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Electric Safety Electrical Hazards Professor Mohamed A. El-Sharkawi Safety Facts • More than 1000 people are killed each year in the U.S. due to electric current, and several thousand more are injured. • Deaths occurs at lower voltages and currents. • Current flows inside the body can cause deep burns and cardiac arrest. • Individual may not let go of the power circuit due to involuntary muscle contraction. • Lungs, brain and heart are the most sensitive organs affected by current. (c) M. A. El-Sharkawi, University of Washington Primary and Secondary Shocks • Secondary shock: Does not produce direct physical harm, but may cause involuntary muscle reactions. • Primary shock: A shock of a magnitude such that it may produce direct physical harm. Fibrillation, respiratory tetanus and muscle contractions are all due to primary shocks (c) M. A. El-Sharkawi, University of Washington Safety Facts (ac current) • For less than 10 mA at the skin level, the person merely feels a "funny" sensation • For currents above 10 mA, the person freezes to the circuit and is unable to let go • For currents of 100 mA to 1 A, the likelihood of sudden death is very high • More than 1 A, the heart experience a single contraction, and internal heating is significant. (c) M. A. El-Sharkawi, University of Washington Effects of ac and dc currents Ref: IEEE Standard 524a-1993 Current (mA) Reaction dc ac Men Women Men Women No sensation on hand 1.0 0.6 0.4 0.3 Tingling (Threshold of perception) 5.2 3.5 1.1 0.7 Shock: Uncomfortable, muscular control not lost 9.0 6.0 1.8 1.2 Painful shock, muscular control is not lost 62.0 41.0 9.0 6.0 (c) M. A. El-Sharkawi, University of Washington Threshold Limit Ref: IEEE Standard 1048-1990 Current in mA Threshold Let-go: Worker cannot release wire 0.5% of population Men Women Men Women 9 6 16 10.5 23 15 Respiratory Tetanus: Breathing is arrested Ventricular Fibrillation: Weak and out of synch heart pulses 50% of population 100 (c) M. A. El-Sharkawi, University of Washington 67 Biological effect of current • Tingling: When a person lightly touches a charged object and the current within the touch perception threshold. If the person grip to the object, the current spread out over a wide contact area and the person may not feel anything. (c) M. A. El-Sharkawi, University of Washington Biological effect of current • Let go: The maximum current a person can tolerate and still manage to release a gripped conductor. (c) M. A. El-Sharkawi, University of Washington Let-go level Early Days! • “Muscular reaction at the let-go current value is increasingly severe and painful, as shown by the subject during laboratory tests” • IEEE Spectrum. Feb, 1972 (c) M. A. El-Sharkawi, University of Washington Biological effect of current • Respiratory tetanus: A current above the let-go threshold passes through the chest can cause involuntary contraction of the muscles, which will arrest breathing as long as the current continues to flow. (c) M. A. El-Sharkawi, University of Washington Biological effect of current • Ventricular fibrillation: A current passes through the chest can disturb the heart’s own electrical stimulation and cause it to assume an uncontrolled vibration. The heart may cease to beat. (c) M. A. El-Sharkawi, University of Washington Factor affecting human Safety • • • • • • Voltage level Current flowing in person Resistance of body Frequency of source Duration of shock Pathway of current (c) M. A. El-Sharkawi, University of Washington 1. Effect of voltage • The higher the voltage the higher the current! • 100-400 V ac is the most lethal voltage – High enough to cause significant current flow in the body – Can cause muscles to contract tightly on the energized equipment. • At higher voltages, fierce involuntary muscle contractions may throw the victim away from the hazard. (c) M. A. El-Sharkawi, University of Washington 2. Effect of Current • High current causes heating damage to tissues. • 10 A passing directly through the heart can cause cardiac arrest. Heart muscle fibers beat out of sync, so no blood is pumped • The spinal cord may also be affected, altering respiration control. 