This article introduces you to the different types of electrical measuring instruments and test sets used in a substation. The purpose of this article is to enable you to recognize and understand the functions of different instruments. Later, you will learn how to actually use and read these instruments on the job.
Figure 1: Digital MultimeterLet us begin by explaining what a DMM (digital multimeter) is. A DMM is simply an electronic tape measure for making electrical measurements. It may have any number of special features, but mainly a DMM measures volts, ohms, and amperes.
A 3 1⁄2 digit meter can display three full digits ranging from 0 to 9, and one "half" digit, which displays only a 1 or is left blank. A 3 1⁄2 digit meter will display up to 1,999 counts of resolution. A 4 1⁄2 digit meter can display up to 19,999. It is more precise to describe a meter by counts of resolution than by digits. Todays 3 1⁄2 digit meters may have enhanced resolutions of up to 3,200, 4,000, or 6,000 counts. For certain measurements, 3,200-count meters offer better resolution. For example, a 1,999 count meter would not be able to measure down to 1/10 V if you were measuring 200 V or more. However, a 3,200-count meter would display a tenth of a volt up to 320 V. This is the same resolution as a more expensive 20,000-count meter until the 320 V reading is exceeded.
Analog meter specifications are determined by the error at full scale, not at the displayed reading. Typical accuracy for an analog meter is 2% or 3% of full scale. At 1/10 of full scale, these become 20 or 30% of reading. Typical basic accuracy for a DMM is between (0.7%+1) and (0.1%+1) of reading or better.
The waveforms associated with AC voltages can be either sinusoidal or non-sinusoidal (sawtooth, square, ripple, etc.). Quality DMMs display the "RMS" (root mean square) value of these voltage waveforms. The RMS value is the effective, or equivalent, DC value of the AC voltage. Most DMMs are "average responding," meaning accurate RMS readings are present if the AC voltage signal is a pure sine wave. Average responding meters are not capable of measuring non-sinusoidal signals accurately. Non-sinusoidal signals are accurately measured using DMMs designated as "true-RMS" up to the DMMs specified crest factor. Crest factor is the ratio of a signals peak-to-RMS value. It is 1.414 for a pure sine wave, but is often much higher for a rectifier current pulse, for example. As a result, an average responding meter will often read much lower than the actual RMS value.
A DMMs ability to measure AC voltage can be limited by the frequency of the signal. Many DMMs can accurately measure AC voltages with frequencies from 50 Hz to 500 Hz, but a DMMs AC measurement bandwidth may be hundreds of kilohertz wide. Such a meter may read a higher value because it is "seeing" more of a complex AC signal. DMM accuracy specifications for AC voltage and AC current should state the frequency range and its accuracy level.
Figure 2: Voltage Tester
Walk onto any site where electricians are working, and you will probably see some voltage testers. These handy devices which often fit in a shirt pocket are popular because they quickly give an indication of voltage presence. That makes them very handy for general voltage checks, especially when working with control systems. Thus, voltage testers are popular with electricians. Yet, these devices are not all the same. The differences show up in safety, reliability, and convenience.
If you were to look at all the voltage testers on the market and note their differences, you would quickly see that they divide into two general categories: solenoid-based testers and electronic testers. Solenoid-based testers have a long tradition; they were the first voltage testers available and are still widely in use today.
When voltage passes some threshold, the tester will indicate that a voltage is present. Below that threshold, the tester will not indicate a voltage at all. The thresholds are markedly different between the two categories of testers, and that fact carries important implications for safety and convenience. Let us compare voltage testers in these two categories more closely so you can draw your own conclusions about what to have in your toolbox or what to clip to your pocket.
An important concern with solenoid-based testers is their relatively low input impedance. They may be rated for 10 kilohms at the upper end, but they often come as low as 1 kilohm. Applying Ohms law, you can see solenoid-based testers can easily make their presence felt in a circuit as loads and subsequently interfere with the operation of that circuit. The relatively high current draw of solenoid-based testers means significantly more heat enough that the testers can quickly overheat, even to the point of destruction (Figure 21) if the voltage is measured a little too long. In fact, you must allow for cooldowns (on the order of half a minute) as you take readings with solenoid-based testers. If your PLC goes down and the plant manager is screaming about production being lost forever, you are at the mercy of this limitation of the tester.
Figure 3: Solenoid-Based Tester After OverheatingSolenoid-based testers generally are unable to comply with IEC 1010 due to excessive current draw, poor dielectric withstand performance, and impulse destruction due to transients originating from the mains. This is one reason many companies forbid the use of voltage testers on anything but 120 V control circuits, while some forbid them altogether. In a moment, we will look at reasons to reconsider those restrictions.
This high current in solenoid-based testers has another downside. Applying Ohms law to the low-impedance solenoid-based tester shows that you can easily carry a lethal current through the tester. Wearing insulated gloves can eliminate the shock hazard, but you still will be breaking a circuit under load each time you use that tester and you will be risking an arc hazard each and every time. Yes, there are riskier things you can do than use a solenoid-based tester. However, there are also safer things you can do, such as use an electronic voltage tester instead.
