to the coil. With the use of hairsprings, the coil returns to its initial position when there is no current. The springs also tend to resist the movement of the coil when there is current through the coil. As the current through the coil increases, the magnetic field generated around the coil increases. The stronger the magnetic field around the coil, the farther the coil moves. This is a good basis for a meter. But how will you know how far the coil moves? If a pointer is attached to the coil and extended out to a scale, the pointer moves as the coil moves, and the scale can be marked to indicate the amount of current through the coil. Two other features are used to increase the accuracy and efficiency of this meter movement. First, an iron core is placed inside the coil to concentrate the magnetic fields. Second, curved pole pieces are attached to the permanent magnet to ensure that the turning force on the coil increases steadily as the current increases. 2023-02-06 B-07c Maintenance Practices Page 14 of 255 CASA Part 66 - Training Materials Only  Assembled meter movement 2023-02-06 B-07c Maintenance Practices Page 15 of 255 CASA Part 66 - Training Materials Only  Ammeters The DC Ammeter An ammeter is a device that measures current. Electric currents are measured in amperes (A), hence the name. Smaller values of current can be measured using a milliammeter or a microammeter. A milliameter Aviation Australia Ammeter connection 2023-02-06 B-07c Maintenance Practices Page 16 of 255 CASA Part 66 - Training Materials Only  DC ammeters are sensitive to current direction. They must always be connected with the correct polarity. The terminal marked + (usually red) must be connected towards the positive side of the circuit, and the terminal marked - (usually black or blue) is connected to the negative side. Remember that all current must pass through the load and the meter. It is always connected in series with the circuit path you wish to test. Connecting an ammeter in parallel would not only give you an incorrect measurement, it would also damage the ammeter because too much current would pass through it. Ammeter Shunts If the range of current to be measured is greater than the full-scale current of a meter, a shunt must be installed in parallel with the meter. A shunt is a type of resistor that is connected in parallel with a meter that increases the amount of current it can measure. The load current flowing through a shunt produces a voltage drop that is proportional to the current. Shunts are designed to carry a fixed high proportion of the current to be measured, say 99% or 99.9% compared to 1% or 0.1% through the meter coil. Aviation Australia Ammeter shunts 2023-02-06 B-07c Maintenance Practices Page 17 of 255 CASA Part 66 - Training Materials Only  Ammeter Shunt Resistance A standard d’Arsonval meter movement may have a current sensitivity of 1 mA and a resistance of 50 Ω. If the meter is going to be used to measure more than 1 mA, then a simple shunt resistor is required to accomplish the task. The purpose of the shunt resistor is to bypass current that exceeds the 1-mA limitation of the meter movement. To illustrate this, assume the 1-mA meter in question is needed to measure 10 mA. The shunt resistor used should carry 9 mA while the remaining 1 mA is allowed to pass through the meter. Because the shunt resistance and the 50-Ω meter resistance are in parallel, the voltage drop across both of them is the same: Esh = Em. © Aviation Australia Ammeter shunt resistance Using Ohm’s law, this relationship can be rewritten as: E SH = I SH × R SH EM = IM × RM I SH × R SH = I M × RM IM × RM 1mA × 50 Ω T hus, R SH = = = 5.56 Ω I SH 9 mA Shunts may be located in the ammeter case or externally. Remotely located external shunts are used in high-current cables, such as generator feeder lines. 2023-02-06 B-07c Maintenance Practices Page 18 of 255 CASA Part 66 - Training Materials Only  Multi-Range Ammeter With the addition of several shunt resistors in the meter case and a switch to select the desired resistor, the ammeter can measure several different maximum current readings or ranges. © Aviation Australia Multi-range ammeter circuit This ammeter has five ranges (100 μA; 1, 10 and 100 mA; 1 A) selected by a switch. With the switch in the 100-μA position, all the current being measured goes through the meter movement. None of the current goes through any of the shunt resistors. If the ammeter is switched to the 1-mA position, the current being measured will have parallel paths with the meter movement and all the shunt resistors (R1, R2, R3 and R4). Now, only a portion of