| Ohm's Law (again!)A common phrase heard in reference to 
                    electrical safety goes something like this: "It's not 
                    voltage that kills, it's current!" While there is an 
                    element of truth to this, there's more to understand about 
                    shock hazard than this simple adage. If voltage presented no 
                    danger, no one would ever print and display signs saying: 
                    DANGER -- HIGH VOLTAGE!  The principle that "current kills" is 
                    essentially correct. It is electric current that burns 
                    tissue, freezes muscles, and fibrillates hearts. However, 
                    electric current doesn't just occur on its own: there must 
                    be voltage available to motivate electrons to flow through a 
                    victim. A person's body also presents resistance to current, 
                    which must be taken into account.  Taking Ohm's Law for voltage, current, and 
                    resistance, and expressing it in terms of current for a 
                    given voltage and resistance, we have this equation:  
                      The amount of current through a body is 
                    equal to the amount of voltage applied between two points on 
                    that body, divided by the electrical resistance offered by 
                    the body between those two points. Obviously, the more 
                    voltage available to cause electrons to flow, the easier 
                    they will flow through any given amount of resistance. 
                    Hence, the danger of high voltage: high voltage means 
                    potential for large amounts of current through your body, 
                    which will injure or kill you. Conversely, the more 
                    resistance a body offers to current, the slower electrons 
                    will flow for any given amount of voltage. Just how much 
                    voltage is dangerous depends on how much total resistance is 
                    in the circuit to oppose the flow of electrons.  Body resistance is not a fixed quantity. It 
                    varies from person to person and from time to time. There's 
                    even a body fat measurement technique based on a measurement 
                    of electrical resistance between a person's toes and 
                    fingers. Differing percentages of body fat give provide 
                    different resistances: just one variable affecting 
                    electrical resistance in the human body. In order for the 
                    technique to work accurately, the person must regulate their 
                    fluid intake for several hours prior to the test, indicating 
                    that body hydration another factor impacting the body's 
                    electrical resistance.  Body resistance also varies depending on how 
                    contact is made with the skin: is it from hand-to-hand, 
                    hand-to-foot, foot-to-foot, hand-to-elbow, etc.? Sweat, 
                    being rich in salts and minerals, is an excellent conductor 
                    of electricity for being a liquid. So is blood, with its 
                    similarly high content of conductive chemicals. Thus, 
                    contact with a wire made by a sweaty hand or open wound will 
                    offer much less resistance to current than contact made by 
                    clean, dry skin.  Measuring electrical resistance with a 
                    sensitive meter, I measure approximately 1 million ohms of 
                    resistance (1 MΩ) between my two hands, holding on to the 
                    meter's metal probes between my fingers. The meter indicates 
                    less resistance when I squeeze the probes tightly and more 
                    resistance when I hold them loosely. Sitting here at my 
                    computer, typing these words, my hands are clean and dry. If 
                    I were working in some hot, dirty, industrial environment, 
                    the resistance between my hands would likely be much less, 
                    presenting less opposition to deadly current, and a greater 
                    threat of electrical shock.  But how much current is harmful? The answer 
                    to that question also depends on several factors. Individual 
                    body chemistry has a significant impact on how electric 
                    current affects an individual. Some people are highly 
                    sensitive to current, experiencing involuntary muscle 
                    contraction with shocks from static electricity. Others can 
                    draw large sparks from discharging static electricity and 
                    hardly feel it, much less experience a muscle spasm. Despite 
                    these differences, approximate guidelines have been 
                    developed through tests which indicate very little current 
                    being necessary to manifest harmful effects (again, see end 
                    of chapter for information on the source of this data). All 
                    current figures given in milliamps (a milliamp is equal to 
                    1/1000 of an amp):    BODILY EFFECT     DIRECT CURRENT (DC)    60 Hz AC     10 kHz AC
--------------------------------------------------------------- 
Slight sensation     Men = 1.0 mA         0.4 mA        7 mA 
felt at hand(s)    Women = 0.6 mA         0.3 mA        5 mA 
--------------------------------------------------------------- 
