Electric Shock and Trama - Electrical, Computer & Energy ...
Electric Shock and Trauma Lecture 29 April 15, 2019 Read Chapter 8 Volume 1 for Monday Musculoskeletal Effects of EMF Write a short summary In Chapter 8 Section 8.4.2 1. A possible problem for a mid term paper Look at the reference by Ross et.al to see if you can generate a feed back model that will explain the changes in results with frequency and
amplitude. How does 15 Hz fit into the picture? 2. Take anyone of the reviews in chapter 8 and go back into the references to see what is there on subjects like bone healing. Electric Shocks Background 1. In the US electrocution is 5th leading cause of fatal occupational injuries. Cost over 1 billion dollars annually. 2. More than 90% of the injuries occur to men 20 to 34 working in the electrical industry with 4 to 8 years experience. Power lines etc. 3. Other studies with similar data. Averages ages 37.5 and 32.4 4. Most domestic shocks are low voltage ( <1000V)
5. no-let-go for currents through the forearm for men 16ma and for females 11ma. Electric Shock Background 1. Lightening deaths 30/year and about 10 times that number of injuries. 2. RF burns or injuries are becoming more common. 3. The electrical currents in the body are largely carried by ions. Saline fluid conductivities are 1.4 Sm-1 4. The electric fields in tissue are relatively uniform so the currents vary with the conductivity.
Physicochemistry of Tissue Injury 1. Electrical injuries are more complex than just burns. 2. They can include electrochemical reactions, electroporation of cell membranes, mechanical trauma as well as thermal chemical reactions. 3. Cell membrane damage is most common characteristic of electrical tissue damage at low frequencies. 4. This leads to ion imbalances that may be more than there is enough energy to restore leading to cell deaths.
Direct Electric Force Mediated Injury 1. The largest electric fields and forces appear across the membranes and are shape dependent. 2. For a sphere where R is the radius, is the off axis angle and fs the sub- frequency dispersion limit for the cell charging time 3. When the transmembrane potentials get greater than 200-300mv you can get membrane disruption. Interior muscle cells are more likely to be damaged than one close to more conductive tissues. Current Flow for an Applied Voltage
Membrane Damage 1. Floating cells 10-20m the order of 1kV/cm 2. Skeletal muscle up to 8cm and nerve cells to 2m long are at much lower fields as small as 60V/cm 3. Cell healing time predicted to be in seconds, however some healing time have been measure to be on the order of minutes. 4. The results are that fields of 30V/cm to 150V/cm for milliseconds can lead to muscle tissue damage. 5. This kind of damage is often observed electric shock victims
Thermal Burn Injury 1. Heating results in the breaking of chemical bonds. The lowest energy bonds are often associate with protein folding, shapes or connections between proteins. Protein denaturation. is the fraction with kinetic energy above Ea, kBis Boltzmanns constant, T is the temperature. Heat Damage Thermal Injury 1. Lipid damage at 45oC as lipids go into solution. Loss of muscle contraction.
2. Heat damage where current density is the highest. Electroporation Thermal Damage RF and Microwave Heating. 1. The depth of penetration depends on the frequency, conductivity, and
2. The highest temperature may occur well below the surface of the skin. 3. You need to solve the heat transfer equations. Lightning Injury 1. Lightning Characteristics t= 1- 10ms I=30,000 to 50,000A temperatures to 30,000 K 2. Direct Strike Body currents to several hundred amps for 1-10s followed by smaller currents for several milliseconds. Can cause damage and death. 3. For nearby strikes the current spreads out along the ground and
can induce voltages between your feet. At 10m you might get 1,500V between your feet and 2-3 amps through the body for 10s Common Clinical Issues 1. Brief accidental shocks by electrical works lead to neuromuscular dysfunction and pain 2. Acute pain normally goes away in a few days. However, many shock victims may develop problems later with musculoskeletal pain, loss of balance control and neuropsychological symptoms. 3. MRI is often the best way to observe these problems.
Safety Guide Lines The National Electrical Code (NEC) in the U.S. considers 5 mA (0.005Amps) to be a safe upper limit for children and adults; hence the 5 mA Ground Fault Interrupter (GFI) circuit breaker requirement for wet locations. (The Physical Effects of Electricity) The values in Table 1 should be used as a guide instead of absolute data points. For instance, 99% of the female populations have a let go limit above 6 mA with an average of 10.5 mA. 99% of the male populations have a let go above 9 mA, with an average of 15.5 mA. (The Physical Effects of Electricity) Ventricular fibrillation can occur at current levels as low as 30 mA for a two year old child and 60 mA for adults. Most adults will go into ventricular fibrillation at hand to hand currents below 100 mA (0.1 Amp). (The Physical Effects of Electricity)
14 Will the 120 volt common household voltage produce a dangerous shock? It depends! If your body resistance is 100,000 ohms, then the current which would flow would be: I = 120 volts = .0012 A = 1.2 mA 100,000
This is just about at the threshold of perception, so it would only produce a tingle. If one had just played a couple of sets of tennis, and is sweaty and barefoot, then the resistance to ground might be as low as 1000 ohms. Then the current would be: I = 120 volts = .12A = 120 mA 1,000
This is a lethal shock, capable of producing ventricular fibrillation and death! The severity of shock from a given source will depend upon its path through the body. (Nave & Nave) 15 Body Resistance Table 2. Typical Human Body Resistance to Electrical Current Body Area Resistance (ohms)
Dry Skin 100,000 to 600,000 Wet Skin 1,000 Internal body (hand to foot) Ear to Ear
400 to 600 ~100 Table 2 shows some of the typical human body resistances to electrical current. Barring broken skin, bodycircuit resistance, even in contact with liquid, will probably be not less than 500 ohms. However, the current flow at this resistance and 120 volts is 240 mAover twice what is required to cause death. (Biological Effects of Electric Shock) 16
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