Principles And Testing Methods Of Earth Ground Resistance.docx

Principles and testing methods of earth ground resistance August 12th, 2014, Published in Articles: Vector Information from Comtest 137

Poor grounding contributes to downtime but a lack of good grounding is also dangerous and increases the risk of equipment failure. Over time, corrosive soils with high moisture and salt content and high temperatures can degrade ground rods and their connections. So, although the ground system had low earth ground resistance values when initially installed, the resistance of the grounding system can increase if the ground rods are corroded. Grounding testers are indispensable troubleshooting tools to help you maintain uptime. It is recommended that all grounds and ground connections be checked at least annually as a part of your normal predictive maintenance plan. Should an increase in resistance of more than 20% be measured during these periodic checks, the technician should investigate the source of the problem and make the correction to lower the resistance by replacing or adding ground rods to the ground system. What is a ground? The US National Electrical Code (NEC) Article 100 defines a ground as: “a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth”. Grounding actually encompasses two different subjects: earth grounding and equipment grounding. Earth grounding is an intentional connection from a circuit conductor, usually the neutral, to a ground electrode placed in the earth. Equipment grounding ensures that operating equipment within a structure is grounded properly. These two grounding systems must be kept separate except for connections between the two systems. This prevents differences in voltage potential from a possible flashover from lightning strikes. The purpose of a ground is to provide a safe path for the dissipation of fault currents, lightning strikes, static discharges, EMI and RFI signals and interference.

The US National Fire Protection Agency (NFPA) and Institute of Electrical and Electronics Engineers (IEEE) recommend a ground resistance value of 5 or less. The goal in ground resistance is to achieve the lowest ground resistance value possible that makes sense economically and physically. What affects the grounding resistance? Four variables affect the ground resistance of a ground system: length or depth of the ground electrode; the diameter of the ground electrode; the number of ground electrodes and ground system design. Length/depth of the ground electrode Driving ground electrodes deeper is a very effective way to lower ground resistance. Soil is not consistent in its resistivity and can be unpredictable. The resistance level can generally be reduced by an additional 40% by doubling the length of the ground electrode. It is sometimes impossible to drive ground rods deeper – in areas composed of rock, for instance. In these cases, alternative methods including grounding cement are viable. Diameter of the ground electrode Increasing the diameter of the ground electrode has very little effect in lowering the resistance. For example, you could double the diameter of a ground electrode and your resistance would only decrease by 10%. Number of ground electrodes Using multiple ground electrodes provides another way to lower ground resistance. More than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods must be at least equal to the depth of the driven rod. The ground electrodes’ spheres of influence will intersect and the resistance will not be lowered without proper spacing. Table 1 provides various ground resistances which can be used as a rule of thumb.

Type of soil

Table 1: Ground resistances for use as a rule of thumb. Earthing resistance Soil resistivity Ground electrode depth Earthing strip (metre) RE (metre) ΩM 3 6 10 5 10 20

Very moist soil, swamplike Farming soil loamy and clay soils

30

10

5

3

12

6

3

100

33

17

10

40

20

10

Sandy clay soil Moist sandy soil Concrete 1:5 Moist gravel Dry sandy soil Dry gravel Stoney soil Rock

150 300 400 500 1000 1000 30 000 107

50 66 – 160 330 330 1000 –

25 33 – 80 165 165 500 –

15 20 – 48 100 100 300 –

60 80 160 200 400 400 1200 –

30 40 80 100 200 200 600 –

15 20 40 50 100 100 300 –

Ground system design Simple grounding systems consist of a single ground electrode driven into the ground. The use of a single ground electrode is the most common form of grounding. Complex grounding systems consist of multiple ground rods, connected, mesh or grid networks, ground plates, and ground loops. These systems are typically installed at power generating substations, central offices, and cellphone tower sites. Complex networks dramatically increase the amount of contact with the surrounding earth and lower ground resistances. Soil resistivity measurement Soil resistivity is necessary when determining the design of the grounding system for new installations (green field applications) to meet your ground resistance requirements. Ideally, you would find a location with the lowest possible resistance. Poor soil conditions can be overcome with more elaborate grounding systems. The soil composition, moisture content and temperature all impact soil resistivity. Soil is rarely homogenous and its resistivity will vary geographically and at different depths. Moisture content changes seasonally, varies according to the nature of the sublayers of earth and the depth of the permanent water table. It is recommended that the ground rods be placed as deep as possible into the earth as soil and water are generally more stable at deeper strata. Calculating soil resistivity The measuring procedure described here uses the Wenner method and uses the formula: ρ=2πAR where: ρ = the average soil resistivity to depth A in: ohm-cm. π = 3,1416. A = the distance between the electrodes in cm.

R = the measured resistance value in ohm from the test instrument. Measuring soil resistance To test soil resistivity, connect the ground tester as shown in Fig. 1. Four earth ground stakes are positioned in the soil in a straight line, equidistant from one another. The distance between earth ground stakes should be at least three times greater than the stake depth. The Fluke1625 earth ground tester generates a known current through the two outer ground stakes and the drop in voltage potential is measured between the two inner ground stakes. The tester automatically calculates the soil resistance using Ohm’s Law (V=IR).

Fig. 1: Test current paths in the stakeless method.