the current will go through the meter movement and the rest of the current will go through the shunt resistors. When the meter is switched to the 10-mA position, only resistors R1, R2 and R3 shunt the meter. Since the shunting resistance is less than with R4 in the circuit (as was the case in the 1-mA position), more current will go through the shunt resistors and less current will go through the meter movement. As long as the current to be measured does not exceed the range selected, the meter movement will never have more than 100 μA of current through it. Shunt resistors are made with close tolerances, typically less than 1%. Since a shunt resistor is used to protect a meter movement and to allow accurate measurement, it is important that the resistance of the shunt resistor is known very accurately. 2023-02-06 B-07c Maintenance Practices Page 19 of 255 CASA Part 66 - Training Materials Only  Ammeter Range Selection Part of the correct use of an ammeter is the proper use of the range selection switch. If the current to be measured is larger than the scale of the meter selected, the meter movement will have excessive current and will be damaged. Therefore, it is important to always start with the highest range when you use an ammeter. If the current can be measured on several ranges, use the range that results in a reading near the middle of the scale. The three images below illustrate these points. © Aviation Australia Multi-range analogue ammeter range selection (A) shows the initial reading of a circuit. The highest range (250 mA) has been selected and the meter indication is very small. It would be difficult to properly interpret this reading with any degree of accuracy. (B) shows the second reading, with the next largest range (50 mA). The meter deflection is a little greater. It is possible to interpret this reading as 5 mA. Since this approximation of the current is less than the next range, the meter is switched as shown in (C). The range of the meter is now 10 mA and it is possible to read the meter indication of 5 mA with the greatest degree of accuracy. Since the current indicated is equal to (or greater than) the next range of the ammeter (5 mA), the meter should not be switched to the next range. 2023-02-06 B-07c Maintenance Practices Page 20 of 255 CASA Part 66 - Training Materials Only  Voltmeters The Voltmeter A voltmeter is a high-resistance instrument used to measure the voltage between two points in an electric circuit. It works with the same type of meter movement as the ammeter, but employs a different circuit external to the meter movement. Analogue voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analogue-to-digital converter. Analogue and digital voltmeters D'Arsonval meter movements are sensitive devices, meaning they have full-scale deflection current ratings as low as 50 µA, with an (internal) wire resistance of less than 1000 Ω. This makes for a voltmeter with a full-scale rating of only 50 mV (50 µA × 1000 Ω). In order to build voltmeters with practical scales from such sensitive movements, we need to reduce the voltage to a level the movement can handle. The basic d’Arsonval meter movement can be converted to a DC voltmeter by connecting a multiplier (RS) in series with the meter movement. The purposes of the multiplier (RS) in voltmeter design are: To extend the voltage range of the meter movements To limit the current through the d’Arsonval meter movement to a maximum full-scale deflection current. 2023-02-06 B-07c Maintenance Practices Page 21 of 255 CASA Part 66 - Training Materials Only  Aviation Australia Multiplier in voltmeter design Multi-range Voltmeter Generally, it is useful to have multiple ranges established for an electromechanical meter such as voltmeter, allowing it to read a broad range of voltages with a single movement mechanism. This is accomplished through the use of a multi-pole switch and several multiplier resistors, each one sized for a particular voltage range. Aviation Australia Multi-range voltmeter The five-position switch contacts only one resistor at a time. In the bottom (full clockwise) position, it contacts no resistor at all, providing an ‘off’ setting. Each resistor is sized to provide a particular full- scale range for the voltmeter, all based on the particular rating of the meter movement (1 mA, 500 Ω). The end result is a voltmeter with four different full-scale ranges of measurement. Of course, in order to make this work sensibly, the meter movement's scale must be equipped with labels appropriate for each range. 