Figure 4: Electronic Voltage TesterThis higher impedance has a downside though: an electronic tester might indicate voltage on a non-energized conductor.
This can happen when one conductor induces a voltage in another conductor parallel to it. This voltage indication can be a disadvantage by showing a false positive. Then again, it can work to your advantage. For one thing, it will not lull you into a false sense of safety that an energized conductor is de-energized. Suppose the solenoid-based tester does not show you the 80 V sitting on that wire, and you grab it. It is better to be too safe than not safe enough.
Another issue that arises in reference to induced voltages is the fact that the same technology used in the electronic testers also dominates todays control systems. We seldom ask our HVAC or industrial control systems to open and close electromechanical coils to operate control logic. Instead, we ask them to provide signals that electronic devices will interpret as the presence or absence of voltage, which is essentially what electronic voltage testers do. Doesnt it make sense to use a tester that matches the control system you are using?
One mistake people make with test equipment is trying to save money on leads and probes. This can be a very expensive "cost-savings." Cheap and poorly constructed input accessories are prone to failure of the materials that are supposed to protect you. Remember that you often hold accessories in your hand while conducting measurements. If a failure occurs, the result could be extremely hazardous to you.
It is important to maintain quality throughout your measurement tool kit. Choose accessories that are suitable for industrial work, keep an eye on them for abrasions and other damage that eventually will occur with use. This way, you will never have to worry about the failure of a test lead or probe, or the consequences thereof. Look for the IEC rating (e.g., CAT II) (Figure 23) on your tester, and buy leads and other accessories that meet or exceed that rating.
Figure 5: Lead Rating
Additional features can make your tester a bit more useful. However, these may add weight and cost. If these features are important to you though, consider them when buying a voltage tester.
The continuity tester consists of a battery-powered buzzer and two leads. When the two leads are connected, a complete circuit is made and the buzzer will sound. If the circuit is not complete, the buzzer will not sound. Another type of continuity tester is a flashlight with two leads attached. When a complete circuit is made, the flashlight illuminates. A multimeter is another type if continuity meter. Figure 6 shows two types of continuity testers.
Figure 6: Continuity Testers
Continuity testers are used only on de-energized circuits. They are not to be used on energized circuits as damage to the continuity tester and possible injury may result.
Figure 7: Digital Low-Resistance Ohmmeter (DLRO)The resistance of electrical contacts or connections should be very low so that the current flows without arcing or burning. Low-resistance ohmmeters are used to check the contact resistance of:
There are two types of low-resistance ohmmeters used: the microohmmeter and milliohmmeter. The microohmmeter is an older style of meter with a dial-type meter that measures resistance in microohms (10-6ohms). The milliohmmeter is a newer style electronic meter with a digital readout. It measures resistance in milliohms (10-3ohms).
Figure 8: Clamp-On AmmetersThe ammeters used in most substations are clamp-on ammeters. Unlike multimeters, a clamp-on ammeter does not have to be connected in the circuit to measure current. Instead, the jaws of the ammeter open to fit around the conductor and are closed, or "clamped," around the conductor. The ammeter measures the circuits current by measuring the conductors magnetic field. Most clamp-on ammeters measure AC current, while a special type of clamp-on ammeter called a Hall Effects ammeter can measure both AC and DC currents.
Figure 9: High-Voltage Detector (Tic Tracer)
Hand-held thermal imagers, like the one shown in Figure 10, collect heat signatures from a range of motors from 1,000 down to 5 hp. A thermal imager is good for spot checks, to determine if motors and associated panels and controls are operating when they are too hot, and troubleshooting, to track down the specific failed component. It can also check for phase imbalances, bad connections, and abnormal heating on the electrical supply.
Figure 10: Thermal ImagerA motors heat signature will tell you a lot about its quality and condition. If a motor is overheating, the windings will rapidly deteriorate. In fact, every increase of 10C on a motors windings above its designed operating temperature cuts the life of its windingsinsulation by 50%, even if the overheating is only temporary.
If a temperature reading in the middle of a motor housing comes up abnormally high, a thermal image of the motor should be taken to determine more precisely what is producing the high temperature: windings, bearings, or couplings.
These are the three primary causes for abnormal thermal patterns; the majority of other causes produce high temperatures and typically are the result of a high-resistance contact surface, usually either a connection or a switch contact. These normally appear warmest at the spot of highest resistance, with patterns cooling off as distance from the spot increases. The thermal image in Figure 11 shows a classic pattern in the center phase connection on the line-side of the breaker; note how the conductor cools off at the top of the image.
Load imbalances, whether normal or out of specification, appear equally warm throughout the phase or part of the circuit that is undersized or overloaded. Harmonic imbalances create a similar pattern. If the entire conductor is warm, it could be undersized or overloaded. The rating and actual load should be checked to determine which is the cause.