Threshold of         Men = 5.2 mA         1.1 mA       12 mA 
perception         Women = 3.5 mA         0.7 mA        8 mA 
--------------------------------------------------------------- 
Painful, but          Men = 62 mA           9 mA       55 mA 
voluntary muscle    Women = 41 mA           6 mA       37 mA 
control maintained                                           
--------------------------------------------------------------- 
Painful, unable       Men = 76 mA          16 mA       75 mA 
to let go of wires  Women = 51 mA        10.5 mA       50 mA 
--------------------------------------------------------------- 
Severe pain,          Men = 90 mA          23 mA       94 mA 
difficulty          Women = 60 mA          15 mA       63 mA 
breathing                                                    
--------------------------------------------------------------- 
Possible heart        Men = 500 mA        100 mA             
fibrillation        Women = 500 mA        100 mA             
after 3 seconds                                              
--------------------------------------------------------------- 
 "Hz" stands for the unit of Hertz, 
                    the measure of how rapidly alternating current alternates, a 
                    measure otherwise known as frequency. So, the column 
                    of figures labeled "60 Hz AC" refers to current that 
                    alternates at a frequency of 60 cycles (1 cycle = period of 
                    time where electrons flow one direction, then the other 
                    direction) per second. The last column, labeled "10 kHz AC," 
                    refers to alternating current that completes ten thousand 
                    (10,000) back-and-forth cycles each and every second.  Keep in mind that these figures are only 
                    approximate, as individuals with different body chemistry 
                    may react differently. It has been suggested that an 
                    across-the-chest current of only 17 milliamps AC is enough 
                    to induce fibrillation in a human subject under certain 
                    conditions. Most of our data regarding induced fibrillation 
                    comes from animal testing. Obviously, it is not practical to 
                    perform tests of induced ventricular fibrillation on human 
                    subjects, so the available data is sketchy. Oh, and in case 
                    you're wondering, I have no idea why women tend to be more 
                    susceptible to electric currents than men!  Suppose I were to place my two hands across 
                    the terminals of an AC voltage source at 60 Hz (60 cycles, 
                    or alternations back-and-forth, per second). How much 
                    voltage would be necessary in this clean, dry state of skin 
                    condition to produce a current of 20 milliamps (enough to 
                    cause me to become unable to let go of the voltage source)? 
                    We can use Ohm's Law (E=IR) to determine this:  E = IR  E = (20 mA)(1 MΩ)  E = 20,000 volts, or 20 kV  Bear in mind that this is a "best case" 
                    scenario (clean, dry skin) from the standpoint of electrical 
                    safety, and that this figure for voltage represents the 
                    amount necessary to induce tetanus. Far less would be 
                    required to cause a painful shock! Also keep in mind that 
                    the physiological effects of any particular amount of 
                    current can vary significantly from person to person, and 
                    that these calculations are rough estimates only.  With water sprinkled on my fingers to 
                    simulate sweat, I was able to measure a hand-to-hand 
                    resistance of only 17,000 ohms (17 kΩ). Bear in mind this is 
                    only with one finger of each hand contacting a thin metal 
                    wire. Recalculating the voltage required to cause a current 
                    of 20 milliamps, we obtain this figure:  E = IR  E = (20 mA)(17 kΩ)  E = 340 volts  In this realistic condition, it would only 
                    take 340 volts of potential from one of my hands to the 
                    other to cause 20 milliamps of current. However, it is still 
                    possible to receive a deadly shock from less voltage than 
                    this. Provided a much lower body resistance figure augmented 
                    by contact with a ring (a band of gold wrapped around the 
                    circumference of one's finger makes an excellent 
                    contact point for electrical shock) or full contact with a 
                    large metal object such as a pipe or metal handle of a tool, 
                    the body resistance figure could drop as low as 1,000 ohms 
                    (1 kΩ), allowing an even lower voltage to present a 
                    potential hazard:  E = IR  E = (20 mA)(1 kΩ)  E = 20 volts  Notice that in this condition, 20 volts is 
                    enough to produce a current of 20 milliamps through a 
                    person: enough to induce tetanus. Remember, it has been 
                    suggested a current of only 17 milliamps may induce 
                    ventricular (heart) fibrillation. With a hand-to-hand 
                    resistance of 1000 Ω, it would only take 17 volts to create 
                    this dangerous condition:  E = IR  E = (17 mA)(1 kΩ)  E = 17 volts  Seventeen volts is not very much as far as 