2023-02-06 B-07c Maintenance Practices Page 22 of 255 CASA Part 66 - Training Materials Only  With such a meter design, each resistor value is determined by the same technique, using a known total voltage, movement full-scale deflection rating and movement resistance. For a voltmeter with ranges of 1, 10, 100 and 1000 V, the multiplier resistances would be as follows: R4 = 500 Ω R3 = 9.5 kΩ R2 = 99.5 kΩ R1 = 999.5 kΩ Voltmeter Loading Effects When a voltmeter is used to measure the voltage across a circuit component, the voltmeter circuit itself is in parallel with the circuit component. Since the parallel combination of two resistors is less than either resistor alone, the resistance seen by the source is less with the voltmeter connector than without. Therefore, the voltage across the component is lower whenever the voltmeter is connected. The decrease in voltage may be negligible or appreciable, depending on the sensitivity of the voltmeter being used. This effect is called voltmeter loading and the resulting error is called loading error. In view A, a source of 150 V is applied to a series circuit consisting of two 10-kΩ resistors. View A shows the voltage drop across each resistor is 75 V. View B shows the voltmeter connected across the circuit. In the 150-V range, the voltmeter to be used has a total internal resistance of 10 kΩ. The parallel combination of R2 and the meter now present a total resistance of 5 kΩ. Because of the addition of the voltmeter, the voltage drops change to 100 V across R1 and 50 V across R2. Notice that this is not the normal voltage drop across R2. Actual circuit conditions have been altered because of the voltmeter. A voltmeter should have a high resistance compared to the circuit being measured, to minimise the loading effect. © Aviation Australia Voltmeter loading effects 2023-02-06 B-07c Maintenance Practices Page 23 of 255 CASA Part 66 - Training Materials Only  Voltmeter Safety Precautions When using voltmeters safety precautions must be observed to prevent injury to personnel and prevent 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. De-energise 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 for electrical and electronic devices. Aviation Australia Voltmeter safety precautions 2023-02-06 B-07c Maintenance Practices Page 24 of 255 CASA Part 66 - Training Materials Only  Alternating Current Meters AC Meters A DC meter, such as a DC ammeter, connected in an AC circuit indicates 0 because the meter movements used in a d’Arsonval type movement are restricted to direct current. They, like permanent-magnet motors, are devices whose motion depends on the polarity of the applied voltage. In order to use a DC-style meter movement such as the D’Arsonval design, the alternating current must be rectified into DC. There are two basic types of rectifiers: the half-wave rectifier and the full- wave rectifier. Both of these are depicted in block diagram form below. The illustration also shows a simplified block diagram of an AC meter. In this depiction, the full-wave rectifier precedes the meter movement. The movement responds to the average value of the pulsating DC. Aviation Australia Simplified block diagram of AC meter 2023-02-06 B-07c Maintenance Practices Page 25 of 255 CASA Part 66 - Training Materials Only  d'Arsonval Meter with Rectifiers The d’Arsonval meter can be used to measure AC current and voltage by connecting a rectifier. Frequently, it is more desirable to use a full-wave rectifier in AC voltmeters because it shows a higher sensitivity rating compared to a half-wave rectifier. The most frequently used circuit for full-wave rectification is the bridge-type rectifier. Aviation Australia d'Arsonval meter with rectifiers 2023-02-06 B-07c Maintenance Practices Page 26 of 255 CASA Part 66 - Training Materials Only  Value of d’Arsonval Multimeter When AC is converted to pulsating DC by a rectifier, the d'Arsonval movement reacts to the average value of the pulsating DC (the average value of half of the sine wave). Average value is 63.7% of peak. Although the meter movement reacts to the average value of the AC, the value used when working with the AC sine wave is the effective value (RMS value). Therefore, a different scale is used on an AC meter. The scale is marked with the effective value even though it is the average value to which the meter is reacting. That is why an AC meter gives an incorrect reading if it is used to measure DC. Aviation Australia Full-wave rectifier voltmeter 2023-02-06 B-07c Maintenance Practices Page 27 of 255 CASA Part 66 - Training Materials Only  Ohmmeters The Ohmmeter Though mechanical ohmmeter (resistance meter) designs are rarely used today, having