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Let Your Light Shine Matt. 5:16 Let your light shine before men in such a way they may see your good works and glorify your Father who is ion Heaven. Destination (region of the world) Electrical standards for the region Voltage Frequency Plug connection arrangements Electrical Load Evaluation Critical Life Support/ Uninterruptible Power Source/non-Critical Linear Loads Resistive Inductive Non-Linear Loads Prime Mover Generators Gasoline Powered Diesel Powered Wind Hydro Alternative Energy Sources Solar Hybrid Systems Electrical Distribution Systems Grounding Lightning Protection Safety Learning Objectives An overview of electrical standards around the world How do I get plugged in? Evaluate the Electrical Energy Usage How is electricity being used? Appropriate Electrical Energy Generation How can I generate the needed Electrical energy? Proper Electrical Power Distribution How do I get the energy where it is needed? Shocking Experiences How do I protect myself and the equipment? General Observations on Electrical Power Generation and Distribution Power Generation 1. Fossil Fuel is EXPENSIVE. Diesel fuel for example is at least $4.50 USD in most of the developing world. 2. Gasoline powered equipment should be avoided because of the danger in handling gasoline. Also gasoline powered products are generally of low quality in the developing world and resulting in premature equipment failure. 3. Solar energy is plentiful in many regions and should be utilized. Obviously, solar energy is a daytime function thus batteries are a necessity for non sunny times. 4. Micro/Mini hydro can be source of renewable energy in the proper setting. Righta-way authorizations, permissions to use or modify stream flow, high capital investment, and security at the site because it is often remote to an inhabited facility are obstacles to a successful installation. 5. Wind power can be a source of renewable energy at the proper settings. Obviously, a constant wind flow of a least 7-9 MPH in necessary for justifying the installation. Wind turbines have a high profile to the environment and security of the installation should be considered. The wind turbine should be at least 20 feet about the canopy of vegetation or highest obstruction. Power Distribution 1. Copper wire is EXPENSIVE. Aluminum wire has inherent problems with wire terminations. 2. Proper coordination of circuit breakers, wire size, and circuit capacity is essential to safely squeeze every electron through a power distribution system. Conclusion EXTREME conservation of electrical usage with high energy efficient devices is essential to a sustainable, affordable installation. Renewable energy generation should be used where feasible to reduce the use of fossil fuel and its associated daily costs. Configurations – Voltage and Frequency Standards Americas – except Bolivia Single Phase – 240/120 VAC; 60 Hz Three phase – 208/120 VAC WYE connected; 60 Hz 240/120 VAC DELTA connected; 60 Hz Centered Tap Grounded European Three Phase – 380/220 VAC WYE connected; 50 Hz West Africa Three Phase – 400/230 VAC WYE connected; 50 Hz East Africa Three Phase – 415/240 VAC WYE connected; 50 Hz IMPORTANT NOTE! 240 Volts Line to Line as we find in USA is NOT the same as 240 Volts Line to Neutral found in most International settings. Governing Electrical Code North America – National Electrical Code 2005 (revised every 3 years) European – International Electrical Regulations Sixteenth Edition Outside of USA Plug Configurations See www.interpower.com Voltage Transformation Use a step-down transformer from 240 to 120 as needed. Make sure the derived 120VAC is referenced to ground. See www.toddsystems.com Frequency Transformation 60/50 Hz There is no economical device that transforms 60 Hz power to 50 Hz. In order to verify that a device rated for 60 Hz will work on 50 Hz, perform the following test: Measure the current “in rush” and “full load” current at both 60 Hz and 50Hz. If current at 50 Hz is 10% greater than at 60 Hz or the current at 50 Hz exceeds the nameplate rating at 60 Hz, the machine will fail prematurely. This applies to devices with motors, transformers, ballasts such as fluorescent lighting fixtures, and any other devices with inductive characteristics. EXTREME WARNING! Select system voltage and frequency based on the regional standard and not the USA standard as a long term design decision. Electrical Load or Device List Each electrical device needs to be evaluated based on the following criteria: DESCRIPTION TYPE VA/w/Hp VOLTS Description Descriptive name for the device AMPS PHASE CRITICAL/UPS NON- DUTY CRITICAL CYCLE Type Resistive – Applied Voltage and load current in phase Heater elements Inductive – Load Current lags the applied voltage by 90 degrees Motor Fluorescent light ballast Transformer Electronic – (non-linear) Load current non linear with respect to the applied voltage Devices using switching power supplies – computers VA – volt-amps W – Watts Hp – Horsepower Volts Nominal Applied Voltage as recommended by the manufacturer – nameplate data Amps Full load