                    electrical systems are concerned. Granted, this is a 
                    "worst-case" scenario with 60 Hz AC voltage and excellent 
                    bodily conductivity, but it does stand to show how little 
                    voltage may present a serious threat under certain 
                    conditions.  The conditions necessary to produce 1,000 Ω 
                    of body resistance don't have to be as extreme as what was 
                    presented, either (sweaty skin with contact made on a gold 
                    ring). Body resistance may decrease with the application of 
                    voltage (especially if tetanus causes the victim to maintain 
                    a tighter grip on a conductor) so that with constant voltage 
                    a shock may increase in severity after initial contact. What 
                    begins as a mild shock -- just enough to "freeze" a victim 
                    so they can't let go -- may escalate into something severe 
                    enough to kill them as their body resistance decreases and 
                    current correspondingly increases.  Research has provided an approximate set of 
                    figures for electrical resistance of human contact points 
                    under different conditions (see end of chapter for 
                    information on the source of this data):  
                      
                      Wire touched by finger: 40,000 Ω to 
                      1,000,000 Ω dry, 4,000 Ω to 15,000 Ω wet. 
                      Wire held by hand: 15,000 Ω to 50,000 Ω 
                      dry, 3,000 Ω to 5,000 Ω wet. 
                      Metal pliers held by hand: 5,000 Ω to 
                      10,000 Ω dry, 1,000 Ω to 3,000 Ω wet. 
                      Contact with palm of hand: 3,000 Ω to 
                      8,000 Ω dry, 1,000 Ω to 2,000 Ω wet. 
                      1.5 inch metal pipe grasped by one hand: 
                      1,000 Ω to 3,000 Ω dry, 500 Ω to 1,500 Ω wet. 
                      1.5 inch metal pipe grasped by two hands: 
                      500 Ω to 1,500 kΩ dry, 250 Ω to 750 Ω wet. 
                      Hand immersed in conductive liquid: 200 Ω 
                      to 500 Ω. 
                      Foot immersed in conductive liquid: 100 Ω 
                      to 300 Ω.  Note the resistance values of the two 
                    conditions involving a 1.5 inch metal pipe. The resistance 
                    measured with two hands grasping the pipe is exactly 
                    one-half the resistance of one hand grasping the pipe.  
                      With two hands, the bodily contact area is 
                    twice as great as with one hand. This is an important lesson 
                    to learn: electrical resistance between any contacting 
                    objects diminishes with increased contact area, all other 
                    factors being equal. With two hands holding the pipe, 
                    electrons have two, parallel routes through which to 
                    flow from the pipe to the body (or visa-versa).  
                      As we will see in a later chapter, 
                    parallel circuit pathways always result in less overall 
                    resistance than any single pathway considered alone.  In industry, 30 volts is generally 
                    considered to be a conservative threshold value for 
                    dangerous voltage. The cautious person should regard any 
                    voltage above 30 volts as threatening, not relying on normal 
                    body resistance for protection against shock. That being 
                    said, it is still an excellent idea to keep one's hands 
                    clean and dry, and remove all metal jewelry when working 
                    around electricity. Even around lower voltages, metal 
                    jewelry can present a hazard by conducting enough current to 
                    burn the skin if brought into contact between two points in 
                    a circuit. Metal rings, especially, have been the cause of 
                    more than a few burnt fingers by bridging between points in 
                    a low-voltage, high-current circuit.  Also, voltages lower than 30 can be 
                    dangerous if they are enough to induce an unpleasant 
                    sensation, which may cause you to jerk and accidently come 
                    into contact across a higher voltage or some other hazard. I 
                    recall once working on a automobile on a hot summer day. I 
                    was wearing shorts, my bare leg contacting the chrome bumper 
                    of the vehicle as I tightened battery connections. When I 
                    touched my metal wrench to the positive (ungrounded) side of 
                    the 12 volt battery, I could feel a tingling sensation at 
                    the point where my leg was touching the bumper. The 
                    combination of firm contact with metal and my sweaty skin 
                    made it possible to feel a shock with only 12 volts of 
                    electrical potential.  Thankfully, nothing bad happened, but had 
                    the engine been running and the shock felt at my hand 
                    instead of my leg, I might have reflexively jerked my arm 
                    into the path of the rotating fan, or dropped the metal 
                    wrench across the battery terminals (producing large 
                    amounts of current through the wrench with lots of 
                    accompanying sparks). This illustrates another important 