Additional measurements, where the stake’s axes are turned 90°, are always recommended because measurement results are often distorted and invalidated by underground metal, underground aquifers etc. A profile is produced that can determine a suitable ground resistance system by changing the depth and distance several times. Soil resistivity measurements are often corrupted by the existence of ground currents and their harmonics. Fall-of-potential measurement The fall-of-potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site. The earth electrode of interest must be disconnected. The tester is then connected to the earth electrode. Then, two earth stakes are placed in the soil in a direct line – away from the earth electrode, for the 3-pole fall of potential test. Spacing of 20 m is normally sufficient. Placing the stakes It is essential that the probe be placed outside the sphere of influence of the ground electrode under test and the auxiliary earth to achieve the highest degree of accuracy when performing a 3pole ground resistance test or the effective areas of resistance will overlap and invalidate any measurements. Table 2 is a guide for setting the probe (inner stake) and auxiliary ground (outer stake). Reposition the inner stake (probe) 1 m in either direction and take a fresh measurement to test the accuracy of the results and to ensure that the ground stakes are outside the spheres of influence. If there is a significant change in the reading (30%), you should increase the distance between the ground rod under test, the inner stake (probe) and the outer stake (auxiliary ground) until the measured values remain fairly constant when repositioning the inner stake (probe). Stakeless measurement The Fluke 1625 earth ground tester can measure earth ground loop resistances for multi grounded systems using only current clamps. This test technique eliminates the dangerous step of disconnecting parallel grounds, as well as the process of finding suitable locations for auxiliary ground stakes. You can also perform earth ground tests in places you have not considered before: inside buildings, on power pylons or anywhere you don’t have access to soil. With this test method, two clamps are placed around the earth ground rod or the connecting cable and each is connected to the tester (see Fig. 2). Earth ground stakes are not used at all. A known voltage is induced by one clamp, and the current is measured using the second clamp. The tester automatically determines the ground loop resistance at this ground rod. If there is only one path to ground, the stakeless method will not provide an acceptable value and the fall-of-potential test method must be used.The earth ground tester works on the principle that in parallel/multi-

grounded systems, the net resistance of all ground paths will be extremely low compared to any single path (the one under test). So, the net resistance of all theparallel return path resistances is effectively zero. Stakeless measurement only measures individual ground rod resistances in parallel to earth grounding systems. If the ground system is not parallel to earth, you will either have an open circuit or be measuring ground loop resistance.

Fig. 2: Setup for the stakeless method.

Ground impedance measurements When attempting to calculate possible shortcircuit currents in power plants and other highvoltage/current situations, determining the complex grounding impedance is important since the impedance will be made up of inductive and capacitive elements. Because inductivity and resistivity are known in most cases, actual impedance can be determined using a complex computation. Since impedance is frequency dependent, the Fluke 1625 uses a 55 Hz signal for this calculation to be as close to voltage operating frequency as possible. This ensures that the measurement is close to the value at the true operating frequency. Power utility technicians testing high voltage transmission lines are interested in two things.The ground resistance in case of a lightning strike and the impedance of the entire system in case of a short circuit on a specific point in the line. Short circuit in this case means an active wire breaks loose and touches the metal grid of a tower. At central offices When conducting a grounding audit of a central office there are three different measurements required. Before testing, locate the master ground bar (MGB) within the central office to determine the type of grounding system. The MGB will have ground leads connecting to the multi-grounded neutral (MGN) or incoming service, the ground field, water pipe and structural or building steel (see Fig. 3).

Fig. 3: The layout of a typical central office. First, perform the stakeless test on all the individual grounds coming from the MGB (see Fig. 4). The purpose is to ensure that all the grounds are connected, especially the MGN. It is important to note that you are not measuring the individual resistance, but the loop resistance of what you are clamped around. Connect the earth ground tester and both the inducing and sensing clamps, which are placed around each connection to measure the loop resistance of the MGN, ground field, water pipe and the building steel. Second, perform the 3-pole fall-of-potential test of the entire ground system, connecting to the MGB (see Fig 5). To get to remote earth, many phone companies use unused cable pairs going out as much as a mile. Record the measurement and repeat this test at least annually.

Fig. 4: Stakeless testing of a central office. Thirdly, measure the individual resistances of the ground system using the selective test of the earth ground tester (see Fig. 6). Connect the tester. Measure the resistance of the MGN; the value is the resistance of that particular leg of the MGB. Then measure the ground field. This reading is the actual resistance value of the central office ground field.

Now move on to the water pipe and repeat for the resistance of the building steel.You can easily verify the accuracy of these measurements through Ohm’s Law. The resistance of the individual legs, when calculated, should equal the resistance of the entire system given (allow for reasonable error since all ground elements may not be measured).

