Electrical Safety

1. Electrical Safety

2. Acknowledgements • Shawn Goodman, Ph.D. – University of Iowa • Lori Leibold, Ph.D. – UNC-CH • Lessons In Electric Circuits, Volume I: DC by Tony R. Kuphaldt • In electronic form, this text can be accessed at http://www.allaboutcircuits.com/vol_1/index.html

3. Physiological Effects of Electricity

4. What happens when electric current passes through human tissue? • Burns • Physiologically the same as damage caused by an open flame BUT the electricity can burn tissue beneath the surface of the skin (including internal organs) • Involuntary muscle contraction • Muscles triggered by an external (shock) current will involuntarily contract • If the conductor delivering current faces the palm, the victim will be completely unable to let go of the wire • More likely with DC than AC • Often takes a while after current is stopped to regain voluntary control over muscles (e.g., tasers)

5. What happens when electric current passes through human tissue? • Disruption of the nervous system • Can override the tiny electrical neural impulses normally generated by the neurons • Diaphragm muscle controlling the lungs, and the heart can be "frozen" in a state of tetanus by electric current • The heart’s pacemaker is particularly vulnerable; even small currents can cause fibrillation

6. Physiological Effects of Electricity • AC vs. DC • DC tends to make the heart stand still • AC tends to to cause fibrillation • A "frozen" heart has a better chance of regaining a normal beat pattern than a fibrillating heart • Defibrillating equipment supplies a DC jolt of current, which halts fibrillation and gives the heart a chance to recover

7. Physiological Effects of Electricity Arc-Flash Injury 80% of electrical injuries and fatalities caused by heat, light, and pressure (blast) waves. Intense heat produced may cause severe burns, especially on unprotected flesh. The blast produced by vaporizing metallic components can break bones and irreparably damage internal organs.

8. Quick Review Electric current is capable of producing deep and severe burns in the body due to power dissipation across the body's electrical resistance. When involuntary contraction of muscles controlling the fingers causes a victim to be unable to let go of an energized conductor, the victim is said to be "froze on the circuit." Diaphragm (lung) and heart muscles are similarly affected by electric current. Even small currents can interfere with the heart's pacemaker neurons, causing the heart to flutter instead of beat. DC is more likely to cause muscle tetanus than AC, making DC more likely to "freeze" a victim in a shock scenario. AC is more likely to cause a victim's heart to fibrillate, which is a more dangerous condition for the victim after the shocking current has been halted.

9. Shock Current Path Why can birds rest on high-voltage power lines without getting shocked? High voltage source

10. Shock Current Path Both feet are touching the same wire, making them electrically common. There is no voltage between them to create current flow through the bird's body.

11. Shock Current Path The person is safe if he only touches one wire, right? WRONG! Don’t try this at home… Unlike birds, people are usually standing on the ground when they contact a “live” wire

12. Shock Current Path Power system intentionally connected to earth ground… System ground is a metallic rod or plate buried deep in the ground for maximum contact with the earth.

13. Shock Current Path He has unintentionally completed a circuit. path for current through the earth

14. Circuit Grounding If grounding a circuit provides an easy point of contact for someone to get shocked, why have it in the circuit at all? Wouldn't a ground-less circuit be safer?

15. Circuit Grounding

16. Circuit Grounding

17. But this could easily change with an accidental ground

18. Summary Electric shock can only occur when contact is made between two points of a circuit; when voltage is applied across a victim's body. Power circuits usually have a designated point that is "grounded:" firmly connected to metal rods or plates buried in the dirt to ensure that one side of the circuit is always at ground potential (zero voltage between that point and earth ground). A ground fault is an accidental connection between a circuit conductor and the earth (ground). Though dirt is a poor conductor, it can conduct enough current to injure or kill a human being. Normal footwear is not good enough to provide protection from shock by insulating its wearer from the earth.

19. Ohm’s Law Again “It's not voltage that kills, it's current!” So why do we have signs like these?

20. Ohm’s Law Again “It's not voltage that kills, it's current!” That’s about like saying, “It's not guns that kill, it's bullets!” Images modified from www.tonyrogers.com/.../webready/22_bullet_a.jpg

21. Ohm’s Law Again “It's not voltage that kills, it's current!” Correct: It is electric current that burns tissue, contracts muscles, and fibrillates hearts. However, current doesn't flow without a voltage to motivate it! A person's body also presents resistance to current, which must be taken into account.

22. E I = R Ohm’s Law Again High voltage means potential for large amounts of current through your body. Large amounts of current will injure or kill you. The more resistance a body offers to current, the slower electrons will flow for any given amount of voltage.

23. Ohm’s Law Again Factors affecting body resistance: Percentage of body fat Fluid intake Point of contact Presence of blood, sweat, or tears. The more resistance a body offers to current, fewer electrons will flow for any given amount of voltage.

24. Ohm’s Law Again How much current is harmful? DC men = 5.2 mA women = 3.5 mA 60 Hz AC men = 1.1 mA women = 0.7 mA EFFECT Perceptual Threshold Pain Unable to let go of wires Difficulty breathing Possible heart fibrillation How much 60 Hz AC voltage does it take to make someone “froze to the circuit” ? men = 62 mA women = 41 mA men = 6.0 mA women = 9.0 mA men = 76 mA women = 51 mA men = 16 mA women = 11 mA men = 23 mA women = 15 mA men = 90 mA women = 60 mA men = 100 mA women = 100 mA men = 500 mA women = 500 mA

25. Ohm’s Law Again Dry Fingers 400 kW 20 mA E = IR E = (20 mA)(400 kW) E = 8,000 volts ??? V

26. Ohm’s Law Again Wet Fingers 4 kW 20 mA E = IR E = (20 mA)(4 kW) E = 80 volts ??? V

27. Ohm’s Law Again Wet Fingers & Ring 1 kW 20 mA E = IR E = (20 mA)(1 kW) E = 20 volts ??? V

28. 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.