Failed components typically look cooler than ones that are functioning normally. The most common example is probably a blown fuse. In a motor circuit, this can result in a single-phase condition and, potentially, costly damage to the motor.
Figure 11: Thermal Image of a Hot Connection
Figure 11 shows a thermal image of a drive cabinet with hot connections on both the A and the B phases. The exact cause cannot be determined solely from the image, although we can tell it could be a load or balance issue.
Figure 12: Thermal Image Bearing Problem
Figure 12 depicts a warm bearing (or seal) on a pump. Clearly, the access is tight but the bearing and its housing can still be compared.
Figure 13 Left
Figure 13 Right
Figure 13: Thermal Image Heat Transfer
Figure 13 Left depicts another bearing problem where heat is transferred into the coupling on the right side.
Figure 13 Right, on the other hand, shows the motor itself heating up due to reduced airflow or misalignment.
Figure 14:Megger MJ159
Meggers come in all varieties. Some are single-voltage devices, like 500 V meggers, and others offer a range of test potentials, from 100 to 5,000 V. Some are manually operated, and others are battery or motor-operated.
The higher voltage applied by a megger allows a more accurate measurement of highly resistive items, such as insulation. According to Ohms law, current is equal to the applied voltage divided by the resistance. Since most digital multimeters use a 6 or 9 V (or smaller) power supply, it is possible to get an infinite reading, which indicates an open circuit, whereas a megger might give a reading of 500 MΩ. The difference in results is due to the higher voltage available in the megger.
Meggers should not be used to measure small resistances or electronic components for the same reason it is used for high resistive values. The voltage produced by a megger is far too high for most standard electronic components to sustain.
Refer to Test Precautions for further explanations and precautions.
The MJ159 is powered by a low-voltage, hand-cranked, brushless AC generator that is connected, after rectification, to a DC-to-DC converter. The generator is designed to be easy to turn, even at full load. Nominal test voltages are 100, 250, 500, and 1,000 V, selectable by the user. This switch is located on the left front side of the megger.
This megger's insulation measuring range is 0.1 Ω to 2,000 MΩ. Automatic discharge for capacitive circuits under test is provided. The instrument also has a 5,000Ω range for testing electrical installations. The voltage measuring range is 0 to 600 V, and it is calibrated for AC voltage. This feature allows monitoring of decaying voltage following the testing of equipment possessing capacitance.
The needle is a day-glow orange, and the scales are white on a black scale face. The needle is moved using a moving coil with taut band suspension.
There are three shrouded terminal sockets labeled (-), (+), and (G) provided on the side of the meter for attachment of the test leads. These terminals are located just above the scale. Since the resistance of the test leads is included in the calibration of the meter, the test leads supplied by the manufacturer should be used. After the test leads have been connected to the instrument terminals, the carrying handle folds down neatly over them to protect the user.
Meggers must only be used by suitably trained and competent people.
The instruments can give an electric shock. Highly capacitive circuits (e.g., long cables) charged to several kilovolts could create a potentially lethal charge. Circuit connections must not be touched when testing.
Care must be taken to prevent capacitive circuits from becoming disconnected during a test, leaving the circuit in a charged state.
The instruments voltmeter and automatic discharge should be regarded as additional safety features and not substitutes for maintaining normal safe working practices.
Fuse replacements must be of the correct type and rating. Specifications are listed in the user's manual.
If any part of the instrument is damaged, it should not be used but returned to the manufacturer or an approved organization for repair.
Without the test leads being connected to the instrument, but with the rotary selector switch set to the 1 kV range, the TEST button should be pressed and held down while turning the generator handle at>180 RPM. The meter pointer should remain over the ∞ (infinity) position on the scale, proving that there is no leakage from the instrument itself.
Connect one of the test leads to the+and the other to the-terminals on the side of the instrument case and ensure that their clips are not touching anything.
Press the TEST button again and keep it pressed while turning the generator handle at>180 RPM. The meters needle should be observed. The needle should rest over the "∞" (infinity) position on the scale. If it does not, the test leads may be faulty and should be inspected more closely for damage. Replace them, if necessary, with calibrated leads available as optional accessories.
Next, the test lead clips should be connected together. Press the Test button and turn the generator handle. The meter should read zero. If it indicates infinity or a high resistance value, the leads may be open circuited and should be inspected. Replace them if necessary. (Shorting the leads together and obtaining a zero reading shows that the instrument is working.)
1. Set the selector switch to the "Ω" position.
2. Connect the leads across the isolated circuit.
3. Press and hold the Test button and turn the generator handle at>180 RPM. The resistance is indicated on the "Ω" scale.
The guard terminal is at the same potential as the negative test terminal. Since the leakage resistance is in parallel with the resistance being measured, the use of the guard terminal causes the current flowing through surface leakage to be diverted from the measuring circuit.