Fig. 6: Measure the individual resistances of the ground system using the selective test. These test methods provide the most accurate measure of central offices because it gives you the individual resistances and their actual behaviour in a ground system. Although accurate, the measurements would not show how the system behaves as a network because, in the event of a lightning strike or fault current, everything is connected. Additional tests First, perform the 3-pole fall-of-potential test on each leg off the MGB and record each measurement. Using Ohm’s Law again, these measurements should be equal to the resistance of the entire system. From the calculations you will see that you are between 20 and 30% off the total RE value. Table 2: A guide to setting the inner and outer stakes. Depth of the ground Distance to the inner stake Distance to the outer stake electrode 2m 15 m 25 m 3m 20 m 30 m 6m 25 m 40 m

10 m

30 m

50 m

Finally, measure the resistances of the various legs of the MGB using the selective stakeless method. It works like the stakeless method, but differs in the way we use the two separate clamps. We place the inducing voltage clamp around the cable going to the MGB and, since the MGB is connected to the incoming power, which is parallel to the earth system, we have achieved that requirement. Place the sensing clamp around the ground cable leading out to the ground field. When we measure the resistance, this is the actual resistance of the ground field plus the parallel path of the MGB. Because it should be very low ohmically, it should have no real effect on the measured reading. This process can be repeated for the other legs of the ground bar such as water pipe and structural steel. To measure the MGB via the stakeless selective method, place the inducing voltage clamp around the line to the water pipe (since the copper water pipe should have very low resistance) and your reading will be the resistance for only the MGN.

How is a Megger tester used? A: Quick Answer

An insulation resistance tester (commonly called a Megger after a prominent company that manufactures them) is operated by hooking it up to a piece of electrical equipment and running voltage across the insulation on the equipment. The Megger calculates how well the electrical insulation on the equipment is working by checking how much current is flowing between the Megger to the machine. The higher the resistance, the less current will flow, amounting to better electrical insulation shielding. This is important, because poor insulation leads to equipment breaking down. Continue Reading

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To test for insulation resistance, make sure to isolate the equipment from any other sources of current. Turn off the equipment, and disconnect it from other components. If it is not possible to completely isolate the equipment, observe what other conducting parts it is connected to, such as cables or wires, and be aware that the insulation resistance rating will be lower than due to these other components. Ground and discharge the equipment to prevent sparks or electrocution both before and after testing. Attach two leads from the Megger across the equipment's insulation, with one lead on one side and one on the opposite side, and run the voltage on the Megger for at least one minute. The device will calculate the resistance using Ohm's Law and give the resistance rating in megaOhms. Learn more about Electrical Sources: ecmweb.com engineersedge.com openelectrical.org Ads Voltage tester tietzsch.de 1000 Volt / CAT IV voltage tester -- up to 36 000 Volt AC/DC high voltage tester Moisture measurement www.muetec.de Manufacturer of moisture measuring technology for bulk materials

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eHow Home Home Repair & Maintenance Electrical Repairs How to Test Cable With a Megger

How to Test Cable With a Megger By Eric W. Thompson eHow Contributor 

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The megohmmeter, also known as a "megger," reads high voltage resistance measures in electrical circuits and motors. It is more commonly used in industrial electrical installations than the ohmmeter because it is more accurate in detecting subtle anomalies in high voltage circuits. This is due in part because the megohmmeter produces higher voltages than the ohmmeter. Megging an electrical circuit is simple enough to do yourself, though you should be very careful to follow the safety procedures exactly as instructed. Things You'll Need  

Electrical wires Red warning tape



Yellow caution tape









Flip the breaker off for the circuit you will be working on and tape the breaker shut with red warning tape. This ensures that no one will turn the breaker on while you are working on the circuit. Apply yellow caution tape around the end of the circuit that you will be working on; then detach the other end of the circuit from the device it is intended to feed. You will be adding voltage to the circuit with the megger, so the yellow caution tape will keep people from getting too close to the exposed wire. Place one megger lead to the exposed wire and the other lead to the system ground. This can be any metal surface that is touching the grounding bolt to which the green grounding cable is attached. Crank the megger and read the meter. A reading between 2 and 1,000 megohms is acceptable; anything less than 2 megohms means that you have a problem with the insulation.

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References 

"Electrician's Pocket Manual": (Pocket References (McGraw-Hill)); Rex Miller; 2005

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In order to measure the insulation resistance in electrical wiring and electrical motors, electricians use the megohmmeter, also known as a megger. It differs from an ohmmeter in that, unlike the ohmmeter, it is able to measure resistances in ohm values of more than one million. This capacity is practical when measuring resistance in high-voltage situations, because accurate resistance measurement requires that you provide a higher voltage than can supplied by the ohmmeter. Things You'll Need  

A megohmmeter Electrical wires



Caution tape

    

Shut off all power connected to the cables you are testing. Disconnect the cables from the power source. Disconnect the cables from the device for which they are intended to supply power. Remove any wire nuts or tape used to temporarily cap the cables. Establish a safety barrier to keep anyone from touching the bare copper on the ends of the cables, as your megger will be supplying high voltage during the testing period. Touch either of the megohmmeter's leads to the ground being used for that electrical system. For example, if the system you were testing included a motor control cabinet, you would touch a lead to the cabinet itself, since it is grounded. Touch the other lead to the bare copper wire of the cable you are testing.

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Turn on and read the meter. Anything reading between 2 megohms and 1000 megohms is usually considered a good reading, unless other problems have been noted. Anything less than 2 megohms indicates an insulation problem.