29. Ohm’s Law Again 30 voltsgenerally considered a conservative threshold value for dangerous voltage. If working with voltage > 30 volts, increase body resistance by using insulated tools, gloves, boots, and other gear.

30. Ohm’s Law Again Dumb. Ouch.

31. Ohm’s Law Again Dumber. @$%&!!.

32. Quick Review Harm to the body is a function of the amount of 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 bad to wear when working around electric circuits. It provides 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.

33. Safe Practices If at all possible, shut off the power to a circuit before performing any work on it. In industry, securing a circuit is commonly known as placing it in a Zero Energy State. All properly designed circuits have "disconnect" switch mechanisms for securing voltage from a circuit. Sometimes called "circuit breakers."

34. Safe Practices For maximum safety of personnel working on the load, a temporary ground connection could be established on the top side of the load, to ensure that no voltage could ever be dropped across the load: A ground connection on both sides of the load is electrically equivalent to short-circuiting across the load with a wire.

35. Safe Practices • After an electrical system has been put into a Zero Energy State, double-check to see if the voltage really has been secured in a zero state. • Try to power up the system using the usual starting switch. • Check for the presence of dangerous voltage with a measuring device before actually touching any conductors in the circuit. • Check your measuring device to see that it indicates properly on a known source of voltage. • Make momentary contact with the conductor(s) with the back of the hand before grasping it or a metal tool in contact with it. (Why?)

36. Emergency Response • If you see someone lying unconscious or "froze on the circuit," what do you do? • 1) Safely disconnect victim from the source of electric power. • Don’t touch the victim; there may be enough voltage dropped across the victim’s body to freeze you too. • Best option: Shut off the power by opening the appropriate disconnect switch or circuit breaker. • Other option: Dislodge the victim by prying or hitting them away with a dry wooden board or piece of nonmetallic material. Could try to drag victim away using an extension cord.

37. Emergency Response • If you see someone lying unconscious or "froze on the circuit," what do you do? • Safely disconnect victim from the source of electric power. • Check for breathing & pulse; administer CPR as necessary. • Closely monitor until trained emergency personnel arrive. There is danger of physiological shock, so keep the victim warm and comfortable. • Victims may suffer heart trouble up to several hours after being shocked. Be vigilant.

38. Common Sources of Electrical Hazard • Wet skin • Bathroom • Power receptacles should be away from bathtubs, showers and sinks • Never use a telephone or radio while sitting in the bathtub • Swimming pools • Use Ground-Fault Current Interrupting (GFI or GFCI) receptables • Extension cords and power tools • Look out for cracks or abrasions

39. Common Sources of Electrical Hazard • Downed power lines • 2400 Volts plus Voltage gradient

40. << << Safe Circuit Design Consider a clinical electrical appliance such as an audiometer. The wires conducting power to the audiometer are insulated from touching the metal case (and each other) by rubber or plastic. Audiometer with stylish metal case “Hot” wire Plug 120 V “Neutral” wire

41. << << Safe Circuit Design If one of the wires inside the audiometer accidentally contacts the metal case, the case will be electrically common to the wire. Whether or not this presents a shock hazard depends on which wire touches the case: “Hot” wire Plug 120 V “Neutral” wire

42. << << Safe Circuit Design If one of the wires inside the audiometer accidentally contacts the metal case, the case will be electrically common to the wire. Whether or not this presents a shock hazard depends on which wire touches the case: “Hot” wire Plug 120 V “Neutral” wire

43. << << Safe Circuit Design Engineers try to design appliances so as to minimize hot conductor contact with the case. “Hot” wire Plug 120 V “Neutral” wire

44. << << Safe Circuit Design However, this preventative measure is effective only if power plug polarity can be guaranteed. Appliances designed this way come with "polarized" plugs, one prong of the plug being slightly narrower than the other. Power receptacles are also designed like this, one slot being narrower than the other. Consequently, the plug cannot be inserted "backwards," “Hot” wire Plug 120 V “Neutral” wire

45. << << Safe Circuit Design Another option: use a third conductor to connect that case to ground The third prong on the power cord provides a direct electrical connection from the case to earth ground, making the two points electrically common with each other (no voltage can be dropped between them) “Hot” wire Plug 120 V << “Neutral” wire “Ground” wire

46. << << Safe Circuit Design If the hot conductor accidentally touches the metal appliance case, it will create a direct short-circuit back to the voltage source through the ground wire, tripping any overcurrent protection devices. “Hot” wire Plug 120 V << “Neutral” wire “Ground” wire

47. << << Safe Circuit Design Better yet, make the outside case nonconductive. Such appliances are called double-insulated. Audiometer with stylish plastic case “Hot” wire Plug 120 V << “Neutral” wire “Ground” wire

48. Quick Review Power systems often have one side of the voltage supply connected to earth ground to ensure safety at that point. The "grounded" conductor in a power system is called the neutral conductor, while the ungrounded conductor is called the hot. Grounding in power systems exists for the sake of personnel safety, not the operation of the load(s). Electrical safety of an appliance or other load can be improved by good engineering: polarized plugs, double insulation, and three-prong "grounding" plugs are all ways that safety can be maximized on the load side.