Current at the rated Voltage and Frequency Phase Single phase or three phase Critical/UPS Generally refers to life support or process critical device requiring the electrical source to be constant in voltage and frequency being with no interruptions powered by an Uninterruptible Power Supply (UPS). A UPS can take the following forms: Single Conversion Line Interactive True On-Line Non-critical Loads not requiring power source backup Duty Cycle Estimate of device usage in a 24 hour period Connected Load The summation of the VA of each device will yield the total connected load. Using the estimation of the Duty Cycle, multiply the Total Connected Load with an estimate of the Duty Cycle of the devices, this will yield an estimate of the expected Running Load. In my experience, the Running Load is about 30% of the Total Connected load. Example of a Simple Load Study Spreadsheet QTY 2 1 3 2 3 Description Office Appliances Desk Top Computers Server Computer Lap top Computer Desk Top Printer Small HP Type Printer Watts Total Watts 400 600 200 720 120 800 600 600 1440 360 4 10 30 1 1 3 10 16 50 2 2 6 8000 BTU Split A/C Unit Variable Speed Fan 40 Watt Single Tube Fluorescent Light 1/2 Hp water pump Refrigerator - 15 ft3 energy efficient water heater Guest House Appliances 5000 BTU wall A/C Variable Speed Fan 40 Watt Single Tube Fluorescent Light Refrigerator - 15 ft3 energy efficient Toaster water heater 850 180 3400 1800 80 1125 850 2400 2400 1125 850 7200 550 180 5500 2880 80 850 1000 2400 4000 1700 2000 14400 Total Estimated Connected Load 20% Load Factor 25% Load Factor 30% Load Factor 35% Load Factor 40% Load Factor 51055 10211 12764 15317 17869 20422 25 KVA (65%) Load Factor 45 KVA (65%) Load Factor 65 KVA (65%) Load Factor 16250 29250 42250 Example spreadsheet for solar installation in Eretria Load Study Eritrea Hours per Description Refrigerator Laptop Computer Laptop Computer Printer Fax Codan Radio Charger for Palm Pilot Regional BGAN Terminal Mini M Satellite Phone Fan Low Wattage Lighting Type Motor Electronic Electronic Electronic Electronic Electronic Electronic Electronic Electronic Motor Resistive VA 120 192 192 280 187 250 60 156 360 60 75 1.15 for AC 138 221 221 322 215 288 69 179 414 69 86 Day 2 1 1 0.25 0.25 0.5 1 0.5 0.25 10 4 Watt Hr/Day 276 221 221 80.5 54 144 69 90 103.5 690 345 Total Watt -Hr/Day System Eff 0.70 5.8KWHr/m2/day 2293 3275 565 80 watts per 7 panel 120 watts per panel Batteries 1 day storage 50% discharge 12 volt @ 225 Ahr Total required Understanding Electrical Power Measurements True Power (W) The term is used to express the rate of doing work or converting energy. True power is the actual power used in an electrical circuit. True power is measured in Watts (W), Kilowatts (KW), or Megawatts (MW). In any DC circuit, or in an AC circuit in which voltage and current are in phase, such as resistive loads, true power is equal to voltage times current. Reactive Power (VAR) Reactive power (VAR) is power supplied to reactive load. The unit of reactive power is volt-amps reactive instead of watts as in true power. VAR represents a pure reactive load (inductor or capacitor) component or load. Reactive power supplied to a reactive component such as an inductor or capacitor should average out to zero and is not converted to sound, rotary motion, light or heat. The function of power in a reactive circuit is to produce a magnetic field around a coil or to charge a capacitor. Apparent Power (VA) Apparent power is the product of the voltage and current in a circuit calculated without considering the phase shift that may be present the voltage and current in a circuit. Apparent power is expressed in volt-amps (VA), kilovolt amps (KVA), or megavolt amps (MVA). Because apparent power considers circuit current regardless of how it is used, apparent power is a measure of component or system capacity. This is the reason why transformers are sized in VA rather than Watts. The transformer must deliver current at a set voltage regardless of the application that uses current. With small single-phase AC motor circuits, apparent power is much higher that true power. Power Factor (pf) Power factor (pf) is the ratio of true power used in an AC circuit to apparent power delivered to the circuit. Power factor is commonly expressed as a percentage. The lower the power factor, the less efficient the circuit and the higher the overall operating cost. The overall operating cost is increased because every component in the system, such as transformer and conductor sizes, must be sized for the higher current caused by lower 5 6550 13100 2700 5 power factor. Power factor is lagging for an inductor load, leading for a capacitive load, and in phase for a resistive load. Generators with Prime Movers Although there are two basic types of generators – synchronous and induction – the former is used almost universally for isolated operation. A generator produces electricity when magnetic flux lines are cut by a rotating wire coil (rotor). The magnetic flux lines are produced by the magnetic field present between the North and South poles of a permanent or electromagnet. The