100-1000 mA is sufficient to induce respiratory arrest and/or cardiac arrest. • Thermal heating of tissues increases with the square of the current (I2R). (c) M. A. El-Sharkawi, University of Washington 3. Effect of Body Resistance • A palm resistance can range from 100 to 1 M. • Nerves, arteries and muscle are low in resistance. • Bone, fat and tendon are relatively high in resistance. • Across the chest of an average adult, the resistance is about 70-100 . (c) M. A. El-Sharkawi, University of Washington Body Resistance (c) M. A. El-Sharkawi, University of Washington Body Resistance in Ohms Ref: IEEE Standard 1048-1990 Hand-to-hand Hand-to-feet Resistance Dry condition Wet condition Wet condition Maximum 13,500 1,260 1,950 Minimum 1,500 610 820 Average 4,838 865 1221 (c) M. A. El-Sharkawi, University of Washington 4. Effect of Source Frequency • 50-60 Hz current has a much greater ability to cause ventricular fibrillation than D.C. current. • At 50-60 Hz, involuntary muscle contractions may be so severe that the individual cannot let go of the power source. • As the frequency gets above about 500 kHz, little energy passes through the internal organs. (c) M. A. El-Sharkawi, University of Washington 5. Effect of Duration • The longer the duration, the more severe the internal heating of tissues. • Keep in mind that with 110-240 V, the individual may become incapable of letting go. (c) M. A. El-Sharkawi, University of Washington Dalziel Formula • Charles Dalziel carried out research on the time-current relationship for the primary shock. K I t • I: ventricular fibrillation current in mA • t: time duration of the current in seconds • K: is a constant that depends on the weight of the test subject – for people weighing less than 70 kg (154 lb), K=116 – for people weighing more than 70 kg, K=157. (c) M. A. El-Sharkawi, University of Washington 1600 Less than 70 kg More than 70 kg fibrillation current in mA 1400 1200 1000 800 600 400 200 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 time in mS (c) M. A. El-Sharkawi, University of Washington 0.8 0.9 1 6. Effect of Pathway • If the current passes through the brain or heart, the likelihood of death increases significantly. (c) M. A. El-Sharkawi, University of Washington Example • A child climbs a tree to retrieve his kite that is tangled on a power line. The voltage of the power line is 240 V. The resistance of the wire from the source to the location of the kite is 0.2 ohms. The tree has a resistance of 500 ohms. The soil resistance is 300 ohms. Assume the child resistance is 2000 ohms. If the child touches the power line with his left hand, estimate the current through his body. Also, estimate the time it takes to induce ventricular fibrillation. (c) M. A. El-Sharkawi, University of Washington Line resistance Rl Solution Child resistance Rb Tree resistance Rt Source Ground resistance Rg True Ground V 240 I 85.7 mA Rl Rb Rt Rg 0.2 2000 500 300 2 2 K 116 t 1.83 s I 85.7 (c) M. A. El-Sharkawi, University of Washington Ground Resistance (c) M. A. El-Sharkawi, University of Washington Definition of ground resistance • Ground Resistance: Determines the amount of current flown through an object. (c) M. A. El-Sharkawi, University