largely been superseded by digital instruments, their operation is nonetheless intriguing and worthy of study. The purpose of an ohmmeter, of course, is to measure the resistance placed between its leads. To measure resistance, the leads of the meter are connected across an external resistance to be measured. This resistance reading is indicated through a mechanical meter movement which operates on electric current. The ohmmeter must then have an internal source of voltage to create the necessary current to operate the movement, and also have appropriate ranging resistors to allow just the right amount of current through the movement at any given resistance. Ohmmeter 2023-02-06 B-07c Maintenance Practices Page 28 of 255 CASA Part 66 - Training Materials Only  Ohmmeter Scale If you look at the ohmmeter scale in the image, you will realise that it is very uneven and is reversed to the scales of the voltmeter and ammeter. This is because the meter is a current-operated device, and a resistor of very low value that is being measured allows a high current to flow, giving a large amount of meter deflection. Conversely, a high resistance that is being measured means a low current flow and low deflection. The uneven scale comes from the inverse relationship of the Ohm’s law formula: R = E/I. The image shows the Ohm scale on an analogue multimeter. Ohm scale on an analogue multimeter 2023-02-06 B-07c Maintenance Practices Page 29 of 255 CASA Part 66 - Training Materials Only  Ohmmeter Design 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 shorted together as shown. © Aviation Australia Ohmmeter design 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 the ohms 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 contact each other and discharge the ohmmeter battery. When the variable resistor (rheostat) is adjusted properly, with the leads shorted, the pointer of the meter comes to rest exactly on the zero position. The purpose of the variable resistor in this illustration is to adjust the current so that the pointer is at exactly 0 when the leads are shorted. This is used to compensate for changes in the internal battery voltage due to aging. When the test leads of an ohmmeter are separated, the pointer of the meter returns 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 (infinity) of the scale. 2023-02-06 B-07c Maintenance Practices Page 30 of 255 CASA Part 66 - Training Materials Only  In this regard, the ohmmeter indication is ‘backwards’ because maximum indication (infinity) is on the left of the scale, while voltage and current meters have 0 at the left of their scales. 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 with an ohmmeter is shown below. Aviation Australia Ohmmeter use 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. This causes the current produced by the 3-V battery of the meter to flow through the circuit being tested. Assume the meter test leads are connected at points a and b in the diagram. The amount of current that flows through the meter coil depends on the total resistance of resistors R1 and R2, and the resistance of the meter. Since the meter has been pre-adjusted (zero), the amount of coil movement now depends solely on the resistance of R1 and R2. The inclusion of R1 and R2 raises the total series resistance, decreasing the current and thus decreasing the pointer deflection. The pointer will come to rest at a scale figure indicating the combined resistance of R1 and R2. 2023-02-06 B-07c Maintenance Practices Page 31 of 255 CASA Part 66 - Training Materials Only  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. Note: Ohmmeters should never be connected to an energised circuit (that is, a circuit with its own source of voltage). Any voltage applied to the test leads of an ohmmeter invalidates its reading. Multi-range Ohmmeter A practical ohmmeter usually has several operational ranges. These typically are indicated by R × 1, R × 10, R × 100, R × 1000, R × 100 000 and R × 1,000,000. These range selections are interpreted differently than that of an ammeter or voltmeter. The reading on the ohmmeter scale is multiplied by the factor indicated by the range setting. For example, if the pointer is set on the scale figure 20 Ω and the range switch is set at R × 100, the actual resistance measurement is 20 × 100, or 2 kΩ. Obviously, the larger the range, the less accurate the reading, particularly on the left side of the scale as the scale graduations are very cramped up. To measure small resistance values, you must use a higher ohmmeter current than is needed for measuring large resistance values. Shunt resistors are