                    lesson regarding electrical safety; that electric current 
                    itself may be an indirect cause of injury by causing you to 
                    jump or spasm parts of your body into harm's way.  The path current takes through the human 
                    body makes a difference as to how harmful it is. Current 
                    will affect whatever muscles are in its path, and since the 
                    heart and lung (diaphragm) muscles are probably the most 
                    critical to one's survival, shock paths traversing the chest 
                    are the most dangerous. This makes the hand-to-hand shock 
                    current path a very likely mode of injury and fatality.  To guard against such an occurrence, it is 
                    advisable to only use on hand to work on live circuits of 
                    hazardous voltage, keeping the other hand tucked into a 
                    pocket so as to not accidently touch anything. Of course, it 
                    is always safer to work on a circuit when it is 
                    unpowered, but this is not always practical or possible. For 
                    one-handed work, the right hand is generally preferred over 
                    the left for two reasons: most people are right-handed (thus 
                    granting additional coordination when working), and the 
                    heart is usually situated to the left of center in the chest 
                    cavity.  For those who are left-handed, this advice 
                    may not be the best. If such a person is sufficiently 
                    uncoordinated with their right hand, they may be placing 
                    themselves in greater danger by using the hand they're least 
                    comfortable with, even if shock current through that hand 
                    might present more of a hazard to their heart. The relative 
                    hazard between shock through one hand or the other is 
                    probably less than the hazard of working with less than 
                    optimal coordination, so the choice of which hand to work 
                    with is best left to the individual.  The best protection against shock from a 
                    live circuit is resistance, and resistance can be added to 
                    the body through the use of insulated tools, gloves, boots, 
                    and other gear. Current in a circuit is a function of 
                    available voltage divided by the total resistance in 
                    the path of the flow. As we will investigate in greater 
                    detail later in this book, resistances have an additive 
                    effect when they're stacked up so that there's only one path 
                    for electrons to flow:  
                        
                      Now we'll see an equivalent circuit for a 
                    person wearing insulated gloves and boots:  
                        
                      Because electric current must pass through 
                    the boot and the body and the glove to 
                    complete its circuit back to the battery, the combined total 
                    (sum) of these resistances opposes the flow of 
                    electrons to a greater degree than any of the resistances 
                    considered individually.  Safety is one of the reasons electrical 
                    wires are usually covered with plastic or rubber insulation: 
                    to vastly increase the amount of resistance between the 
                    conductor and whoever or whatever might contact it. 
                    Unfortunately, it would be prohibitively expensive to 
                    enclose power line conductors in sufficient insulation to 
                    provide safety in case of accidental contact, so safety is 
                    maintained by keeping those lines far enough out of reach so 
                    that no one can accidently touch them.  
                      
                      REVIEW: 
                      Harm to the body is a function of the 
                      amount of shock current. Higher voltage allows for the 
                      production of higher, more dangerous currents. Resistance 
                      opposes current, making high resistance a good protective 
                      measure against shock. 
                      Any voltage above 30 is generally 
                      considered to be capable of delivering dangerous shock 
                      currents. 
                      Metal jewelry is definitely bad to wear 
                      when working around electric circuits. Rings, watchbands, 
                      necklaces, bracelets, and other such adornments provide 
                      excellent electrical contact with your body, and can 
                      conduct current themselves enough to produce skin burns, 
                      even with low voltages. 
                      Low voltages can still be dangerous even 
                      if they're too low to directly cause shock injury. They 
                      may be enough to startle the victim, causing them to jerk 
                      back and contact something more dangerous in the near 
                      vicinity. 
                      When necessary to work on a "live" 
                      circuit, it is best to perform the work with one hand so 
                      as to prevent a deadly hand-to-hand (through the chest) 
                      shock current path.  |