of Washington I Object R Center of Earth Ground Resistance of Hemisphere I I J area of hemisphere 2 r 2 I Conductor J ( x) Surface of earth r I 2 x 2 ; xr Field intensity E ( x) J ( x); is ground resistivity (c) M. A. El-Sharkawi, University of Washington xr Ground Resistance of Hemisphere x b x b I 1 1 Vab E ( x) d x J ( x) d x 2 a b x a x a Vab 1 1 Rab I 2 a b I Conductor Surface of earth r Equipotential surface For a=r, b= a Vab b Va Vb Rg 2 r (c) M. A. El-Sharkawi, University of Washington Ground Resistance Ref: IEEE Standard 1048-1990 IEEE Standard 524a-1993 Soil Composition Resistivity (Ohm meter) Wet Organic Moist Dry Bedrock 10 100 1000 10,000 (c) M. A. El-Sharkawi, University of Washington Example • Compute the ground resistance of a hemisphere with 2m diameter buried in a wet organic soil. • Also compute the ground resistance at 2m, 10m and 100m away from the center of the hemisphere. (c) M. A. El-Sharkawi, University of Washington 10 Rg 1.6 2 r 21 Solution At 2m 1 1 10 1 1 Rab1 0.8 2 a b 2 1 2 1.8 Resistance (Ohm) 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 50 100 Distance (meter) 150 (c) M. A. El-Sharkawi, University of Washington Voltage Source I Measuring Ground Resistance I Voltmeter V Surface of earth x Measured Voltage (V) Object under test Current electrode Potential probe Vf xf (c) M. A. El-Sharkawi, University of Washington Distance of the voltage probe from object (x) Ground Resistance of Objects Object Rod Ground Resistance 2l r ln 2 l r l is the length of the rod r is the radius of the rod 4r r is the radius of the disk Circular plate (disk) at the surface Buried wire Parameters 2 l l l ln ln 2d r l is the length of the wire r is the radius of the wire d is the depth at which the wire is buried (c) M. A. El-Sharkawi, University of Washington Ground Resistance of people Assume the foot is a circular plate Rf 4r A r Rf 4 Rf Rf Remote earth A 2 4 For standing person Rg Rf Rf Rf Rf (c) M. A. El-Sharkawi, University of Washington 0.02 3 0.5 R f 1.5 Touch and Step Potential • Step Potential: Potential between the two feet. • Touch Potential: Potential between a hand and any other part of the body. (c) M. A. El-Sharkawi, University of Washington (c) M. A. El-Sharkawi, University of Washington Static Wire Ground Wire Cross arm High voltage wire Insulators High voltage wire Steel Tower Tower ground or local (c) M. A. El-Sharkawi, University ofground Washington Flashover Flashover Energized line (c) M. A. El-Sharkawi, University of Washington Touch Potential I I Rman Itg Iman Rg 0.5 Rf Itg Rg Iman 0.5 Rf I man I (c) M. A. El-Sharkawi, University of Washington Rg Rg Rman 0.5 R f Example • A power line insulator is partially failed and 10A passes through the tower structure to the ground. Assume that the tower ground is a hemisphere with a radius of 0.5 meter, and the soil surrounding the hemisphere is moist. – Compute the voltage of the tower. – Assume that a man with a body resistance of 3k touches the tower while standing on the ground. Compute the current passing through the man. – Use Dalziel formula and compute the man’s survival time. (c) M. A. El-Sharkawi, University of Washington Solution Compute the ground resistance of the hemisphere 100 Rg 32 2 r 2 0.5 V I Rg 10 32 320 V To compute the current through the man, first compute Rf R f 3 300 (c) M. A. El-Sharkawi, University of Washington Solution The current through the man is given in Equation (8.12) I man I Rg Rg Rman 32 10 100 mA 0.5 R f 32 3000 150 According to Dalziel formula, the man can survive for 2 K 157 t 2.5 s I man 100 2 (c) M. A. El-Sharkawi, University of Washington Step Potential a b Iman (c) M. A. El-Sharkawi, University of Washington Step Potential Rman