needed to provide multiple ranges on the ohmmeter to measure a range of resistance values, from very small to very large. For each range, a different value of shunt resistance is switched in. The shunt resistance increases for higher ohm ranges and is always equal to the centre scale reading on any selected range. In some meters, a higher battery voltage is used for the highest ohm range. A common circuit arrangement is shown below. © Aviation Australia Multi-range ohmmeter circuit 2023-02-06 B-07c Maintenance Practices Page 32 of 255 CASA Part 66 - Training Materials Only  Safety Ohmmeter The safety ohmmeter is specifically designed for ultra-safe resistance testing in explosive devices (e.g. squib resistance, igniters, detonators, flares). It is used in volatile and potentially explosive atmospheres. A safety ohmmeter uses a very small current to test resistance (typical current limited to 0.5 mA). Safety ohmmeter used for squib resistance test 2023-02-06 B-07c Maintenance Practices Page 33 of 255 CASA Part 66 - Training Materials Only  Insulation Tester (Megohmmeter) It is sometimes necessary to test the integrity of insulation on aircraft conductors. Insulation resistance testing may be required by maintenance data as part of scheduled maintenance or it may be necessary during an electrical defect investigation. This task is performed using a megohmmeter such as the one shown below. A digital insulation tester (megohmmeter) Degraded insulation allows a leakage current to flow to ground or to an adjacent conductor through weaknesses in the insulation. The integrity of insulation degrades with age and exposure to flexing, abrasion, heat and contaminants. Leakage current across degraded insulation causes fluctuation of supply to electrical/avionics equipment. It creates sparks as it jumps the gap between conductors. The sparks cause radio frequency interference (RFI) and increase the risk of fire or explosion. Serviceable insulation has a resistance value of millions of ohms. Insulation resistance is measured between the isolated conductor and ground, or between the isolated conductor and an adjacent isolated conductor. A very small leakage current exists even across serviceable insulation because even new insulation is not perfect. A conventional ohmmeter or megohmmeter operates by applying a regulated voltage across its test terminals and measuring the resulting current across the resistance. It calculates and displays resistance as the resultant of Voltage/Current. 2023-02-06 B-07c Maintenance Practices Page 34 of 255 CASA Part 66 - Training Materials Only  A conventional ohmmeter cannot be used to verify the integrity of insulation because the voltage across its probes (2-3 V DC) is too low to drive leakage current across the insulator. A valid test of the insulation on a conductor must stress the insulation at a voltage considerably greater than the normal working voltage of the circuit. A typical megohmmeter can be set to apply a voltage as high as 1000 V DC across insulation. Megger is the brand name of an obsolete hand-cranked analogue insulation tester that was once widely used. It has been out of production for many years. Use of the name now causes confusion between modern Megger Insulation Testers and modern Megger Bonding Testers. Design of an Insulation Tester An insulation tester measures and displays resistance values in the megaohm range. The typical range of resistances measured by a modern digital insulation tester, when selected to the ‘Insulation Test’ function, is about 0.1 to 4000 MΩ. The insulation tester shown in the photograph can also be selected to function as a conventional ohmmeter and as an AC and DC voltmeter. Aviation Australia Digital insulation tester schematic 2023-02-06 B-07c Maintenance Practices Page 35 of 255 CASA Part 66 - Training Materials Only  A modern digital insulation tester is battery powered, usually by a 9-V battery. Refer to the diagram. An insulation tester produces high DC test voltages by the following process: 1. Battery voltage is regulated to a stable value. 2. When the TEST button is pushed, regulator output is inverted to low-voltage AC. 3. The inverter drives the primary winding of a step-up transformer. 