To determine if the resistance fuse is blown:
1. Disconnect the test leads and set the rotary switch to the "Ω" position.
2. Press the Test button and keep it pressed while turning the generator handle.
3. The reading obtained should be beyond full scale.
4. If the reading is approximately zero on the resistance scale, the 500 mA fuse is blown and should be replaced.
To determine if the insulation fuse is blown:
1. Connect the test leads together and set the rotary switch to the "MΩ" position.
2. Press the "Test" button and keep it pressed while turning the generator handle.
3. The reading obtained should be zero on the insulation scale.
4. If the reading is ∞, the 7 A fuse has blown and should be replaced.
If a hand-cranked megohmmeter is used, the reading should be taken while it is cranked at rated speed. Readings should be taken at the end of the 30-second or 60-second periods even though the pointer is still climbing. Then, if all future tests are made using the same time period, a good comparison of the insulations trend can be made.
Note that good insulation shows a continual increase in resistance over a period of 5 to 10 minutes. The absorption current discussed above is the cause of this. Good insulation shows this charge effect over a longer period of time than just the time required to charge the insulations capacitance. If the insulation contains much moisture or contaminants though, the absorption effect is masked by a high leakage current that stays at a fairly constant value, keeping the resistance reading low. Remember, R = E/I.
The timed resistance test also is of value because it is independent of equipment size. The increase in resistance for clean and dry insulation occurs in the same manner whether a motor is large or small; therefore, the readings of several motors can be compared, and standards for new ones can be established regardless of the motors' horsepower.
The ratio of two timed resistance readings (such as a 60-second reading divided by a 30-second reading) is called a dielectric absorption ratio, which is useful in recording information about insulation. The intended ratio used is a 10-minute reading divided by a 1-minute reading; the value obtained is called the Polarization Index.
With hand-cranked megger instruments, it is easier to run the test for only 60 seconds, taking the first reading at 30 seconds. If a power-operated megger is used, the Polarization Index can be obtained after the test is run for a full 10 minutes, with readings taken at 1 and 10 minutes. Table 2 shows the ratio values for each test length and the corresponding relative conditions of insulation that they indicate.
In some cases with motors, values that are 20% higher than shown here indicate a dry, brittle winding that will fail under shock or during starts. Forpreventive maintenance, the motor winding should be cleaned, treated, and dried to restore winding flexibility.
Insulation resistance tests should be conducted regularly on a predetermined basis depending on the type, location, and importance of the equipment. Test results are important, and graphs are helpful in establishing trends in the insulations condition.
Keeping accurate records is essential to a logical analysis of insulation condition. Insulation resistance tests should be made regularly and in the same manner, using comparable test equipment. These tests may be made on a monthly, semi-annual, or annual basis, as conditions demand. The resistance, wet- and dry-bulb temperatures, and date should be recorded.
The megohmmeter readings should be corrected to a base temperature, such as 25 C, and plotted on semilog paper for easy determination of any trends (as shown in Figure 15).
Figure 15: Megger Readings
Figure 16: Battery-Operated MeggerThere are many different models and manufacturers of battery-operated meggers. The extensive use of electronics makes it possible to find models capable of almost any output voltage, even ones above 1,000 V using a 9 V DC battery source. The example picked for this text is a Megger BM121. It has a nominal test voltage of 500 V. It is almost identical to the Megger BM122, which has a nominal test voltage of 1,000 V.
This instrument is also equipped with an automatic discharge feature, which will automatically discharge capacitive circuits through the instruments internal resistance (360 kΩ) when the test button is released after an insulation test.
The circuit being tested must be switched off, de-energized, and isolated before any tests are made. If an external voltage 25 V AC or DC is detected, normal testing is inhibited and the display flashes a "V" warning. This feature does not function when either the instrument is turned off or the batteries are exhausted.
1. The test push button must not be held down while connecting the test leads.
2. During an insulation test, the circuit must not be touched.
3. After an insulation test, capacitive circuits must be allowed to discharge before disconnecting the test leads.
4. Test leads, probe, and crocodile clips must be in good order, clean, and with no broken or cracked insulation.
5. Replacement fuses must be of the correct size, type, and rating.
Figure 17: Battery-Operated Megger Front Panel
1. Before each use of the instrument, visually inspect the test leads to confirm that their condition is good, with no damaged or broken insulation
2. Check the continuity of the test leads by firmly shorting the leads together and reading the test lead resistance measurement directly from the display.
1. Turn the instrument on by briefly pressing and releasing the test button.
2. Firmly connect the test leads together and note their combined resistance.
3. Connect the test leads across the circuit under test.
4. Take the reading directly from the displayed continuity range (maximum reading 99.9 Ω).
5. Subtract the test lead resistance value from the measured reading.
1. Turn on the instrument by briefly pressing and releasing the test button.
2. Firmly connect the test leads to the isolated item/circuit under test.
3. Press and hold the test button. The display will change to the MΩ range and show the insulation value. As an additional safety feature, the BM122 display flashes its "1,000 V" symbol before performing the test.
