Laboratory Safety. Physiological Effects of Electricity. by Rodger Ziemer, UCCS ECE Department

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1 Laboratory Safety by Rodger Ziemer, UCCS ECE Department Physiological Effects of Electricity Shocks Involving 60 Hz, Sinusoidal Voltages 1. Effects of 60 Hz sinusoidal voltages are related to the rms value of the current and the duration of the shock. 2. Body resistance is about 500 ohms between any two limb extremities; corresponds to the resistivity of sea water which is about one ohm-meter. 3. Skin contact resistance also determines the amount of current - depends on voltage, frequency, current duration, contact area, contact pressure, skin condition, and moisture level. Varies from 1000's of ohms to essentially zero. 4. Measurements on human volunteers and cadavers indicate that 500 ohms limb-to-limb is a conservative value. Effects on Humans of 60 Hz Voltages 1. Threshold of perception for finger tapping contact at 60 Hz is about 0.2 ma. 2. Let-go current (mind still functions, but cannot control muscles voluntarily): 10.5 ma for women (50% of subjects) and 16 ma for men. 6 ma has been determined as the safe value for let-go current. 3. Asphyxia passage of continuous current through the chest cavity causes the chest muscles constantly to contract, interfering with breathing. Lab Safety: Physiological Effects of Electricity 1

2 4. Respiratory arrest breathing arrested. Shocks affecting the medulla of the brain stem are effective in causing. 5. Ventricular fibrillation uncoordinated, asynchronous contraction of the ventricular muscle fibers of the heart. A person becomes unconscious in less than 10 seconds and suffers brain damage in 3 to 6 minutes after onset of ventricular fibrillation. CPR can be used to restore some circulation, but a defibrillator (electrical shock of heart muscles) is needed to get heart restarted. From experiments with animals, extrapolated to humans, the current value through the chest cavity that will cause ventricular fibrillation is given by A safe current limit of 500 ma for shocks less than 0.2 seconds in duration and 50 ma for shocks longer than 2 seconds in duration has been proposed. 6. Asystole - heart does not beat. Currents above 1 A through the chest cavity can cause such a condition. Effects of Frequency In the following table the current values relative to the 60 Hz values are given. Frequency (Hz) I 100 = ma rms, 0.2 < t < 2 s t Threshold of Perception Let-go Ventricular Fibrillation 15 to Lab Safety: Effects of Frequency 2

3 Frequency (Hz) a. Insufficient data Electrical burns, lightning, and high voltage shocks 1. Burns caused by arcing to the skin and/or heating of the point of contact due to current. 2. Lightning death can be caused by direct strike, by currents flowing in the ground or by a sideflash from a nearby object struck by lightning. Transient arc currents characterizing lightning can reach peak values of 30,000 A; the peak occurs within 5 microseconds and is over within 100 microseconds. 3. High voltage shocks can be considered to be due to any voltage over 600 V. Shocks due to such voltages usually leave physical evidence such as burns on the body. Power line contacts Threshold of Perception Let-go Ventricular Fibrillation 1, , * 10, * a 1. Usually due to overhead distribution or cross-country transmission lines. Such lines are usually bare and operate at line-to-ground voltages of 2,400 to 19,900 volts. Distribution systems within inhabited areas are at lower heights than cross-country distribution systems and usually operate at 7,200 V. Lab Safety: Effects of Frequency 3

4 2. Contrary to beliefs of some persons, injury or death must invariably involve contact with such lines rather than arcing. Striking an arc requires 75,000 V per inch or a gap distance of 0.1 inch for a 7,200 V line. Once struck, an arc can be maintained with a voltage gradient of only 50 V per inch. 3. Will a person in a vehicle in contact with an electrical transmission line be electrocuted? Only if that person forms a transmission path to ground. (Note that birds sitting on transmission lines are not electrocuted.) Equipment Grounding 1. It is important that the metallic case of equipment be grounded to prevent an internal equipment fault from using an electrical path from equipment to person to ground. This is the reason for the three-wire plugs found on many hand-held electrical tools. 2. Insulator cases preclude a path being made from internal fault to case to person to ground. 3. Electrical currents in water can electrocute! For example, a hair dryer dropped into a bathtub full of water can electrocute a child in the water, whether the appliance is switched off or not. The path from appliance through the water (and the child - the human body is mainly water) and to ground through the plumbing will cause the electricity to flow through the human body - whether it electrocutes or not depends on the resistivity of the total path and the resultant current that flows. Note that distilled water is an insulator, but tap water and bodily fluids are not pure water! Lab Safety: Effects of Frequency 4

5 Hazards in Electrical Engineering Laboratories 1. Can come about due to improperly grounded equipment, large capacity capacitors charged up, goofing around, curious children, etc. 2. A few simple rules to follow when you are doing your laboratory work: No food or drinks in the laboratories; No children allowed in laboratories; No goofing around in laboratories; If a piece of equipment appears to be malfunctioning, notify your lab instructor immediately - do not try to fix it! In some cases your lab instructor will ask to inspect your circuit before you apply the power. Reference [1] E. K. Greenwald (editor), Electrical Hazards and Accidents, New York: Van Nostrand Reinhold, Lab Safety: Hazards in Electrical Engineering Laboratories 5