Rth Vth b a Rf Rf (c) M. A. El-Sharkawi, University of Washington Iman Example • During a weather storm, an atmospheric discharge hits a lightning pole. The pole is grounded through a hemisphere, and the maximum lightning current through the pole is 20,000A. – A person is playing golf 30 meters away from the center of the hemisphere. The distance between his feet is 0.3m, and his leg-to-leg resistance is 2k. Assume the soil surrounding the hemisphere is moist. Compute the current through the person and his step potential. – If that person is 3m away from the center of the hemisphere, repeat the solution. (c) M. A. El-Sharkawi, University of Washington Solution At 30m. For moist soil, = 100 m. I 1 1 20,000* 100 1 1 Vth 105 V 2 ra rb 2 30.3 30 R f 3 300 Vth 105 I man 40.4 mA 2 R f Rman 600 2000 Vstep I man * Rman 40.4* 2000 80.8 V (c) M. A. El-Sharkawi, University of Washington Solution At 3m I 1 1 20,000* 100 1 1 Vth 9.646 kV 2 ra rb 2 3.3 3 Vth 9646 I man 3.71 A 2R f Rman 600 2000 Vstep I man * Rman 3.71* 2000 7.42 kV (c) M. A. El-Sharkawi, University of Washington Prevention and Protection from Electric Shock Problem: Microshock due to Capacitive Coupling: Internal Electric Circuit V Internal Electric Circuit Equipment chassis Leakage capacitance Leakage capacitance (c) M. A. El-Sharkawi, University of Washington V Problem: Macroshock due to Faulted Equipment Conductive enclosure V If Rman Rg man Rg Fault If If Energized Electric Circuit CB V In=0 a If Example: Rg man=100, Rg=20, Rman=1000 If V 120 107 mA Rman Rg man Rg 1000 100 20 Rg man Circuit breaker will not interrupt this hazardous current (c) M. A. El-Sharkawi, University of Washington If Rg Solution: Grounding Chassis through the Neutral Wire Internal Electric Circuit V (c) M. A. El-Sharkawi, University of Washington Problem: Stray Voltage due to Resistance of Neutral Wire I Internal Electric Circuit Rh V Rn In Rn I man I Rn Rman Iman (c) M. A. El-Sharkawi, University of Washington Example • The resistance of the neutral wire is 0.1 Ohm, and the equipment current is 10 A. Assume the resistance of the man plus its ground resistance is 2000 Ohm. • Compute the current through the man assuming that the service transformer is farther away from the house, where Rn = 2 Ohm. • Repeat the problem assuming the appliance is about 3 kW. (c) M. A. El-Sharkawi, University of Washington Solution Rn 0.1 I man I 10 0.5 mA Rn Rman 0.1 2000 3 kW produce about 30A Rn 2 I man I 30 30 mA Rn Rman 2 2000 Circuit breaker will not interrupt this hazardous current (c) M. A. El-Sharkawi, University of Washington Solution: Three-prong: Separated Neutral and Ground I Internal Electric Circuit Ileak V Ileak Ileak In Equipment Grounding Conductor (EGC) (c) M. A. El-Sharkawi, University of Washington Problem: Macroshock due to Faulted Equipment Conductive enclosure Panel circuit breaker If If Internal Electric Circuit If V If EGC In=0 If If Example: Rg1=Rg2=10. If V 120 6 A Rg1 Rg 2 20 VEGC I f Rg1 6 *10 60 V Rg2 Rg1 If 6A is very small current and within the level of normal loads. The smallest circuit breaker in most premises would not open and the fault will not be cleared. (c) M. A. El-Sharkawi, University of Washington earth is not an effective path for fault current Solution: NEC 250.24 Conductive enclosure Neutral and ground bonding If Internal Electric Circuit Service Panel If If V Rwire If EGC If2 Rg2 If EGC VEGC I f 1 Rg1 I f Rwire Rg1 Rg1 Rg1 Rg 2 Rwire (c) M. A. El-Sharkawi, University of Washington If1 Example Example: Rg1=Rg2=10. I f V Rg1 Rg 2 Rwire R g1 Rg 2 * Rwire 120 20.5 246 A 10 This is large fault current and will be easily detected and cleared by the panel circuit breaker. (c) M. A. El-Sharkawi, University