stronger the magnetic flux lines and the faster the rotation, the higher the voltage produced. Synchronous Generators The generators are called “synchronous” because the mechanical rotational of the rotor is directly related to the phase angle of the AC voltage produced. A synchronous generator produces its own voltage. The frequency produced will be exactly the revolutions per second of the rotor divided by the number of pole pairs. Automatic Voltage Regulator The Automatic Voltage Regulator maintains a no load to full load steady state voltage to tight tolerances. The AVR has a volts/hertz characteristic that proportionally reduces the regulated voltage at reduced speeds. This feature aids the engine during sudden large additions of load. Single phase voltage sensing can aggravate the voltage unbalance across a three phase system by monitoring the voltage and current only in a single phase. Three phase voltage sensing does a mathematical calculation by electronics based on the current and voltage of all three phases and sets the system voltage accordingly. The electrical power produced by the synchronous generator set is derived from a closed loop system consisting principally of the exciter rotor, the main revolving field and the automatic voltage regulator. The process begins when the engine starts to rotate the internal components of the alternator. The residual magnetism in the main rotor produces a small alternating voltage (AC) in the main stator. The automatic voltage regulator rectifies this voltage (converts it to DC) and applies it to the exciter stator. This DC current to the exciter stator creates a magnetic field that, in turn, induces an AC voltage in the exciter rotor. This AC voltage is converted back to DC by the rotating diodes. When this DC voltage appears at the main rotor, a stronger magnetic field that the original residual field is created which induces a higher voltage in the main stator. This higher voltage circulates through the system inducing an even higher DC voltage back at the main rotor. This cycle continues to build up the voltage until it approaches the proper output level of the generator set. At this point the automatic voltage regulator begins to limit the voltage being passed to the exciter stator that, in turn, limits the overall output of the alternator. Diesel Generator Standby Rating – Application for supplying continuous electrical power (at variable load) in the event of a utility power failure. No overload is permitted on these ratings. The generator is peak rated. Generally, this rating is for 4 hours or less. Prime Power Rating – Application for supplying continuous electrical power (at variable load) in lieu of commercially purchased power. There is no limitation to the annual hours of operation and the generator set can supply 10% overload power for 1 hour in 12 hours. Other Considerations Derate 10% for altitude above 3500 Feet Above sea level Derate 15% for 50 Hz operation if using the 60Hz specifications Derate for power factor less than 0.8 www.caterpillar.com Diesel Engine Mechanical Governor The speed regulating governor characteristics are accomplished by a mechanical rotating mechanism coupled to mechanical linage to the injector pump. Generally, the speed droop on this type of governor is 3% - 5%. Isosynchronous operation is not an option with this type of governor. Diesel Engine Electronic Governor The speed regulating characteristics are modeled in electronics in the form of a Proportional, integral, and derivative control scheme. Speed droop and isosychonous arrangements are switchable. The actuator is generally a current to rotational transducer. www.woodward.com Wind Power The use of wind energy has been around for well over a thousand years. However, there are certain physics that guide us on what it can and cannot do. The proper name of a wind generator is actually “Wind Energy Converter” that being a device that converts the potential energy in the wind to another form of energy. This can either be mechanical or electrical. When the wind blows, the rotor blade stops a percentage of the wind. That percentage is what is converted into energy. According to physics, the maximum amount of wind energy that can be converted is 59.3%. This is known as the Betz Limit. www.eere.energy.gov/windandhydro/wind_how.html There are a number of types of wind generators. Research has been done on virtually every possible concept with the objective of producing the maximum amount of power for the lowest cost at the highest possible reliability. Conventional experimentation has found that the horizontal axis upwind or down wind design to be the best concept. The most common designs include: 1. Horizontal upwind: The generator shaft is positioned horizontally and the wind hits the blade before the tower. 2. Horizontal downwind: The generator shaft is positioned horizontally and the wind hits the tower first then the blade. 