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A megger is a type of megohmmeter used for testing electrical insulation in industrial settings. Although a conventional ohmmeter can perform well in most commercial settings, it is incapable of handling the high-voltage capacity of the megger. This is due to the fact that meggers can supply the voltage required to test the large conductors necessary for high-voltage circuits. While megging a circuit is quite easy, the safety issues at hand must be properly addressed, as any major mistakes could result in electrocution. Things You'll Need  

Electrical wires Padlock

 

Yellow caution tape Flat-head screwdriver



Cut off power to the circuit to be tested by flipping the breaker and locking it off with a padlock. Surround the work area with yellow warning tape to protect anyone from accidentally touching the bare end of the cable. Touch one megger probe to the bare end of the cable, then touch the other probe to the grounding screw of the system in which the cable will terminate. Turn the megger handle manually and look for a rendering between 2 and 1,000 megohms. Insulation issues are indicated by any reading that is below 2 megohms. Terminate the bare wire into its intended device using a flat-head screwdriver. Remove the yellow warning tape and the padlock. Restore the power to the circuit.

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Megger Group Limited is a manufacturer of electric test equipment and measuring instruments for electrical power applications. Although all of its products are manufactured in the United States and England (Dallas, Texas; Valley Forge, Pennsylvania; and Dover, England), Megger is an international company with sales and technical offices throughout the world. Established in 1903, Megger offers more than 1,000 products for a wide array of uses, including cable fault locating, protective relay testing, power quality testing, circuit breaker testing, insulation testing and telecommunications and data integrity testing.

ISO 9001 

According to the Megger website, every Megger facility is ISO 9001 certified. Established by the International Organization for Standardization, ISO 9000 is a family of standards for "quality management systems." ISO 9001 is the only standard in the 9000 family that can be certified. ISO 9001 standards include maintaining records that

document where raw materials and products were processed, planning stages of development for new products and testing those products at each stage and establishing formal procedures for situations dealing with non-conformance.

ISO 14001 

Megger's facility in Dover, England, is also ISO 14001 certified. ISO 14000 is a family of standards for environmental management systems. ISO 14001 lists requirements that organizations need to follow to minimize any negative impact on the environment during production. ISO 14001 requirements include identifying and controlling the environmental impact of an organization's activities, products and services; improving the environmental performance of an organization's activities, products and services; and implementing a systematic approach to settling environmental objectives and targets; achieving these goals and demonstrating that they have been achieved. ISO 14004 establishes the guidelines to accomplish the requirements of ISO 14001.

Auditing 

According to the Megger website, the organization continually strives to improve the quality of its products by periodic audits performed by independent experts. To be ISO 9001 certified, an organization must undergo regular audits and meetings to evaluate the conformance and effectiveness of its systems. By identifying problems and keeping a record of these checks, Megger is able to adjust and improve upon its facilities.

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References   

Megger.com: About International Organization for Standardization: ISO 9000 essentials International Organization for Standardization: ISO 14000 essentials

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International Organization for Standardization

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Megohmmeters, or meggers, are testing meters used to measure the current leakage from an electrical cable by means of administering an insulation-resistance test. A megger sends voltage through a cable and measures any current leakage in megohms (1,000 ohms). Routine megger tests of aged electrical cable is a prudent way to ensure your circuits are safe from arc faults that can cause circuits to be blown, and safe from fire or electrical shock. Properly interpreting megohmmeter readings of an insulation-resistance test can help you identify faulty or damaged insulation that can lead to these problems. Things You'll Need 

Megohmmeter



Perform a time-resistance testing method using a megger, and take successive readings at specific intervals. A time-resistance test is performed over five to 10 minutes. Record the results at 60 seconds, and then in 30-second intervals thereafter.

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Compare the results of each reading. The time-resistance method is based on absorption. Good insulation allows for an increase of resistance (ohms) over time, showing the charge effect over a time period longer than the capacitance of the insulation. Damaged or contaminated insulation will will mask absorption with current leakage, keeping resistance low, identifying a potential problem. If the megger reads a resistance under 1 (1,000 ohms) on your meter after the initial 60second interval, the cable has failed and the cable should be removed. If the megger reads a resistance between 1-1.25 on your meter, then the cable passes. Any reading above 1.25 is considered excellent. Continue to monitor the megger over 30-second intervals. If the resistance reading continues to increase, the cable insulation is in excellent condition. If the resistance levels out, that could be the sign that insulation is failing.

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References  

AEMC.com: Tech Info: Understanding Insulation Resistance Testing ECM: Understanding Insulation Resistance

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Photo CreditAblestock.com/AbleStock.com/Getty Images

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eHow Home Home Repair & Maintenance Electrical Repairs How to Test Electric Motor Windings

How to Test Electric Motor Windings By Daniel Ray eHow Contributor 

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When a bad motor winding cannot be found by a physical inspection, many times an ohmmeter may show that there is a defective winding. Most repair shops will use a "megger" or a ground insulation tester. The megger will help to detect areas of deteriorating insulation where the windings may soon short. Most individuals do not own a megger, but probably do have an ohmmeter. While the ohmmeter may not show potential failures, it can be used to determine if the windings are good or bad. Things You'll Need 

Volt ohmmeter with probes

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Common hand tools

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Remove all power from the motor and isolate the circuit. Tag the circuit breaker to prevent accidental restoration of power. Open motor housing to access windings. Clean the windings and motor interior with shop air and inspect for any damage. Check winding insulation for cracks or brittleness. Connect the probes to the ohmmeter and set the meter to a midrange setting. Touch the two leads together and make sure that the meter indicates continuity, or zero ohms. Consult the motor schematic or a diagram of the motor's windings and use the meter to measure the resistance of the windings. Each winding leg should have some small resistance reading. The meter should not show an "open" or a "short" when a winding leg is measured. Use the schematic and take readings of all of the windings. Write down each measurement and make comparisons when measurements are complete.