4. Release the test button before removing the test leads. This enables the instrument to discharge the circuit under test.
Figure 18: High-Voltage MeggerThe high-voltage megger has many different controls and indications:
The raise voltage control is used to regulate the output voltage. Markings on the raise voltage control denote the percentage of the units maximum output voltage to which the voltage may be increased. The raise voltage control should be set at zero when not in use. It should always be returned to zero immediately upon completion of a test.
The multiplier control allows for resistance readings in several ranges when performing insulation testing. To obtain the proper reading from the megger, simply multiply the reading on the scale by the value indicator at the range selected. The result when multiplied by the bottom reading of the voltmeter scale will give the total resistance of the sample in megohms.
Example: In performing an insulation resistance test, the megohmmeter indication is 4 on the megohm scale, with the multiplier control set at x100. The proper reading is therefore 400 megohms. The lower scale on the voltmeter is 2.5, so the final reading is 2.5 x 400, or 1,000 MΩ.
The megohmmeter indicator should indicate zero when the unit is off. If not, minor adjustments may be made only when the HV control is off.
A jumper link connected between the Guard and Ground posts: The resistance of the material between the high-voltage electrode and the low-voltage electrode is directly proportional to the applied voltage and inversely proportional to the resultant current flow. The voltage stress on the insulation is affected by fringing at its edges, resulting in two types of leakage current flowing. With a grounded Guard hookup, all leakage current flowing directly to ground will be bypassed around the current meter and, therefore, not observed. Only leakage current flowing along the insulator will be observed and accurately measured by the current meter.
A jumper link connected between the Ground and Positive posts - This front panel hookup connection will only be used when sensitive leakage current measurements are not necessary or when only flashover tests to ground are required. The leakage current paths are not separated in this configuration, so all leakage current is observed on the current meter.
1. Select a location for the unit where the meters are placed at eye level to allow maximum accuracy in readings.
2. Set the Raise Voltage control to zero and check to ensure that the AC Power switch is in the "off" position.
3. Ground the case before connecting the input power. The Ground post on the control panel may be used also.
4. Plug the line cord into 115 V, 50/60 Hz outlet. If a two-prong adapter is used, be sure to ground the third wire.
5. Turn the AC Power switch on (the "up" position). Depress the Press-To-Test button to commence the test.
For performing resistance testing, the following procedure should be used:
1. Plug the line cord into a three-wire grounded-type receptacle. For a 115 V AC plug, plug a three-prong plug into a three-prong adapter. For a 220 V plug, connect the green wire to ground, the white wire to the B line, and the black wire to the A line.
2. Connect the red test lead to the Pos terminal and the shielded black test lead to the Neg re-entry.
3. Connect the Ground strap to the Guard terminal if the specimen under test is isolated from ground. Connect the Ground strap to the Pos terminal if the specimen under test is not isolated from ground. The red test lead will act as a return in this case.
4. Connect the black and red test clips to the specimen under test. Connect the red clip to the high side and the black clip to the low side.
5. Turn the AC Power switch on. Check that the AC Power indicator lights are on.
6. Select the lowest range on the Multiplier switch to start. The range must be increased step-by-step until the desired range is reached. This is done to protect meter movement.
7. Depress the Push-To-Test button.
8. Adjust the Raise Voltage control until the desired voltage level is indicated on the kV meter.
9. Select the proper Multiplier range.
10. Record the resistance reading as indicated on the meter.
11. To obtain the correct resistance value, the megohm reading must be multiplied by the correct factor, as indicated on the kV meter.
12. Always release the Push-To-Test button so that the test sample is shorted to ground for operator safety.
13. Turn the AC Power switch off. Note that the AC Power indicator is out.
14. Disconnect the test leads from the equipment under test.
The major area of application for high-potential testing is checking equipment that has been scheduled out-of-service for evaluation and repair. This test must not be made on vital equipment that, if damaged, would disrupt the operation of a plant or pose a safety hazard.
Unless otherwise specified by the manufacturer or testing agency, the following recommended maximum allowable test voltages may be used for a nondestructive high-potential test: 1.5 x rated line-to-line voltage for an AC test voltage and 1.7 x 1.5 x rated line-to-line voltage for an equivalent DC test voltage.
When making connections for a high-potential test, all wiring of the apparatus under test should be connected to the hot terminal of the tester, and the framework of the apparatus should be connected to the ground terminal.