3. Vertical Axis: The generator shaft is positioned vertically with the blades pointing up with the generator mounted on the ground or a short tower. There are two basic types of airfoils (blades) a lifting and drag type. 1. The drag style airfoil is typically what you see with an old Dutch wind mill or American water pumping wind mill. The blades are generally a flat plat which the wind hits and causes to rotate. This type of design is great for very low wind areas and will develop a lot of torque to perform an operation. However, in medium to higher winds, their capabilities to produce energy are limited. 2. The lifting style airfoil is what you see in most modern wind turbines and on airplanes. A properly designed airfoil is capable of converting significantly more power in medium and higher winds. Actually, with this design, the fewer number of blades the more efficient this design can be. Two European companies actually produced one bladed machines however, dynamic balance issues prevented them from becoming a commercial success Locating a wind generator is extremely important to the performance of the machine. It is the difference between a machine that give you lots of energy and a garden sculpture. The ideal location for a wind turbine is 20’ above any surrounding object within a 250 foot radius. This generally means your property should be at least one acre in size. You should have at least a 9 MPH average wind speed at your location. www.windenergy.com Hydropower The basic principle of hydropower is that if water can be piped from a certain level to a lower level, then the resulting water pressure can be used to do work. If the water pressure is allowed to move a mechanical component then that movement involves the conversion of the potential energy of the water into mechanical energy. Hydro turbines convert water pressure into mechanical shaft power, which can be used to drive an electricity generator, a grinding mill or some other useful device. Hydropower is a very clean source of energy. It does not consume but only uses the water, after use it is available for other purposes (although on a lower horizontal level). The conversion of the potential energy of water into mechanical energy is a technology with a high efficiency (in most cases double that of conventional thermal power stations). The main advantages of hydropower are: • • • • • power is usually continuously available on demand, given a reasonable head, it is a concentrated energy source, the energy available is predictable, no fuel and limited maintenance are required, so running costs are low (compared with diesel power) and in many cases imports are displaced to the benefit of the local economy, it is a long-lasting and robust technology; systems can last for 50 years or more without major new investments. Against these, the main shortcomings are: • • • it is a site specific technology and sites that are well suited to the harnessing of water power and are also close to a location where the power can be economically exploited are not very common, there is always a maximum useful power output available from a given hydropower site, which limits the level of expansion of activities which make use of the power, river flows often vary considerably with the seasons, especially where there are monsoon-type climates and this can limit the firm power output to quite a small fraction of the possible peak output, Hydropower Feasibility To know the power potential of water in a river it is necessary to know the flow in the river and the available head. The flow of the river is the amount of water (in m3 or liters) which passes in a certain amount of time a cross section of the river. Flows are normally given in cubic meters per second (m3/s) or in liters per second (l/s). Head is the vertical difference in level (in meters) the water falls down. The theoretical power (P) available from a given head of water is in exact proportion to the head H and the flow Q. P=Q × H × c c = constant The constant c is the product of the density of water and the acceleration due to gravity (g). If P is measured in Watts, Q in m3/s and H in meters, the gross power of the flow of water is: P=1000 × 9.8 × Q × H This available power will be converted by the hydro turbine in mechanical power. As a turbine has an efficiency lower than 1, the generated power will be a fraction of the available gross power. www.microhydropower.net Hybrid system Combination of power sources interfaced together Components of a Hybrid System Voltage Regulators Each of the transformer types below has application in developing world facilities. Voltage swings of -25% to +10% are common throughout the daytime. The largest voltage swings occur at 6:00 PM to 7:00 PM in the early evening because of residential lighting loads. Industrial loads are often a small percentage of electrical power usage especially in the rural settings. Extremely long and under sized power lines aggravate this problem of voltage sags throughout the day. The voltage regulator needs to carefully coordinated with the type of electrical load that is connected so as not to cause additional problems with voltage instability. www.superiorelectric.com Transfer Switches An ATS (automatic Transfer Switch) with built-in control logic monitors your normal