Earth Ground Tester : Tutorial 1

                                           
                                                                Fig : 1 : Earth Ground Test

Published by : Yousuf Ibrahim Khan
Date : 9-9-2012

Earth Ground Testing :

Most facilities have grounded electrical systems, so that in the event of a lightning strike or utility overvoltage, current will find a safe path to earth. A ground electrode provides the contact between the electrical system and the earth. To ensure a reliable connection to earth, electrical codes, engineering standards, and local standards often specify a minimum impedance for the ground electrode. The International Electrical Testing Association specifies ground electrode testing every three years for a system in good condition with average up-time requirements.

Why Ground ? Why Test ?

Poor grounding not only contributes to unnecessary downtime, but a lack of good grounding is also dangerous and increases the risk of equipment failure.Without an effective grounding system, we could be exposed to the risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas. If fault currents have no path to
the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people. The following organizations have recommendations and/or standards for grounding to
ensure safety:

• OSHA (Occupational Safety Health  Administration)
• NFPA (National Fire Protection Association)
• ANSI/ISA (American National Standards Institute and Instrument Society of America)
•  TIA (Telecommunications Industry Association)
•  IEC (International Electrotechnical Commission)
• CENELEC (European Committee for Electrotechnical Standardization)
•  IEEE (Institute of Electrical and Electronics Engineers)

However, good grounding isn’t only for safety; it is also used to prevent damage to industrial plants and equipment. A good grounding system will improve the reliability of equipment and reduce the likelihood of damage due to lightning or fault currents. Billions are lost each year in the workplace due to electrical fires. This does not account for related litigation costs and loss of personal and corporate productivity.

Multimeter : Terms and Definitions

                                    Fig : 1 Manual (left) and Auto (right) Ranging in Multimeters

Published By : Yousuf Ibrahim Khan
Date : 9-9-2012

A, ampere or amp — The basic unit of electric current.

AC, alternating current — An electric signal in which the current and voltage vary in a repeating pattern over time; the most common type of voltage. 

analog meter — A mechanical measuring device using a needle moving across a graduated scale or dial.

APO- Auto-Power — Off  Automatically shuts down unit after a certain amount of time to preserve battery life. Most meters with APO may be disabled or set to a certain amount of time before shutting off.

auto ranging — A DMM that automatically selects the range with the best resolution and accuracy in response to the sensed values.

calibration — To adjust the meter measured value to a recognized artifact or standard.

capacitance — Ability of a component to hold an electrical charge, usually stated in microfarads.

capacitor — Electronic component which stores energy and then discharges it rapidly; blocks DC and allows AC to pass through.

clamp-on — DMM with jaws that allow it to fit around a conductor to measure AC or DC current without breaking the circuit.

contact — A connection between two conductors that allows a flow of current.

continuity — A continuous path for current flow in a closed circuit.

current — The flow of an electrical charge through a conductor; measured in amperes or amps.

DC, direct current — a direct, steady voltage; typically produced through electromagnetism, chemicals (batteries), light, heat or pressure.

data hold — Feature of a DMM that allows continued display of the last reading taken after probes have been removed.

diode — Electronic device in circuits that allows current to flow easily in only one direction and blocks flow in the opposite direction.

DMM, digital multimeter — An instrument that uses an LCD display typically capable of measuring voltage, current and resistance.

F, farad — The basic unit of capacitance.

frequency — The number of cycles per second that a wave form repeats; measured in hertz. (Line voltage in the U.S. is 60 Hz.)

ground — A large conducting body (earth) used as a common return for fault current in a circuit.

H, hertz — One cycle per second; the unit of frequency.

harmonics — A signal with a frequency which is  an integer multiple of the fundamental frequency (60Hz); may damage or degrade the performance of electrical devices.
 
harmonic distortion — Diminishes power quality; caused by non-linear loads such as variable speed motor drives, electronic lighting ballasts and computers. 

impedance — Total opposition to current flow; includes resistance, capacitance and reactance.

load — Any device which consumes power in a circuit.

manual ranging — DMM that requires the user to manually select the range, using the meter’s dial.

min/max — Feature that allows a meter to capture and store the highest and lowest readings during a specific measurement.

ohm — The basic unit of resistance, specified as equal to that of a conductor in which one amp of current is produced by one volt of potential across its terminals. 

OL, overload — Signal amplitudes or frequencies above the specified limits of the instrument; typically displayed as ―OL on the display of a DMM.

peak hold — Feature of DMM that allows retention of highest reading in a series of measurements.

polarity — The positive or negative direction of DC voltage or current.

resolution — Increments in value that can be displayed by a DMM; the greater the resolution the more precise the readout.

resistance — Opposition to current; measured in ohms.

Sleep mode — Automatically shuts down unit not in use to preserve battery life. 

short — Any connection that has relatively low resistance or any resistance between two points below a preselected threshold. Typically, this is unintended.