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Photo CreditAll images and illustrations by Dan Ray

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eHow Arts & Entertainment Music Other Music How to Use an Ohmmeter to Test an Amp

How to Use an Ohmmeter to Test an Amp By Serm Murmson eHow Contributor 

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An amp circuit can malfunction in a number of places. If individual components -- such as resistors, transformers or the amp's speakers -- are broken, the circuit may not be complete, and an ohmmeter can help you identify the location of the problem. An ohmmeter measures the resistance between two points in a circuit. If two points are electrically common, they will ideally have no resistance. If the circuit is broken, the circuit will have infinite resistance. If you know the expected resistance between two points, you will be able to locate the problematic parts of your amp's circuit.  

Turn off the amp. The ohmmeter supplies its own test current. Turn your ohmmeter to the range of resistance you expect to find between two points. For example, if you expect to find a resistance in the thousands of ohms, set the ohmmeter to the kilo-ohms scale.

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Touch the leads of your ohmmeter together. The ohmmeter should read zero. If it doesn't, turn the zero dial so that the ohmmeter reads zero when the leads are together.

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Connect the two leads across the component you wish to measure. The polarity of the leads does not matter, as resistance does not depend on direction. Observe the reading on your ohmmeter. If you see a reading of "OL" or a "1" in the far left on a digital multimeter, your scale is too low for the present reading. This indication can vary depending on your ohmmeter model. In this case, turn the dial on your ohmmeter to a larger scale. Connect the leads of your ohmmeter between two points that are electrically common. The resistance between these points should be extremely low. If the reading on your ohmmeter is infinite, your circuit is broken between these two points.

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Tips & Warnings 

Some components, such as transistors, only conduct under certain conditions. Make sure you understand how each component functions so that you know what to expect when using your ohmmeter.



Capacitors can store large voltages even when your amplifier is turned off. If you need to test a capacitor, discharge it first by shorting it with an insulated screwdriver.

References   

Cleveland Institute of Electronics: Analog Multimeter Basics and Measuring Resistance Northwestern University: Using a Multimeter St. Lawrence University: Using a Digital Multimeter

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eHow Home Home Repair & Maintenance Electrical Repairs How to Test Megger Motors

How to Test Megger Motors By Eric W. Thompson eHow Contributor 

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A Megger is the brand name of a special megohmmeter capable of detecting very small resistance disturbances in high-voltage electrical circuits. It is most commonly used in industrial installations with multi-phase power supplies. It can give an accurate reading of the measure of resistance and continuity available in a high-voltage circuit and electrical motor. Using the Megger to test a three-phase motor is rather easy, though the safety hazards for this procedure are quite high. Be sure to follow good safety measures to prevent electrical shock. This is important, because the high-voltage current in three-phase systems is far more lethal than a mere low-voltage shock received in residential setting. In other words, one brief exposure to this kind of electrical current is enough to kill you. Things You'll Need  

Megger Padlock

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Flathead screwdriver

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Locate the circuit that feed the motor, and turn it off. Secure the breaker with a lock to preclude the breaker from being switch back on. Take the cover from the off using a flathead screwdriver. Remove the leads from the motor with the flathead screwdriver. Place one of the Megger leads on the end of either of the wires, and place the other Megger lead anywhere on the motor's housing to evaluate grounding continuity. Turn the Megger handle and read the meter. A reading between 20 to 30 megohms indicates that the circuit and the motor are in good order. Continue the same procedure for each wire and winding.

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References 

"Electrician's Pocket Manual" (Pocket References (McGraw-Hill)); Rex Miller; 2005

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Transducer Instrumentation is the heart of industrial applications. Instrumentation is the art and science of measuring and controlling different variables such as flow, level, temperature, angle, displacement etc. A basic instrumentation system consists of various devices. One of these various devices is a transducer. A transducer plays a very important role in any instrumentation system. An electrical transducer is a device which is capable of converting the physical quantity into a proportional electrical quantity such as voltage or electric current. Hence it converts any quantity to be measured into usable electrical signal. This physical quantity which is to be measured can be pressure, level, temperature, displacement etc. The output which is obtained from the transducer is in the electrical form and is equivalent to the measured quantity. For example, a temperature transducer will convert temperature to an equivalent

electrical potential. This output signal can be used to control the physical quantity or display it. Note that any device which is able convert one form of energy into another form is called as a transducer. For example, even a speaker can be called as a transducer as it converts electrical signal to pressure waves (sound). But an electrical transducer will convert a physical quantity to an electrical one.

Types of Transducer There are of many different types of transducer, they can be classified based on various criteria as: Types of Transducer based on Quantity to be Measured    

Temperature transducers (e.g. a thermocouple) Pressure transducers (e.g. a diaphragm) Displacement transducers (e.g. LVDT) Flow transducers

Types of Transducer based on the Principle of Operation       

Photovoltaic ( e.g. a solar cell ) Piezoelectric Chemical Mutual Induction Electromagnetic Hall effect Photoconductors

Types of Transducer based on Whether an External Power Source is required or not

Active Transducer Active transducers are those which do not require any power source for their operation. They work on the energy conversion principle. They produce an electrical signal proportional to the input (physical quantity). For example, a thermocouple is an active transducer.