power supply and senses any interruptions. When the utility power fails, the ATS automatically starts the engine and transfers the load after the generator has reached proper voltage and frequency. This happens in a matter of seconds after the power failure occurs. When the utility power has been restored, the ATS will automatically switch the load back, and after a time delay, it will shut down the engine. A recommended practice is to wait at least 5 minutes after national power restoration before transferring from emergency backup. During those five minutes, a national utility can experience voltage and current surges, switching transients as capacitor banks are energized, brown outs, frequency swings, etc. as they attempt to pick up the electrical load on the power line. It is essential to monitor all three phases of incoming power. In the developing world, single phase national power brown/black outs are common. If a three phase motor is allowed to continue to run under single phase brown out conditions, it can be destroyed in a matter of minutes. Often times, the distribution system in a missionary community is single phase loads so a single phase brown/black out will leave sections of the facility with poor or no electricity. www.asco.com OutBack Inverter/Chargers Outback Inverter/chargers are the next generation in advanced power management. Each is a DC to AC sine wave inverter, battery charger and AC transfer switch housed within a tough die-cast aluminum chassis. Just like the local utility grid, the inverter produces true sine wave AC electricity for your stand-alone or backup power needs. Computers, TVs and pumps are just some of the examples of modern electronics that last longer and run better when powered with true sine wave electricity from an OutBack inverter. Starting up your air conditioning, washing machine or well pump is worry-free because of our high surge power capability. Batteries and generators are the costly consumables when using inverters to generate electricity. The integrated smart battery charger uses multiple stages to perform quick recharging while prolonging battery life, saving your batteries and generator from unnecessary wear. Automatic switching between AC power sources is seamless due to an AC transfer switch that reacts in less than 16 milliseconds. Unique networked communication is built into all OutBack products providing complete integration. Expanding your system with your growing power needs is as simple as adding additional inverters with modular architecture. Further flexibility is provided with the ability to be connected at any time in either parallel, series or three-phase power configurations. Industry leading OutBack reliability is achieved through simplified design and rugged construction. www.outbackpowersystems.com www.outbackpower.com Batteries – Deep Cycle Deka/MK Battery 8L16, 6 volt 420 Ah Battery Group: L16 Terminal Type: T875 Nominal Voltage (V): 6 volts Capacity at C/20: 420 Ah Operating Temperature: -20F (-29C) to 140F ((60C) Charge Voltage @ 68F (20C) Cycle: 2.35 VPC Float: 2.25 VPC Resistance: 2.0 Milliohms (full charge) Terminal: T875 Made in U.S.A by East Penn Manufacturing Weight: 113 lbs / 51.2 kg Dimensions (LWH): 11.75"x 7"x 17.3" / 298 x 178 x 435 mm www.eastpenn-deka.com Solar Photovoltaic Panels Sharp ND-208U1, 208 watt PV module This poly-crystalline 208 watt module features 12.8% module efficiency for an outstanding balance of size and weight to power and performance. Using breakthrough technology perfected by Sharp's 45 years of research and development, these modules use an advanced surface texturing process to increase light absorption and improve efficiency. Common applications include office buildings, cabins, solar power stations, solar villages, radio relay stations, beacons, traffic lights and security systems. Ideal for grid-connected systems and designed to withstand rigorous operating conditions, Sharp's ND-208U1 modules offer maximum power output per square foot of solar array. Features • High-power module (208W) using 155 mm square poly crystalline silicon solar cells with 12.8% module conversion efficiency • • • • • Sharp's advanced surface texturing process increases light absorption and efficiency while providing a more subdued, and natural look Bypass diode minimizes the power drop caused by shade White tempered glass, EVA resin, and a weatherproof film, plus aluminum frame for extended outdoor use UL Listings: UL1703, UL Sharp modules are manufactured in ISO 9001 certified facilities Electrical Characteristics • • • • • • • • • • • Cell: Poly-crystalline silicon No. of Cells and Connections: 60 in series Open Circuit Voltage (Voc): 36.1V Maximum Power Voltage (Vpm): 28.5V Short Circuit Current (Isc): 8.13A Maximum Power Current (Ipm): 7.3A Maximum Power (Pmax): 208W (+10% / -5%) Module Efficiency Maximum Power: 12.8% Maximum System Voltage: 600 VDC Series Fuse Rating: 15A Type of Output Terminal: Lead Wire with MC Connector Mechanical Characteristics • • • • • • Dimensions (L x W x D): 64.6" x 39.1" x 1.8" (1640mm x 994mm x 46mm) Weight: 46.3 lbs (21 