of Washington NEC 250.24 House Circuit Breaker 120V House meter Neutral EGC 120V 120V Neutral Ground rod Conductive enclosure bonded to ground wire (c) M. A. El-Sharkawi, University of Washington Appliance Distribution Panel (c) M. A. El-Sharkawi, University of Washington (c) M. A. El-Sharkawi, University of Washington 3-prongs receptacles Neutral (N) Hot (H) Ground (G) (c) M. A. El-Sharkawi, University of Washington 3-prongs receptacles and Plug (Europe) (c) M. A. El-Sharkawi, University of Washington (c) M. A. El-Sharkawi, University of Washington (c) M. A. El-Sharkawi, University of Washington Ground Fault Circuit-Interrupters (GFCI) (c) M. A. El-Sharkawi, University of Washington Ground Fault Circuit-Interrupters (GFCI) • Accidents and several deaths involving electric appliances and water occur every year in USA. • Changes were made in the National Electric Code which require the use of (GFCI). (c) M. A. El-Sharkawi, University of Washington Without GFCI (touch potential) House breaker I Internal Electric Circuit Rh Rn In Water is inside Iman (c) M. A. El-Sharkawi, University of Washington V Main idea of GFCI Ihot core winding Ineutral (c) M. A. El-Sharkawi, University of Washington Stray Voltage (c) M. A. El-Sharkawi, University of Washington What is Stray Voltage • Elevated voltage on neutral wires • The stray voltage problems are often associated with low voltage magnitudes – nuisance instead of hazard. • However, in some scenarios, the stray voltage could be lethal. (c) M. A. El-Sharkawi, University of Washington Problem: Voltage Drop on Utility Neutral Tower 1 Tower 2 Tower 3 I Hot Wire I V1 V2 Tower 4 Neutral V3 V4 V2 V1 V ; V3 V1 2V ; V4 V1 3V ;... V4 V3 V2 V1 (c) M. A. El-Sharkawi, University of Washington 3.72 What is Stray Voltage? • When a grounded object carries current, the voltage of the object is non-zero. • This voltage is called Stray voltage • The current that produce the stray voltage is called stray current. (c) M. A. El-Sharkawi, University of Washington Causes of Stray Voltage • The neutral and ground wires are not adequately separated • The neutral wire is deteriorated • The neutral is poorly grounded. • The neutral wire carries excessive currents at the service transformers due to unbalanced loads • Electromagnetic field coupling with metallic object (fence, pole, etc.) (c) M. A. El-Sharkawi, University of Washington Stray Voltage I I Internal Electric Circuit In V Ig In Stray Current (c) M. A. El-Sharkawi, University of Washington Ig What is the Problem? • Stray voltage can cause behavioral problems in farm animals due to their nerve stimulation • May cause sensitive hospital equipment to malfunction. • Swimming pools and outdoor shower shocks • In severe cases, the stray voltage could reach lethal levels if the neutral wire is broken (c) M. A. El-Sharkawi, University of Washington Stray Current (mA) 1-3 3-4 5-6 >6 Effect of Stray Current on Livestock (USDA) Effect on Livestock Signs of awareness by livestock, but no milk production is lost Animal may become more difficult to manage Short term changes in feed/water consumption or milk production Long term changes in feed/water consumption or milk production (c) M. A. El-Sharkawi, University of Washington Case Study 10A Internal Electric Circuit Service panel I=10A In 10A Rn=1Ω Rcow=500Ω Ig Rg1=20Ω Rg2=30Ω Ig Rg3=20Ω Icow Icow (c) M. A. El-Sharkawi, University of Washington V Ig+Icow Analysis of Case Study I g I cow I Rn 1 10 0.203 A Rn Rg1 Rg 2 //( Rcow Rg 3 ) 1 20 30 //( 500 20) I cow 0.203 Rg 2 Rg 2 Rg 3 Rcow 30 0.203 11 mA 30 20 500 According to the table, this level of stray current will have long term changes in animal’s feed and water consumption as well as milk