Example: A 200 hp, 2,300 V, three-phase, 60 Hz induction motor is to be given a high-potential test. The maximum allowable test voltage is determined to be:
Figure 19a shows current-time curves for the 10-minute constant voltage test. Good insulation exhibits a steady decrease in leakage current with time; bad insulation exhibits a rise rather than a decline in leakage current. Any rise in leakage current during this test is a signal to stop the test. Note that the polarization index can be calculated from this test data by dividing the leakage current after 1 minute by that obtained after 10 minutes:
After the initial 10-minute test, the DC voltage should be increased in 8 to 10 uniform steps, each 1 minute long, from the 30 percent value up to the maximum allowable DC test voltage. Each set of current and voltage measurements taken during this part of the test should be plotted immediately after the end of the particular 1-minute interval, even though the current may still be changing. The test should be stopped at the first indication of an upward bend or knee in the curve. A knee in the curve indicates the need for cleaning and drying, and if the test is not stopped, the leakage current may increase to a value that may damage the insulation. Figure 19b shows representative current-voltage curves for the step voltage test. A straight line is indicative of good insulation, whereas a knee indicates the need for reconditioning.
Figure 19: Current Time CurveAs a safety precaution, when using the step voltage test, the leakage-current relay should be adjusted to an initial setting approximately four times the steady leakage current obtained when 30 percent of the maximum allowable test voltage is applied. As increased increments of test voltage cause the leakage current to approach the trip setting of the overcurrent relay, the trip setting should be gradually increased. It is important that the relay setting not be too high, or a sudden failure may cause arcing and extensive damage to the insulation. In any event, the test should be stopped when a sudden increase in leakage current is observed.
Repeated diagnostic DC high-potential tests on an apparatus during reconditioning should always be made with the same terminal of the test equipment connected to the apparatus ground. A good rule to follow is to always connect the negative terminal to ground and the positive terminal to the apparatus wiring.
High-voltage testing, whether AC or DC, represents a potential hazard to life, and every safety precaution recommended by the manufacturer or testing agency must be followed. The voltages used and the available currents are lethal, even though they are only being measured in milliamperes. Electricians rubber gloves must be worn when making these tests and the wiring of the apparatus must be discharged by grounding for 15 minutes or more after completing the test. When making the ground discharge connection, one end of the grounding wire must first be firmly connected to the framework of the apparatus, and then the other end connected to the wiring. At very high test voltages, e.g., 75,000 V, appreciable voltage recovery may occur when the grounding wire is removed, even though the winding has been connected to ground for more than 30 minutes. This voltage is caused by the release of energy as the molecules return slowly to their unstressed equilibrium positions.
Not exactly. Engineers who analyze multimeter safety often discover that failed units were subjected to a much higher voltage than the user thought he was measuring. There are the occasional accidents when the meter, rated for low voltages (1,000 V or less), was used to measure medium voltages, such as 4,160 V. Just as common, the knock-out blow had nothing to do with misuse; it was a momentary high-voltage spike or transient that hit the multimeter input without warning, as shown in Figure 20.
Figure 20: High-Voltage SpikeAs distribution systems and loads become more complex, the possibility of transient overvoltages increases. Motors, capacitors, and power conversion equipment, such as variable speed drives, can be prime generators of spikes. Lightning strikes on outdoor transmission lines also cause extremely hazardous high-energy transients. If you are taking measurements on electrical systems, these transients are "invisible" and largely unavoidable hazards. They occur regularly on low-voltage power circuits, and can reach peak values of many thousands of volts. In these cases, you are dependent for protection on the safety margin already built into your meter. The voltage rating alone will not tell you how well that meter was designed to survive high transient impulses.
Early clues about the safety hazard posed by spikes came from applications involving measurements on the supply bus of electric commuter railroads. The nominal bus voltage was only 600 V, but multimeters rated at 1,000 V lasted only a few minutes when taking measurements while the train was operating. A close look revealed that the train stopping and starting generated 10,000-volt spikes. These transients had no mercy on early multimeter input circuits. The lessons learned through this investigation led to significant improvements in multimeter input protection circuits.
For a number of years, the industry used IEC 348 in designing equipment. That standard has been replaced by IEC 1010. While well-designed IEC 348 meters have been used for years by technicians and electricians, the fact is that meters designed to the new IEC 1010 standard offer a significantly higher level of safety. Let us see how this is accomplished.
The most important single concept to understand about the new standards is the Overvoltage Installation Category system. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. (see Figure 21). The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (AC resistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher energy transients. Thus, a multimeter designed to a CAT III standard is resistant to much higher energy transients than one designed to CAT II standards.
Figure 21: Overvoltage Installation Categories
Within a category, a higher voltage rating denotes a higher transient withstand rating; e.g., a CAT III-1,000 V meter has superior protection compared to a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1,000 V rated meter thinking that it is superior to a CAT III-600 V meter.
Refers to the "origin of installation"; i.e., where low-voltage connection is made to utility power.
Includes electricity meters, primary overcurrent protection equipment, outside and service entrances, service drops from pole to building, the run between meter and panel, overhead line to a detached building, and an underground line to a well pump.
Three-phase distribution, including single-phase commercial lighting equipment used in fixed installations, such as switchgear and polyphase motors,. for the bus and feeder in industrial plants, feeders and short branch circuits, distribution panel devices, lighting systems in larger buildings, and appliance outlets with short connections to service
Single-phase receptacle connected loads
Appliance, portable tools, and other household and similar loads.