kg) Modules/Carton: 2 Carton Size: 68.3" x 43.2" x 4.5" (1735mm x 1097mm x 114mm) Carton Weight: 93.2lbs (42.3 kg) Modules/Pallet: 28 www.affordablesolar.com Solar Charge Controller OutBack MX60 60 amp Charge Controller Rated for up to 60 amps of DC output current, the OutBack MX60 can be used with battery systems from 12 to 60 VDC with a PV open circuit voltage as high as 140 voc. The MX60's set points are fully adjustable to allow use with virtually any battery type, chemistry, and charging profile. The OutBack MX60 allows you to use a higher output voltage PV array with a lower battery voltage - such as charging a 24 vdc battery with a 48 VDC PV array. This reduces wire size and power loss from the PV array to the battery location while maximizing the performance of you system and saving you money! The OutBack MX60 comes standard with an easy to use and understand display. The four line, 80 character, backlit LCD display is used for programming and monitoring of the system's operation including built-in Data Logging with 64 days of memory. SPECIFICATIONS MX60 Output Current Rating 60 amps DC Maximum at 12, 24 or 48 VDC Nominal Battery Voltage 12, 24, 32, 36, 48, 54 or 60 VDC (programmable) PV Open Circuit Voltage 125 VDC Maximum Standby Power Consumption Less than 1 watt typical Charging Regulation Methods Five Stage: Bulk, Absorption, Float, Silent, Equalization Voltage Regulation Setpoints Equalization Voltage Adjustable 1.0 to 5.0 VDC above Bulk Setpoint Temperature Compensation Programmable slope -2.0mV/oC/Cell to -5.0mV/oC/Cell Voltage Step-Down Capability Can charge a 12 or 24 VDC battery from a 48V nominal PV array Power Conversion Efficiency 99.1% @ 40 amps Output 97.3% @ 60 amps Output Digital Display 4 line 80 character backlit LCD Display Remote Interface RJ 45 Modular Connector CAT 5 Cable 8 wire Operating Temperature Range -40 to 60°C Power derated above 25°C Environmental Rating Indoor Type 1 Conduit Knockouts Two 3/4 - 1” on the back; One 1” - 1 1/2 “ on each side; Two 1” - 1 1/2” on the bottom Warranty Two years parts and labor Optional Extended Warranty Dimensions Enclosure: 14.5 “ H x 5.75” W x 5.75” D Shipping box: 17.75” H x 10” W x 7” D Shipping Weight 12 lbs. - 5.4 kg Electrical Power Distribution Load Distribution Panels Sizing Voltage Rating and Configuration Single Phase Three Phase Bus Bar Current Rating Greater than Connected Load Calculations Main Breaker or Lugs Only Number of Circuit Breakers/Spaces Surface or Recessed Mount Indoor or Rainproof www.squared.com/us/products/panelboards.nsf Circuit Breakers Coordinate with wire size and electrical load calculations Wire Size Based on electrical load calculations and voltage drop for a given length Voltage drop not to exceed 5% (recommendation) Grounding The purpose of electrical grounding is stated as follows from the National Electrical Code section 250-1: “Systems and circuit conductors are grounded to limit voltages due to lightning, line surges, or unintentional contact with higher voltage lines, and to stabilize the voltage to ground during normal operation. Equipment grounding conductors are bonded to the system grounded conductor to provide a low impedance path for the fault current that will facilitate the operation of over current devices under ground-fault conditions.” To boil this down to understandable terms, a ground is a conduction connection between electrical circuits or equipment and the earth. A low impedance (resistance) ground path is a ground path that contains very little resistance to the flow of fault current to ground. The purpose of electrical grounding is for the protection of personnel from electrical shocks. Infants and patients in the hospital/clinic setting are particularly vulnerable to electrical shocks. A few milliamperes of current at 120 VAC can be fatal. Electronic grounding is used primarily to provide a clean chassis ground to help maintain signal integrity for sensitive electronic equipment. How can you tell if your system is properly grounded? The following drawing illustrates the necessary components for grounding a single-phase and three-phase electrical service with a grounding electrode. If you are still not sure what arrangement that you have and if you are properly grounded, measure the voltage with a Digital Multimeter (DMM) between the “HOT’ and neutral conductor. Then measure the voltage between the neutral and ground conductor that could be a metal chassis of a piece of electrical equipment. Now measure the voltage between the “HOT” and ground conductor. The voltage between the “HOT” conductor and the neutral and the “Hot” conductor and ground conductor should be the same. The voltage between the neutral and ground conductor should be less than 0.5 volts. Any other voltage combination indicates a problem in the grounding scheme. Note! Personal Safety If a DVM indicates a relatively high voltage or a voltage that is erratic, the voltage may be a false indication of the presence of voltage due to the high internal impedance of the measuring circuit in the DVM. Fluke now sells a STRAY VOLTAGE MODULE for their DVM. This module lowers