True RMS meter — DMM that has the True RMS feature, allowing for accurate measurement of AC voltage in environments with harmonics (see harmonics).

V, volt — The unit of electrical pressure; one volt is the potential difference needed to cause one amp of current to pass through one Ohm of resistance.

Multimeter : Questions and Answers

Fig : 1 Some basic Multimeter Symbols

Published By : Yousuf Ibrahim Khan (B.Sc Engg., AIUB)
Date : 9-9-2012

1. What is port panel in Multimeters ?
ans : The port panel is where you plug in your test leads.

2. Please tell some famous brands in Multimeter Industries ?
ans :   Fluke, Extech, Amprobe, Mastech, Craftsman.

3. Where to put the black lead ?
ans : In the COM (common) port.

4. How can i measure DC voltage ?
ans : So simple question....To measure DC voltage, we place the Red lead into the VΩ port and black lead into the COM port.  Turn the dial or switch to VDC or the 2nd symbol show in Fig.1. If it is a manual ranging meter set it for the proper range. As in the example below, we want to measure a 9V battery so the best range would be the 20 V range. If you have an auto-ranging meter you only need to set the function on the dial to VDC.

Introduction to Multimeter : Tutorial 1

Figure 1 : A Basic Multimeter (DMM Type)

Published By : Engr. Yousuf Ibrahim Khan
Date : 9th September, 2012

A multimeter or a multitester, also known as a VOM (Volt-Ohm meter), is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance. Multimeters may use analog or digital circuits—analog multimeters (AMM) and digital multimeters (often abbreviated DMM or DVOM.) Analog instruments are usually based on a microammeter whose pointer moves over a scale calibrated for all the different measurements that can be made; digital instruments usually display digits, but may display a bar of a length proportional to the quantity being measured. A multimeter can be a hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as electronic equipment, motor controls, domestic appliances, power supplies, and wiring systems. Multimeters are available in a wide range of features and prices. Cheap multimeters can cost less than US$10, while the top of the line multimeters can cost more than US$5,000.

Digital Multimeters : Multimeters are designed and mass produced for electronics engineers. Even the simplest and cheapest types may include features which you are not likely to use. Digital meters give an output in numbers, usually on a liquid crystal display (LCD), shown in the Fig. 1.

Analog Multimeters : An analogue meter moves a needle along a scale. Switched range analogue multimeters are very cheap but are difficult for beginners to read accurately, especially on resistance scales. The meter movement is delicate and dropping the meter is likely to damage it! Each type of meter has its advantages. Used as a voltmeter, a digital meter is usually better because its resistance is much higher, 1 MΩ or 10 MΩ, compared to 200 KΩ for a analogue multimeter on a similar range. On the other hand, it is easier to follow a slowly changing voltage by watching the needle on an anlaogue display. Used as an ammeter, an analogue multimeter has a very low resistance and is very sensitive, with scales down to 50 µA. More expensive digital multimeters can equal or better this performance. Most modern multimeters are digital and traditional analogue types are destined to become obsolete.

Resolution of analog multimeters is limited by the width of the scale pointer, parallax, vibration of the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and errors due to non-horizontal use of the mechanical display. Accuracy of readings obtained is also often compromised by miscounting division markings, errors in mental arithmetic, parallax observation errors, and less than perfect eyesight. Mirrored scales and larger meter movements are used to improve resolution; two and a half to three digits equivalent resolution is usual (and is usually adequate for the limited precision needed for most measurements).

Figure 2 : An Analog Multimeter

Resistance measurements, in particular, are of low precision due to the typical resistance measurement circuit which compresses the scale heavily at the higher resistance values. Inexpensive analog meters may have only a single resistance scale, seriously restricting the range of precise measurements. Typically an analog meter will have a panel adjustment to set the zero-ohms calibration of the meter, to compensate for the varying voltage of the meter battery.

The second tutorial will be available soon....

Top 10 EEE Departments in Bangladesh

Published by : Yousuf Ibrahim Khan
Date : 9th September, 2012


1. Bangladesh University of Engineering and Technology (BUET)
Website : http://www.buet.ac.bd

2. Rajshahi University of Engineering and Technology (RUET)
Website : http://www.ruet.ac.bd

3. Khulna University of Engineering and Technology (KUET)
Website : http://www.kuet.ac.bd

4. Chittagong University of Engineering and Technology (CUET)
Website : http://www.cuet.ac.bd

5. Islamic University of Technology (IUT)
Website : http://www.iutoic-dhaka.bd

6. Dhaka University of Engineering and Technology (DUET)
Website : http://www.duet.ac.bd

7. American International University-Bangladesh (AIUB)
Website : http://www.aiub.edu

8. Ahsanullah University of Science and Technology (AUST)
Website : http://www.aust.edu

9. Military Institute of Science and Technology (MIST)
Website : http://www.mist.ac.bd

10. Shahjalal University of Science and Technology (SUST)
Website : http://www.buet.ac.bd