Used in outlet and long branch circuits, outlets at more than 10 meters (30 feet) from a CAT III source, amd outlets more that 20 meters (60 feet) from CAT IV source.
Used in protected electronic equipment, equipment connected to (source) circuits in which measures are taken to limit transient overvoltages to an appropriately low level.
Any high-voltage, low-energy source derived from a high winding resistance transformer, such as the high-voltage section of a copier.
As shown in Figure 21, a technician working on office equipment in a CAT I location could actually encounter DC voltages much higher than the power line AC voltages measured by the motor electrician in the CAT III location. Yet transients in CAT I electronic circuitry, whatever the voltage, are clearly a lesser threat because the energy available to an arc is quite limited. This does not mean that there is no electrical hazard present in CAT I or CAT II equipment, of course. It just means that the primary hazard is electric shock, not transients and arc blast.
To cite another example, an overhead line run from a house to a detached workshed might be only 120 V or 240 V, but it is still technically CAT IV. Why? Any outdoor conductor is subject to very high-energy lightning-related transients. Even conductors buried underground are CAT IV because, although they will not be directly struck by lightning, a lightning strike nearby can induce a transient due to the presence of high electromagnetic fields.
When it comes to Overvoltage Installation Categories, the rules of real estate apply: location is what really matters.
How can you tell if a meter is a genuine CAT III or CAT II meter? Unfortunately, it is not always easy.Beware of wording such as "designed to meet specifications." Designers plans are never a substitute for an actual independent test. Also it is possible for a manufacturer to self-certify that its meter is CAT II or CAT III without any independent verification. The IEC (International Electrotechnical Commission) develops and proposes standards, but it is not responsible for enforcing those standards.Independent testing is the key to safety compliance.
Look for the symbol and listing number of an independent testing lab, such as UL, CSA, TV (Figure 22), or another recognized approval agency. These symbols can only be used if the product successfully completed testing to the agencys standards, which are based on national/international standards. UL 3111, for example, is based on IEC 1010. In an imperfect world, that is the closest you can come to ensuring that the multimeter you choose was actually tested for safety.
Figure 22: Independent Testing Lab Symbols
What does the CE symbol indicate? A product is marked CE (Conformit Europenne) to indicate its conformance to certain essential requirements concerning health, safety, environment, and consumer protection established by the European Commission and mandated through the use of "directives." There are directives affecting many product types, and products from outside the European Union cannot be imported and sold there if they do not comply with applicable directives. Compliance with the directives can be achieved by proving conformance to a relevant technical standard, such as IEC 1010 for low-voltage products. Manufacturers are permitted to self-certify that they have met the standards, issue their own Declaration of Conformity, and mark the product "CE." The CE mark is not, therefore, a guarantee of independent testing.
Here is what could happen:
A lightning strike (Figure 23) causes a transient on the power line, which in turn strikes an arc between the input terminals inside the meter. The circuits and components to prevent this event have just failed or were missing. Perhaps it was not a CAT III rated meter. Whatever the reason, the result is a direct short between the two measurement terminals through the meter and the test leads.
Figure 23: Lightning-Generated ArcA high fault current, possibly measuring several thousands of amps, flows in the short circuit just created. This happens in thousandths of a second. When the arc forms inside the meter, a very high-pressure shock wave might cause a loud bang!very much like a gunshot or the backfire from a car. At the same instant, the tech sees bright blue arc flashes at the test lead tips and the fault currents superheat the probe tips, which start to burn away, drawing an arc from the contact point to the probe. The natural reaction is to pull back in order to break contact with the hot circuit. However, as the techs hands are pulled back, an arc is drawn from the motor terminal to each probe. If these two arcs join to form a single arc, there will be another direct phase-to-phase short, this time directly between the motor terminals.
This arc can have temperatures approaching 6,000C (10,000F), which is higher than the temperature of an oxy-acetylene cutting torch! As the arc grows, fed by available short-circuit current, it superheats the surrounding air. Both a shock blast and a plasma fireball are created. If the technician is lucky, the shock blast blows him away and removes him from the proximity of the arc; though injured, his life is saved. In the worst case, the victim is subjected to fatal burn injuries from the fierce heat of the arc or plasma blast.
In addition to using a multimeter rated for the appropriate Overvoltage Installation Category, anyone working on live power circuits should be protected with flame-resistant clothing, wear safety glasses or, better yet, a safety face shield, and use insulated gloves.
Transients are not the only source of possible short circuits and arc blast hazards. One of the most common misuses of handheld multimeters can cause a similar chain of events. Suppose a user is making current measurements on signal circuits (Figure 24). The procedure is to select the amps function, insert the leads in the mA or amps input terminals, open the circuit, and take a series measurement. In a series circuit, current is always the same. The input impedance of the amps circuit must be low enough so that it does not affect the series circuits current. The input impedance on the 10 A terminal of a Fluke meter is .01 Ω. Compare this with the input impedance on the voltage terminals of 10 MΩ (10,000,000 Ω).