the input impedance of the meter to eliminate the possibility of static voltages. And alternative is the following: The light bulb test will indicate a current carrying grounding situation. Place a 120 incandescent light across the apparent voltage source and see if it glows or lights up. The bulb has low impedance. The major causes of grounding problems are loose electrical connections, reversal of conductor connections, missing ground wires, improperly installed or missing grounding electrodes. Another cause of electrical noise and grounding problems is the bonding of the neutral connection and the ground conductor on the load side of the service distribution box such as a sub panel or at a piece of electrical equipment. In this case, the neutral currents will flow in both the neutral and ground conductors. All of these examples produce electrical noise and safety hazards for personnel. An additional separate isolated ground rod creates two ground references that are typically at different potentials. The reason for these different potentials is because the resistances of the soil between the two isolated ground rods vary at each location. This results in current circulating (ground loop) in an attempt to equalize potentials. A ground loop is a circuit that has more than one ground point connected to earth ground with a voltage potential difference between the two ground points high enough to produce a circulating current in the ground system. Unfortunately, correcting the above wiring problems with grounding current loops is time consuming and can be frustrating. Nevertheless, the place to start your investigation is at the service transformer or service load distribution box. Check for grounding electrodes and inspect the connections for sound mechanical and electrical bonding. Next, check each outlet for the proper termination of “HOT”, neutral, and ground conductors looking specifically for poor connections, wiring reversals, and missing grounding wires. Often it will be necessary to unplug other equipment during the investigation to attempt to localize the problem. The results of correcting grounding problems can be substantial resulting in better performance and longer life from electrical equipment especially devices using the latest technology. Eliminating electrical shocks from the metal chassis will improve the performance and attitude of personnel. The color green or yellow with green stripe always identifies a conductor used. Yellow with green strip coding is international. Green is often not used or only as a control wire color in the international setting. A neutral conductor carries current from one side of a load back to the source. The color white or natural gray is used for the neutral (grounded circuit) conductor in most places. Check local and country standards for color coding. For example, in Kenya the neutrals are black and white is not used. IMPORTANT NOTE! Failing to know these wiring colors can lead to destroyed equipment and some personally shocking experiences. Lightning Protection As a thunderstorm grows, charges build up in the cloud. The bottom usually develops a large negative charge while high at the top of the cloud a positive charge develops. Ninety percent of all lightning flashes occur within the cloud. When a thunder cloud moves over an area, it can induce an intense charge of opposite polarity on the ground below. This is called the cloud electrical shadow, and results in unequal and constantly changing ground potential. Everything within the electrical shadow accumulates and dissipates this charge at varying degrees: 1. Conductors (metal structures, storage tanks, wiring/piping, and grounding grids) - collect and dissipate these charges most quickly (microseconds). 2. Products within containers (petroleum, military ammunition) - collect and dissipate these charges relatively slowly (the rate is highly dependent on surrounding insulators vs. conductors) 3. Insulators, and some insulated products - collect and dissipate these charges most slowly. 4. The ground (earth)- collects and dissipates these charges on a grand scale but the rate is very dependent on geological factors (soil resistivity, moisture, stratifications, lakes and rivers, etc.) If all of the above components are interconnected (using an effective common grounding mechanism), their charges will rise and fall together, keeping the charge across the system in balance. If the components are not interconnected, then the charges grow and shrink independently causing ground potential differentials, and the creation of “Bound Charges”. If the intensity of a Bound Charge becomes big enough, it will try to dissipate following a path of least resistance which can either be to follow grounding structures or wiring or to arc to a nearby conductor which has less resistance and/or impedance. As the storm intensifies, so do the magnitudes of these charges, and when the air between the cloud and the earth can no longer act as an insulator a cloud-to-ground spark (or lightning strike) occurs. Lightning always “chooses” to follow a path of least resistance/impedance. Thunder