Passive Transducers Transducers which require an external power source for their operation is called as a passive transducer. They produce an output signal in the form of some variation in resistance, capacitance or any other electrical parameter, which than has to be converted to an equivalent current or voltage signal. For example, a photocell (LDR) is a passive transducer which will vary the resistance of the cell when light falls on it. This change in resistance is converted to proportional signal with the help of a bridge circuit. Hence a photocell can be used to measure the intensity of light.

Above shown is a figure of a bonded strain gauge which is a passive transducer used to measure stress or pressure. As the stress on the strain gauge increases or decreases the strain gauge bends or compresses causing the resistance of the wire bonded on it to increase or decrease. The change in resistance which is equivalent to the change in stress is measured with the help of a bridge. Hence stress is measured.

Transducer From Wikipedia, the free encyclopedia This article is about an engineering device. For the similarly named concept in computer science, see Finite state transducer.

A transducer is a device that converts one form of energy to another. Usually a transducer converts a signal in one form of energy to a signal in another.[1] Transducers are often employed at the boundaries of automation, measurement, and control systems, where electrical signals are converted to and from other physical quantities (energy, force, torque, light, motion, position, etc.). The process of converting one form of energy to another is known as transduction.[2]

Contents 

    

1 Transducer types o 1.1 Active o 1.2 Passive o 1.3 Sensors o 1.4 Actuators o 1.5 Bidirectional 2 Ideal characteristics 3 Applications 4 See also 5 References 6 External links

Transducer types Active

Active sensors require an external source which is called an excitation signal. The signal is modulated by the sensor to produce the output signal. it works without an external source . It converts the physical or mechanical form of energy to electrical energy without an external source.[3][2] Passive

Passive sensors generate electric signals in response to an external stimulus without the need of an additional energy source. Such examples are a thermocouple, photodiode, and a piezoelectric sensor.[4] Sensors

A sensor is a device that receives and responds to a signal or stimulus.[5] Transducer is the other term that is sometimes interchangeably used instead of the term sensor, although there are subtle differences. A transducer is a term that can be used for the definition of many devices such as sensors, actuators, or transistors.[6][2] Actuators

An actuator is a device that is responsible for moving or controlling a mechanism or system. It is operated by a source of energy, which can be mechanical force, electrical current, hydraulic fluid pressure, or pneumatic pressure, and converts that energy into motion. An actuator is the mechanism by which a control system acts upon an environment. The control system can be simple (a fixed mechanical or electronic system), software-based (e.g. a printer driver, robot control system), a human, or any other input.[2]

Bidirectional

Bidirectional transducers convert physical phenomena to electrical signals and also convert electrical signals into physical phenomena. Examples of inherently bidirectional transducers are antennae, which can convert conducted electrical signals to or from propagating electromagnetic waves, and voice coils, which convert electrical signals into sound (when used in a loudspeaker) or sound into electrical signals (when used in a microphone). Likewise, DC electric motors may be used to generate electrical power if the motor shaft is turned by an external torque.[2]

Ideal characteristics    

High dynamic range[2] High repeatability Low noise Low hysteresis

Applications

Transducers are used in electronic communications systems to convert signals of various physical forms to electronic signals, and vice versa. In this example, the first transducer could be a microphone, and the second transducer could be a speaker. 

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Electromagnetic: o Antennae – converts propagating electromagnetic waves to and from conducted electrical signals o magnetic cartridges – converts relative physical motion to and from electrical signals o Tape head, disk read-and-write heads – converts magnetic fields on a magnetic medium to and from electrical signals o Hall effect sensors – converts a magnetic field level into an electrical signal Electrochemical: o pH probes o Electro-galvanic fuel cells o Hydrogen sensors Electromechanical (electromechanical output devices are generically called actuators): o Accelerometers o Air flow sensors o Electroactive polymers o Rotary motors, linear motors o Galvonometers

o o o o o o o o

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Linear variable differential transformers or rotary variably differential transformers Load cells – converts force to mV/V electrical signal using strain gauges Microelectromechanical systems Potentiometers (when used for measuring position) Pressure sensors String potentiometers Tactile sensors Vibration powered generators Electroacoustic: o Loudspeakers, earphones – converts electrical signals into sound (amplified signal → magnetic field → motion → air pressure) o Microphones – converts sound into an electrical signal (air pressure → motion of conductor/coil → magnetic field → electrical signal)[2] o Pickup (music technology) – converts motion of metal strings into an electrical signal (magnetism → electrical signal) o Tactile transducers – converts electrical signal into vibration ( electrical signal → vibration) o Piezoelectric crystals – converts deformations of solid-state crystals (vibrations) to and from electrical signals o Geophones – converts a ground movement (displacement) into voltage (vibrations → motion of conductor/coil → magnetic field → signal) o Gramophone pickups – (air pressure → motion → magnetic field → electrical signal) o Hydrophones – converts changes in water pressure into an electrical signal o Sonar transponders (water pressure → motion of conductor/coil → magnetic field → electrical signal) o Ultrasonic transceivers, transmitting ultrasound (transduced from electricity) as well as receiving it after sound reflection from target objects, availing for imaging of those objects. Electro-optical (Photoelectric): o Fluorescent lamps – converts electrical power into incoherent light o Incandescent lamps – converts electrical power into incoherent light o Light-emitting diodes – converts electrical power into incoherent light o Laser diodes – converts electrical power into coherent light o Photodiodes, photoresistors, phototransistors, photomultipliers – converts changing light levels into electrical signals o Photodetector or photoresistor or light dependent resistor (LDR) – converts changes in light levels into changes in electrical resistance o Cathode-ray tubes (CRT) – converts electrical signals into visual signals Electrostatic: o Electrometers Thermoelectric: o Resistance temperature detectors (RTD) – converts temperature into an electrical resistance signal o Thermocouples – converts relative temperatures of metallic junctions to electrical voltage o Thermistors (includes PTC resistor and NTC resistor) Radioacoustic: o Geiger-Müller tubes – converts incident ionizing radiation to an electrical impulse signal

o o

Radio receivers converts electromagnetic transmissions to electrical signals. Radio transmitters converts electrical signals to electromagnetic transmissions.