Figure 24: Multimeter Setup IncorrectIf the test leads are left in the amps terminals and then accidentally connected across a voltage source, the low input impedance becomes a short circuit! It does not matter if the selector dial is turned to volts; the leads are still physically connected to a low-impedance circuit. That is why the amps terminals must be protected by fuses. Those fuses are the only things standing between an inconvenience - blown fuses - and a potential disaster.
Use only a multimeter with its amps inputs protected by high-energy fuses. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer. These fuses are rated at a voltage and with a short-circuit interrupting capacity designed for your safety.
The general rule of thumb is that the closer you are to the power source, the higher the category number, and the greater the potential danger from transients.
It also follows that the greater the short-circuit current available at a particular point, the higher the CAT number.
Another way of saying the same thing is the greater the source impedance, the lower the CAT number. Source impedance is simply the total impedance, including the impedance of the wiring, between the point where you are measuring and the power source. This impedance is what dampens transients.
Finally, if you have any experience with the application of TVSS (transient voltage surge suppression) devices, you understand that a TVSS device installed at a panel must have higher energy-handling capacity than one installed right at the computer. In CAT terminology, the panel board TVSS is a CAT III application, while the computer is a receptacle-connected load and, therefore, a CAT II installation.
As you can see, the concept of categories is not new and exotic. It is simply an extension of the same common-sense concepts that people who work with electricity professionally apply every day.
There is one scenario that sometimes confuses people trying to apply categories to real world applications. In a single piece of equipment, there is often more than one category. For example, in office equipment, from the 120 V/240 V side of the power supply back to the receptacle is CAT II. The electronic circuitry, on the other hand, is CAT I. In building control systems, such as lighting control panels, or industrial control equipment, such as programmable controllers, it is common to find electronic circuits (CAT I) and power circuits (CAT III) existing in close proximity.
What do you do in these situations? As in all real-world situations, use common sense. In this case, that means using the meter with the higher category rating. In fact, it is not realistic to expect people to be going through the category-defining process all the time. What is realistic, and highly recommended, is to select a multi-meter rated to the highest category in which it could possibly be used. In other words, it is always best to error on the side of safety.
Table 5 can help us understand an instruments true voltage-withstand rating. Test values for 50 V/150 V/300 V are not included.
When is 600 V more than 1000 V?
Within a category, a higher "working voltage" (steady-state voltage) is associated with a higher transient, as would be expected. For example, a CAT III 600 V meter is tested with 6,000 V transients while a CAT III 1,000 V meter is tested with 8,000 V transients. So far, so good.
What is not as obvious is the difference between the 6,000 V transient for CAT III 600 V and the 6,000 V transient for CAT II 1,000 V. They are not the same. This is where the source impedance comes in. Ohms law (Amps = Volts/Ohms) tells us that the 2 ÃƒÅ½Ã‚Â© test source for CAT III has six times the current of the 1 ÃƒÅ½Ã‚Â© test source for CAT II.
The CAT III 600 V meter clearly offers superior transient protection compared to the CAT II 1,000 V meter, even though its so-called "voltage rating" could be perceived as being lower. It is the combination of the steady-state voltage (called the working voltage), and the category that determines the total voltage withstand rating of the test instrument, including the all-important transient voltage withstand rating.
A note on CAT IV: Test values and design standards for Category IV voltage testing are addressed in IEC 1010 second edition.
In addition to being tested to an actual overvoltage transient value, multimeters are required by IEC 1010 to have minimum "creepage" and "clearance" distances between internal components and circuit nodes. Creepage measures distance across a surface. Clearance measures distances through the air. The higher the category and working voltage level, the greater the internal spacing requirements are. One of the main differences between the old IEC 348 and IEC 1010 is the increased spacing requirements in the latter.
If you are faced with the task of replacing your multimeter, do one simple task before you start shopping: analyze the worst-case scenario of your job and determine what category your use or application fits into.
First choose a meter rated for the highest category you could be working in. Then, look for a multimeter with a voltage rating for that category matching your needs. While you are at it, do not forget the test leads. IEC 1010 applies to test leads too: they should be certified to a category and voltage as high as or higher than the meter. When it comes to your personal protection, do not let test leads be the weak link.
Figure 25: Synchronous Motor
Figure 26: Wound Rotor MotorThe synchronous machine uses these components to create a DC magnetic field on the rotor. This allows for constant speed characteristics from no-load to full-load condition on the synchronous machine. This differs from an induction motor in that, from no-load to full-load, the induction motor slows down. The wound rotor motor, however, uses brushes and slip-rings to connect resistors to a specially designed three-phase rotor circuit. This design enables the wound rotor motor to vary in torque and speed for a given load condition.