4. A rectifier on the transformer output supplies high-voltage DC to the test terminals. Analogue insulation testers are also used. Older analogue insulation testers incorporate a hand- driven generator to supply high DC test voltages. Test current is low, e.g. 1 mA when set to 1000 V and the test terminals connected across a resistance of 1 MΩ. The short-circuit current of a digital insulation tester is electronically restricted to a low value. This is achieved by reducing the output voltage to a value far below the selected test voltage. The instantaneous output voltage of the insulation tester in the photograph is displayed in the lower right corner of the display. The display in the photograph shows 0 V because the TEST button is not being pushed. Performing an Insulation Resistance Test Most insulation resistance tests on aircraft are performed on feeder lines that interconnect generators, batteries, Transformer Rectifier Units (TRUs) and busbars. Insulation testing may also be required for other heavy-gauge wiring, e.g. starter-generator power cable, electric pumps and ovens. CAUTION: ISOLATION - Disconnect power from the system. All wires to be tested should be disconnected at both ends to ensure that no electrical equipment is subjected to the high-voltage output of the meter. Voltage Selection: Select the test voltage specified in the maintenance data, e.g, 125, 250, 500 and 1000 V. A voltage that is higher than the one specified by the maintenance data could damage the insulation under test. Meter Quick Test: Verify that the meter is functional by pushing the TEST button with the test terminals open (separated) and closed (touching). When the test terminals are open, the meter should read OL for Outside Limits, i.e. a higher value of megohms than the meter can display. When the test terminals are closed, the meter should read OΩ. CAUTION: FIRE/EXPLOSION RISK - Ensure that the insulation under test is not exposed to any flammable materials (such as fuel vapour). Connecting the Insulation Tester: To test the insulation of a specific conductor, connect one test lead to the aircraft ground and the other test lead to the conductor. The maintenance data may also require insulation testing between adjacent conductors. If shielded cable is tested, apply one lead to the centre conductor and the other to the wire shield. 2023-02-06 B-07c Maintenance Practices Page 36 of 255 CASA Part 66 - Training Materials Only  Taking the Reading: Push the red TEST button to apply the selected high voltage across the insulation. The meter displays the insulation resistance. If the parasitic capacitance of the conductors under test is large, there will be a delay of a few seconds before the reading stabilises. The display initially reads a low value of resistance, which increases as the cable capacitance becomes charged. This effect is easier to see on an analogue insulation tester. Parasitic capacitance is the capacitance between insulated conductors because of their proximity to each other and the dielectric properties of electrical insulation. After recording the measurement, discharge the accumulated capacitive charge by keeping both test terminals in contact with the conductors for a few seconds after releasing the TEST switch. The charge dissipates across the resistors in the meter. The display indicates instantaneous static voltage as the cable discharges. Interpreting the Reading Atmospheric humidity affects the resistance reading because moisture is conductive. Typical acceptable values of insulation resistance are: 1 MΩ or higher when humidity is ≥80% 5 MΩ or higher when humidity is 70%-80% 10 MΩ or higher when humidity is ≤70%. Note: Do not perform an insulation test when humidity is 100% because current will flow through condensed surface moisture. This causes an under-read of insulation resistance. Insulation resistance tester 2023-02-06 B-07c Maintenance Practices Page 37 of 255 CASA Part 66 - Training Materials Only  Additional Use of an Insulation Tester (Megohmmeter) Static Discharger Testing An insulation tester (megohmmeter) is used to test the serviceability of static dischargers (static wicks). This is a specialised task within electrical bonding testing. A static discharger is a device attached to an aircraft control surface to dissipate accumulated electrostatic charge into the air. Its purpose is to prevent RFI by preventing sparks from jumping between the control surface and the airframe. Each static discharger consists of a semi-flexible resistive element covered with shrink tubing. A wire bundle protrudes from the trailing end, and the leading end is threaded. The leading end locates in an aluminium tapered sleeve that is rivetted to the control surface. The resistance range between the ends of a serviceable static discharger is between 1 and 100 MΩ, which is far beyond the range of a conventional ohmmeter. The megohmmeter is connected between the tapered sleeve and the trailing end of the discharger. Corrosion protection (paint) must be removed from the sleeve in order to obtain a low resistance connection for the test terminal. Corrosion protection must be re-applied to the sleeve following the test. 2023-02-06 B-07c Maintenance Practices Page 38 of 255 CASA Part 66 - Training Materials Only  Multimeters The Multimeter A multimeter is the most common measuring instrument used by the aircraft technician. 