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Capacitive Transducers or Capacitive Sensors or Variable Capacitance Transducers written by: Haresh Khemani • edited by: Lamar Stonecypher • updated: 10/22/2009 The capacitive transducer is used extensively for the measurement of displacement, pressure etc. Let us see the principle of working of capacitive transducer or sensor also called as variable capacitance transducer.

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What is Capacitve Transducer? The capacitive transducer or sensor is nothing but the capacitor with variable capacitance. The capacitive transducer comprises of two parallel metal plates that are separated by the material such as air, which is called as the dielectric material. In the typical capacitor the distance between the two plates is fixed, but in variable capacitance transducers the distance between the two plates is variable. In the instruments using capacitance transducers the value of the capacitance changes due to change in the value of the input quantity that is to be measured. This change in capacitance can be measured easily and it is calibrated against the input quantity, thus the value if the input quantity can be measured directly.



Capactive Transducer or Capacitive Sensor or Variable Capacitance Transducer



Capacitance of the Capacitive Transducers The capacitance C between the two plates of capacitive transducers is given by: C = εo x εr x A/ d Where C is the capacitance of the capacitor or the variable capacitance transducer εo is the absolute permittivity εr is the relative permittivity The product of εo & εr is also called as the dielectric constant of the capacitive transducer. A is the area of the plates D is the distance between the plates It is clear from the above formula that capacitance of the capacitive transducer depends on the area of the plates and the distance between the plates. The capacitance of the capacitive transducer also changes with the dielectric constant of the dielectric material used in it. Thus the capacitance of the variable capacitance transducer can change with the change of the dielectric material, change in the area of the plates and the distance between the plates. Depending on the parameter that changes for the capacitive transducers, they are of three types as mentioned below.

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1) Changing Dielectric Constant type of Capacitive Transducers In these capacitive transducer the dielectric material between the two plates changes, due to which the capacitance of the transducer also changes. When the input quantity to be measured changes the value of the dielectric constant also changes so the capacitance of the instrument changes. This capacitance, calibrated against the input quantity, directly gives the value of the quantity to be measured. This principle is used for measurement of level in the hydrogen container, where the change in level of hydrogen between the two plates results in change of the dielectric constant of the capacitance transducer. Apart from level, this principle can also be used for measurement of humidity and moisture content of the air.

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2) Changing Area of the Plates of Capacitive Transducers

The capacitance of the variable capacitance transducer also changes with the area of the two plates. This principle is used in the torquemeter, used for measurement of the torque on the shaft. This comprises of the sleeve that has teeth cut axially and the matching shaft that has similar teeth at its periphery.

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3) Changing Distance between the Plates of Capacitive Transducers In these capacitive transducers the distance between the plates is variable, while the area of the plates and the dielectric constant remain constant. This is the most commonly used type of variable capacitance transducer. For measurement of the displacement of the object, one plate of the capacitance transducer is kept fixed, while the other is connected to the object. When the object moves, the plate of the capacitance transducer also moves, this results in change in distance between the two plates and the change in the capacitance. The changed capacitance is measured easily and it calibrated against the input quantity, which is displacement. This principle can also be used to measure pressure, velocity, acceleration etc.

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Image Source http://kalpanatamvada.tripod.com/

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CAPACITIVE TRANSDUCER The capacitive transducer is nothing but the capacitor with variable capacitance. PRINCIPLE: The capacitance of the capacitor varies as the material comes between two plates of capacitor and the presence of something is measured by capacitance. OPERATION: When voltage is applied to the conductive objects the charges get collected on each object. The moving charges causes alternating current in the space between them and current flowia measured as capacitance. ADVANTAGES:   

Construction is simple. Cost of transducer is low. It has very high sensitivity.

Inductive Transducers « Previous Next » Inductive transducers work on the principle of inductance change due to any appreciable change in the quantity to be measured i.e. measured. For example, LVDT, a kind of inductive transducers, measures displacement in terms of voltage difference between its two secondary voltages. Secondary voltages are nothing but the result of induction due to the flux change in the secondary coil with the displacement of the iron bar. Anyway LVDT is discussed here briefly to explain the principle of inductive transducer. LVDT will be explained in other article in more detail. For the time being let’s focus on basic introduction of inductive transducers. Now first our motive is to find how the inductive transducers can be made to work. This can be done by changing the flux with the help of measured and this changing flux obviously changes the inductance and this inductance change can be calibrated in terms of measured. Hence inductive transducers use one of the following principles for its working. 1. Change of self inductance 2. Change of mutual inductance 3. Production of eddy current Let’s discuss each principle one by one.