production. (c) M. A. El-Sharkawi, University of Washington Equipotential Grid in Farms Equipotential grid (c) M. A. El-Sharkawi, University of Washington Service Transformer: Auto Connection to Detect Faults If Hot If vs N y If Neutral (N) If x (c) M. A. El-Sharkawi, University of Washington 3.8 Stray Voltage in Swimming Pools I1 Service Box I2 Rw N2 Vs N1 Rw I2 I5 I3 Earth surface EGC I4 I7 I9 I6 Rg1 Iman Rg2 Rg3 Insulated lining of pool I6 Iman (c) M. A. El-Sharkawi, University of Washington I3 I5 I9 Elimination of Stray Voltage in Swimming Pools Double bushing or isolation xfm Service Box I2 I2 I2 a Earth surface I4=0 I5=0 Iman=0 Rg2 (c) M. A. El-Sharkawi, University of Washington Rg4 EG for Swimming Pool 3 ft Swimming Pool (c) M. A. El-Sharkawi, University of Washington NEC 680.26 (Conductive Shell) Pool’s reinforcing steel Light fixture EGC Barrier Pool’s reinforcing steel EGC Light fixture (c) M. A. El-Sharkawi, University of Washington NEC 680.27.A3 (Insulated Shell) Barrier Light fixture EGC EGC Intentional bond Ground the water? Huh? (c) M. A. El-Sharkawi, University of Washington Stray Voltage in Outdoor Showers I1 Service Box Rw I2 Vs I2 Rw I4 Rman I2 I3 I6 Iman I5 Rg1 Rg2 I5 Iman (c) M. A. El-Sharkawi, University of Washington Iman Rg4 Stray Voltage in Hospitals Ground lead of a catheter Conductive Chassis Rwire I Internal Circuit Rwire Iman Rwire Iman C2 C1 Ic1 Iman Rgman Rg Iman (c) M. A. El-Sharkawi, University of Washington Equipotential Bonding Conductive Chassis of medical equipment Ground lead of a catheter Rwire I Internal Circuit Rwire Rwire Ic Bed frame Rg • For the isolated or the non-isolated systems, equipotential grounding must be established by bonding the patient to the chassis of the equipment and the bed frame (c) M. A. El-Sharkawi, University of Washington Equipotential Grounding Grounding strip of equipotential area Outlet Water pipes and faucet Conductive window frame Patient’s bed Building ground (c) M. A. El-Sharkawi, University of Washington Conductive door frame Severe Conditions • Most stray voltages due to the bonding of the ground and neutral wires at the customers site are small to cause hazardous conditions • However, if the neutral wire is damaged, lethal voltages could exist on objects frames – Not uncommon on lighting poles, fences and side walk fixtures. (c) M. A. El-Sharkawi, University of Washington Hazard of Neutral Damage I2 Hot I=I1+I2 In2 Neutral Rn2 I2 In2 I1 Rg2 Ig2 I In1 I2 Electric load 2 In1+In2 Rn1 I1 Electric load 1 Ig1+Ig2 Ig1 Rg1 Rg Ig1 Ig2 (c) M. A. El-Sharkawi, University of Washington I2 Hot I=I2 In2=0 Neutral I2 Rn1 In1=Is1 I In1=Is1 I2 Electric load 2 Rg2 Electric load 1 Ig2=I2 Rg1 Is2 Is1 Is1 Is2 (c) M. A. El-Sharkawi, University of Washington Rg Example the load current I2=5A, Rg=20, Rg1=Rg2=30 and Rn1=1. Assume that the neutral current between the two loads is broken and load 1 is not energized Compute the stray voltage at both loads. (c) M. A. El-Sharkawi, University of Washington Solution I 2 I s1 I s 2 Rg 20 I s1 I 2 5 2 A Rn1 Rg Rg1 1 20 30 I s 2 I 2 I s1 3 A Vstray1 I s1 * Rg1 2 * 30 60 V Vstray2 I 2 * Rg 2 5 * 30 150 V Notice that although the first load is de-energized, its stray voltage is 60 V due to the broken neutral wire. (c) M. A. El-Sharkawi, University of Washington How to Control Stray Voltage? • The most fundamental requirement in controlling stray voltage is to separate the Ground and Neutral conductors. – The Neutral wire provides the return path from the load back to the power source. – The neutral is grounded only once at the service entrance. – The neutral wire must never be grounded at a second place in the system. (c) M. A. El-Sharkawi, University of Washington