storms and lightning strikes have the following characteristics: • • • • • • Total Cloud Charge: 10 to 40 Coulombs Average Cloud Charge: 30 to 90 Coulombs are discharged Charge Transfer per Flash: 25 Coulombs Discharged Average Transfer per Flash: (EFS) 5 to 30 to 300 kV/m Electric Field Strength: Dependent on humidity, temperature & pressure Average EFS for lightning: 10 kV/m (required to breakdown the insulation threshold of moist air) • • • • • • • • Multiple Upward Streamers: 100 to 300 kV/m (usually under drier conditions) Peak Voltage: One to Ten Billion Volts, 50% at 100 Million Volts Peak Current: 2 to 510 kA (usually the return stroke) 99% < 200 kA 50% @ 30 kA Polarity Negative: > 90% Duration (99%): 30 to 200 ms (average duration of single return stroke 50ms) Number of Strokes per Flash: 1 to 26 50% > 4 10% > 9 Lightning RFI Range: 1 kHz to 100 MHz 95% 200 kHz to 20 MHz Temperature: 50,000 F Pressure 10 atm (causing sonic boom = thunder) Many bad things happen when lightning strikes, resulting in various direct and secondary effects. Direct Effects of Lightning A Direct strike can have the following effects: 1. Heat: fires, structural damage from instant vaporization of trapped moisture (example: explosions of concrete or trees) 2. High voltage and high current surges along conductors over long distances (along lightning rods to grounding rods, and along any electrically or metallically connected equipment) 3. High voltage and high current surges along the ground over shorter distances. 4. If a strike hits an individual it can cause severe injuries or even death. Secondary Effects of Lighting When lightning strikes nearby within microseconds the strike lowers (or neutralizes) the local ground charge and all interconnected conductors. However, the accumulated charge of some objects such as the fluid of a storage tank, does not discharge as quickly, resulting in a temporary “Bound Charge”. People, computers and electronic equipment, transformers and certain electrical equipment, and flammables do not like being hit by lightning, or being anywhere near where it strikes. • • • intense electromagnetic pulses (EMP) earth current transients atmospheric transients Nature of an Earth Electrode Resistance to current through an earth electrode actually has three components. 1. Resistance of the electrode itself and connections to it. Rods, Pipes, masses of metal, structures and other devices are commonly used for earth connections. These are usually of sufficient size or cross section that their resistance is a negligible part of the total resistance. 2. Contact resistance between the electrode and the soil adjacent to it. If the electrode is free from paint or grease, and the earth is packed firmly, the contact resistance is a negligible part of the total resistance. Rust on an iron electrode has little or no effect since the iron oxide is readily soaked with water and has less resistance than most soils. 3. An electrode driven into the earth of uniform resistivity radiates current in all directions in the surrounding soil. Generally, the resistance of the surrounding earth will be the largest of the three components making up the resistance of a ground connection. Whether a soil is largely clay or very sandy can change the earth resistivity very much. In soil, conduction of current is largely electrolytic. So the amount of moisture and salt content of the soil radically affect resistivity. The amount of water in soil varies, of course, with the weather, time of year, nature of sub-soil and depth of permanent water table. How to Improve Earth Resistance 1. Lengthen the earth electrode in the earth. In general, doubling the rod length reduces resistance by about 40%. The diameter of the earth rod has very little effect on its earth resistance. 2. Use multiple rods. Two well-spaced rods driven into the earth provide parallel paths. They are, in effect, two resistances in parallel. The rule for two resistances in parallel does not apply exactly. The resultant resistance is not ½ of the individual rod resistance but the reduction for two equal- resistance rods is about 40%. When you use multiple rods, they must be spaced apart further that the length of their immersion. The use of a grounding ring of bare #2 AWG often serves as a good earthing rod. 3. Treatment of the soil – Chemical treatment of the soil is a good way to improve earth-electrode resistance when you cannot drive deeper ground rods. Magnesium sulfate, copper sulfate, and ordinary rock salt are suitable non-corrosive materials. Chemical treatment is not a permanent way to improve your earthing system though. The chemicals are gradually washed away by rainfall and natural drainage through the soil. www.lecglobal.com Metal Oxide Varistors The most common type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only tiny current flows caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and large current flows. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages. The main use of varistors is to protect electrical and electronic equipment by shunting transients voltages to ground.