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Basic Principles

Basic Principles. No action will occur unless electrical current is flowing No current will flow unless it finds a complete loop out from the source and back to it Current will flow through any and all available loops, although not equally Heating occurs as current flows, due to resistance.

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Basic Principles

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  1. Basic Principles • No action will occur unless electrical current is flowing • No current will flow unless it finds a complete loop out from the source and back to it • Current will flow through any and all available loops, although not equally • Heating occurs as current flows, due to resistance

  2. Ohm’s Law One volt will cause a current of one ampere to flow through a resistance of one ohm

  3. Ohm’s Law I = V/R V = I x R R = V/I where: I is Amperes (current) V is Volts (pressure) R is Ohms (resistance)

  4. Electrical Shock Injuries • Contraction of chest muscles • Temporary paralysis of nerve centers • Ventricular fibrillation • Heart muscle stops contractions • Hemorrhage and destruction of tissues • Burns

  5. The severity of electrical shock injuries is determined by: • The amount of current that flows through the body • The length of time the body receives the current • The path of the current through the body • The type of current (ac or dc) • The frequency of ac current (60 hz or other)

  6. HUMAN RESISTANCE TO ELECTRICAL CURRENT

  7. Electrical Shock Injury Example In mid-July, an electrician is working on an energized 120-volt, 20-amp ac circuit in an industrial plant in Alabama. She is standing on an elevated metal work platform with metal railing that is bolted to the structural steel of the building. She trips on a tool that was left on the platform and grasps the rail with one hand to steady herself. She continues to fall and contacts the energized parts with her other hand. what is the likely outcome?

  8. Since this incident takes place in Alabama in mid-July, it could be assumed that she would be perspiring heavily. her skin resistance might be as low as 1,000 ohms instead of the 100,000 to 600,000 ohms of dry skin. The path of the current would probably be from the hand contacting the electrical parts to the other hand touching the railing.

  9. Using ohm’s law, the current flow through her body would be: V(volts) / R(ohms) = I(amps) 120 volts / 1,000 ohms = 0.12 amps or 120 milliamps Given that the current would probably pass through her heart, the likely outcome would be ventricular fibrillation.

  10. Other Injury Risks • Falls to lower levels after receiving an electrical shock • Burns to the eyes or skin from an arc flash • Fire or explosion

  11. Power Generation, Transmission, & Distribution

  12. Service Entrance & Premises Wiring

  13. Basic Electrical Circuits • The 120-volt alternating current circuit is commonly used to power: • Portable power tools • Fixed-location cord and plug connected Equipment • Lighting • This type of circuit has three wires: • Hot (ungrounded) - colored black • Neutral (grounded) - colored white • Equipment ground - bare or colored green

  14. Portable Power Tool Wiring

  15. Protection & Prevention • Insulation • Overcurrent devices • Grounding • Polarity • Ground-fault circuit interrupter • Special wiring and equipment in hazardous locations • Work practices

  16. Insulation • The insulation on the conductors in cords and cables is intended to prevent contact with live parts

  17. Overcurrent Devices • Are intended to limit the current through an electrical circuit - rated in amperage • Provide protection by interrupting the circuit before the conductors overheat • Are primarily for protection of equipment, but provide some personnel protection also

  18. The two types of overcurrent devices commonly used are: • Fuses • Several types and sizes • Must be replaced after one actuation • Circuit breakers • Two types are magnetic or thermal • May be reset after tripping from overcurrent

  19. Plug Fuses Plug Fuse Panel Plug Fuse

  20. Cartridge Fuses

  21. Circuit Breaker Panel

  22. Grounding • Equipment ground - a path for carrying fault current to quickly operate the overcurrent device to protect personnel • System ground - connection to earth or conducting body that serves in place of earth

  23. Equipment Grounding

  24. Equipment Grounding

  25. System Grounding • AC systems operating at 50 volts or more must be grounded • System grounding is accomplished by connecting the neutral conductor to earth • The purpose of system grounding is to equalize voltage differences from: • Lightning strikes • Line surges • Accidental contact with higher voltage lines

  26. System Grounding

  27. Polarity • Correct polarity means that the hot and neutral conductors are properly connected • If polarity is correct, the hot wire will be connected to the on/off switch of the equipment

  28. Correct Polarity

  29. Reversed Polarity

  30. Hazardous Locations • Class I - flammable gas or vapor • Groups a-d • Divisions 1 & 2 • Class II - combustible dust • Groups e-g • Divisions 1 & 2 • Class III - combustible fibers or flyings • Group h • Divisions 1 & 2

  31. Hazardous Locations

  32. Hazardous Location Equipment • Equipment used in Class I & II hazardous locations is commonly called “explosion-proof” • Explosion-proof enclosures are designed to cool the hot gases from an internal ignition and allow them to escape • The cooled gases will not be hot enough to ignite the flammable vapor in the area

  33. Equipment used in a Class III hazardous location must not develop surface temperatures high enough to cause excessive dehydration or carbonization of accumulated fibers or flyings • Organic material that is carbonized or excessively dry is susceptible to spontaneous combustion

  34. Explosion-Proof Equipment

  35. Explosion-Proof Fitting

  36. Ground Fault Circuit Interrupter • A fast-acting device that protects a person from a potentially serious electrical shock • The GFCI compares the current that flows through the hot and neutral conductors in a 110/120 volt circuit • If there is a fault that causes some current to flow through another path(ground), the GFCI senses the resulting imbalance and quickly shuts off both the hot and neutral conductors

  37. The GFCE detects a ground fault of from 2 to 5 milliamps and reacts within approximately .02 seconds • The GFCI protection is independent of the equipment grounding conductor

  38. GFCI Receptacle GFCI Circuit Breaker Portable GFCI

  39. Work Practices • OSHA requires that safety-related work practices must be used to prevent electric shock or other injuries resulting either from direct or indirect electrical contact when work is performed near or on equipment or circuits which are or may be energized • These must be consistent with the nature and extent of associated hazards

  40. Live parts to which a worker may be exposed must be de-energized before the employee works on or near them unless de-energizing the parts would add to the hazards or be infeasible due to equipment design or operational limitations • When working on electric equipment or circuits that have been de-energized, the circuits must be locked out or tagged or both

  41. Only qualified persons may work on electric parts or equipment that have not been de-energized under the proper procedures • To be qualified, these persons must be capable of working safely on energized circuits and must be familiar with the proper use of personal protective equipment, insulating and shielding materials, and insulated tools

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