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 ammeter, an AC voltmeter and an ohmmeter, all in one package. There are two categories of multimeters: analogue and digital (often abbreviated DMM or DVOM.) Analogue and digital multimeters Most multimeters use a d’Arsonval meter movement and have a built-in rectifier for AC measurement. 2023-02-06 B-07c Maintenance Practices Page 39 of 255 CASA Part 66 - Training Materials Only  Multimeter Circuit Basics In its simplest form, the multimeter is a single meter with its external circuit switched to enable it to measure amps, volts and ohms. © Aviation Australia Basic multimeter circuit With the switch in position A, the shunt is connected in parallel to the meter to allow it to read as an ammeter. The range is determined by the value of the shunt. The test terminals form its positive and negative leads. With the switch in position V, the multiplier is connected in series with the meter, allowing it to read as a voltmeter. Again, the range is determined by the value of the multiplier. As with the ammeter, the test terminals form the positive and negative leads. With the switch in position O, the meter is now connected with the battery in the circuit to measure ohms. However, if you examine the circuit closely, you will notice that the lead polarity has reversed. This is necessary to maintain the correct current flow direction through the meter. 2023-02-06 B-07c Maintenance Practices Page 40 of 255 CASA Part 66 - Training Materials Only  Analogue Multimeter Analogue multimeters typically have a sensitivity of 20 000 Ω/V for measuring DC and 1000 Ω/V for measuring AC. All analogue multimeters have a ‘Zero Ω Adjust’ knob/dial to zero the scale for different ohm ranges and also as the internal battery discharges. There is also a ‘Zero Adjust’ to set the needle to 0. Analogue meters are very useful for testing rheostats/potentiometers for their linear range of resistance. They also load a circuit more, which provides more accurate indications when seeking faults involved with possible high-resistance joints (AC). Typically, a DMM does not load a circuit sufficiently to gain true indications. Analogue multimeter 2023-02-06 B-07c Maintenance Practices Page 41 of 255 CASA Part 66 - Training Materials Only  Digital Multimeter A digital multimeter displays the quantity measured as a number, which prevents parallax errors. It is the most common meter used by aircraft technicians for the following reasons: Easy to use Compact Accurate Auto-ranging Higher sensitivity Usable in any position if necessary. Check calibration compliance prior to use. Digital multimeter 2023-02-06 B-07c Maintenance Practices Page 42 of 255 CASA Part 66 - Training Materials Only  Multimeter Usage The following steps provide the correct process to use a multimeter. As with other meters, incorrect use of a multimeter could cause injury or damage. De-energise and discharge the circuit completely before connecting or disconnecting a multimeter. Never apply power to the circuit while measuring resistance with a multimeter. Connect the multimeter in series with the circuit for current measurements, and in parallel for voltage measurements. Be certain the multimeter is switched to AC before attempting to measure AC circuits. Observe proper DC polarity when measuring DC. When you are finished with a multimeter, switch it to the OFF position, if available. If there is no OFF position, switch the multimeter to the highest AC voltage position. Always start with the highest voltage or current range. Select a final range that allows a reading near the middle of the scale. Adjust the ‘0 Ω’ reading after changing resistance ranges and before making a resistance measurement. Be certain to read AC measurements on the AC scale of a multimeter. Observe the general safety precautions for electrical and electronic devices. 