Change of Self Inductance of Inductive Transducer We know very well that self inductance of a coil is given by

Where, N = number of turns. R

= reluctance of the magnetic circuit. Also we know that reluctance R is given by

where μ = effective permeability of the medium in and around the coil. Where, G = A/l and called geometric form factor. A = area of cross-section of coil. l = length of the coil. So, we can vary self inductance by   

Change in number of turns, N, Changing geometric configuration, G, Changing permeability

For the sake of understanding we can say that if the displacement is to be measured by the inductive transducers, it should change any of the above parameter for causing in the change in self inductance.

Change of Mutual Inductance of Inductive Transducer Here transducers, which work on change of mutual inductance principle, use multiple coils. We use here two coils for the sake of understanding. Both coils have their self inductance as well. So let’s denote their self inductance by L1 and L2. Mutual inductance between these two coils is given by Thus mutual inductance can be changed by varying self inductance or by varying coefficient of coupling, K. The methods of changing self inductance we already discussed. Now coefficient of coupling depends on the distance and orientation between two coils. Thus for the measurement of displacement we can fix one coil and make other movable which moves with the source whose displacement is to be measured. With the change in distance in displacement coefficient of coupling changes and it causes the change in mutual inductance. This change in mutual inductance can be calibrated with the displacement and measurement can be done.

Production of Eddy Current of Inductive Transducer We know that when a conducting plate is placed near a coil carrying alternating current, a circulating current is induced in the plate called “EDDY CURRENT”. This principle is used in such type of inductive transducers. Actually what happens? When a coil is placed near to coil carrying alternating current, a circulating current is induced in it which in turn produces its own flux which try to reduce the flux of the coil carrying the current and hence inductance of the coil changes. Nearer the plate is to the coil, higher will be eddy current and higher is the reduction in inductance and vice versa. Thus inductance of coil varied with the variation of distance between coil and plate. Thus the movement of the plate can be calibrated in terms of inductance change to measure the quantity like displacement. Real Life Application of Inductive Transducer Inductive transducers find application in proximity sensors which are used for position measurement, dynamic motion measurement, touch pads etc. Particularly inductive transducer is used for the detection of type of metal, finding missing parts or counting the number of objects.

What is Potentiometer? Potentiometer Used as the Transducer or Sensor written by: Haresh Khemani • edited by: Lamar Stonecypher • updated: 9/1/2009 Potentiometer is one of most commonly used devise for measurement of the displacement of the body. This article describes what potentiometer is and how it can be used as the transducer.

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Principle of Working of Potentiometers The potentiometer is also called as pots and it one of the most commonly used devise for measuring the displacement of the body. The potentiometer is the electrical type of transducer or sensor and it is of resistive type because it works on the principle of change of resistance of the wire with its length. The resistance of the wire is directly proportional to the length of the wire, thus as the length of the wire changes the resistance of the wire also changes.

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What is Potentiometer? The potentiometer is an electric circuit in which the resistance can be changed manually by the sliding contacts. The typical potentiometer is shown in the figure below. Here the voltage Vs is applied across the two points of the wire A and B. C is the variable contact point between A and B and its position can be changed by the sliding contact. The voltage Vo is measured between the points A and C. As per the resistance law of the conductor, the resistance of the conductor AC changes as the length of the wire AC changes. Accordingly the output voltage Vo between A and C also changes. The point C is the slider whose position is changed by the operator or by the motion of the body whose displacement is to be measured. The relationship between the length of the conductors and the voltage across them can be expressed as: Vo/Vs = AC/AB

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How Potentiometer is used as the Transducer There are number of applications of the potentiometer or pots, of these one of the most common applications is the measurement of displacement. To measure the displacement of the body, this body, which is moving, is connected to the sliding element of the potentiometer. As the body moves, the position of the slider located on the potentiometer also changes so the resistance between the fixed point and the slider changes. Due to this the voltage Vo across these points also changes. The change in voltage or the resistance is proportional to the change in the displacement of the body. Thus the voltage change indicates the displacement of the body. The potentiometer can be used for the measurement of translational as well as well rotational displacement. Since these potentiometers work on the principle of resistance, they are also called as the resistive potentiometers.

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Important Performance Parameters of the Potentiometers 1) Potentiometer resolution: Though the resistance of the conductor is proportional to its length, when the slider moves over the wire the change in the resistance is not continuous function of the contact movement. The smallest increment of the slider that can be recorded across the whole length of the potentiometer is called as the resolution. If resistance winding is used in the potentiometer, its limiting resolution is measured as the reciprocal of the number of turns of the coil. The lower the length that can be recorded higher is the resolution of the potentiometer. 2) Potentiometer linearity: Ideally the resistance measured between the fixed point and the slider point is linear function of the contractor position relative to that end. In actual cases the change in resistance and the distance between the fixed and the moving point is never linear. The maximum allowable deviation between change in resistance and the linear position of the slider is called as the linearity tolerance.

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Reference 1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck 2) Book: Measurement and Instrumentation Principles by Alan S Morris.

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Image Courtesy 1) Book: Measurement and Instrumentation Principles by Alan S Morris.

What is Potentiometer? This is the series of articles that describes what is potentiometer, also called as pots, its types and it can be used as the transducer.

What is Potentiometer? Potentiometer Used as the Transducer or Sensor Types of Potentiometers 7.

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