2023-02-06 B-07c Maintenance Practices Page 43 of 255 CASA Part 66 - Training Materials Only  Current Transformers Current transformers (CTs) are devices used to scale large primary currents to smaller, easy-to- measure secondary currents. The ratio of the windings determines the relation between the input and output currents. CTs of various shapes and sizes are used as an interfacing solution between high currents and instrumentation devices. When measuring AC current, a CT is generally connected in series with the load. The CT typically has a 1 A or 5 A secondary that is connected to the input of the measuring device. Aviation Australia Current transformers CTs are typically available in ratios such as 50:5, 100:5, 300:5, etc. This means if a conductor carrying 50 A passes through a 50:5-ratio CT, a 5-A current flow is produced in the CT leads. It is important to connect the CT leads to an ammeter or short them together. If the leads are left unconnected, a high voltage will be produced and the CT will likely be destroyed. A CT allows the metering equipment to measure current flow at levels which would otherwise be beyond the range of the meter. It is very difficult to produce an output from a DC magnetic field because it does not oscillate, unlike an AC field, which is constantly expanding and contracting. In a DC system, a CT works on the effect of sensed DC magnetic field on a self-transmitted AC magnetic field. A meter current transformer has an oscillating magnetic field and it senses how much it is affected by the DC magnetic field. 2023-02-06 B-07c Maintenance Practices Page 44 of 255 CASA Part 66 - Training Materials Only  Transformer/Rectifier Meters (The Clamp Meter) The output of a transformer is a function of the input and it is often advantageous, when adapting moving-coil instruments for ac measurement, to make use of voltage transformers in place of, or to supplement, series resistors in voltmeters, or current transformers instead of shunts in ammeters. This practice is very widely adopted in multi-range rectifier type instruments, where the non-linear accuracy on the different ranges of measurement. The very small power required by the rectifier/ moving coil ammeter enables such an instrument to be used with a current transformer in which the primary winding provides only a few ampere-turn’s. This feature is exploited in the "clip-on" type of ac ammeter shown in the diagram. This instrument enables the current in a conductor to be measured simply by enclosing the conductor, at any convenient point, in the manner shown. The complete meter comprises a normal permanent magnet moving coil instrument, a full wave metal rectifier and a special type of transformer. The core of the latter is in two parts, viz a main portion (A) and a pivoted limb (B) which can be opened to admit the cable by means of a trigger (C); this trigger operates against a strong spring to ensure adequate pressure at the butt joint of the core when it is released. The cable circled by the core constitutes the primary winding of the transformer and the secondary winding is tapped at various points, through the range switch, to give full scale deflection current through the instrument coil (via the rectifier) when the current through the encircled cable is of specified (rms) values, e.g. 10, 25, 50, 100, 250, 500 and 1,000 amperes. There are different types of clamp meters available which include the following: The current transformer type or ac clamp meter is used to measure AC (alternating current) only Hall Effect type is used to gauge both AC (alternating current) & DC (direct current) 2023-02-06 B-07c Maintenance Practices Page 45 of 255 CASA Part 66 - Training Materials Only  Transformer/Rectifier Meters 2023-02-06 B-07c Maintenance Practices Page 46 of 255 CASA Part 66 - Training Materials Only  Meter General Safety Precautions Meters must frequently be used in operating electric circuits. As a result, the risk of electric shock is often present. To avoid injury to personnel and damage to equipment, follow these basic safety rules when using electrical measuring instruments: Use a meter that meets acceptable safety standards. Use a meter that is calibrated (check currency on calibration label). Use a meter face-up on a horizontal surface – away from this position, the readings will be inaccurate. Use a meter with fused current inputs. This will protect the user and/or equipment should leads inadvertently short. Inspect test leads for physical damage before use. A frayed lead could be very harmful. Use the meter to check the continuity of the test leads. Use test leads with shrouded connectors and finger guards. Only use meters with recessed input jacks. Ensure the correct jacks are used with the correct function (current readings). Select the proper function and range for measurement. If in doubt, select a higher range than required and move the selection down as needed. 2023-02-06 B-07c Maintenance Practices Page 47 of 255 CASA Part 66 - Training Materials Only