HU151 Lecture 6 Electrical Safety Fall 2012/2013

HU151_ Lect7_ Electrical2 Outline Classification of Exposure Electrical Hazards Electrical Hazard Control

HU151_ Lect7_ Electrical3 Classification of Exposure High Voltage >600 volts: typically associated with “outdoor” electrical transmission. Accounts for 60% of electrocutions (OSHA). Low Voltage: <600 volts: typically associated with “indoor” electrical service. Accounts for 32% of electrocutions (OSHA). Low voltage does not imply safe voltage.

HU151_ Lect7_ Electrical4 How Human Nerve Cells Transmit Signals. Dendrites Axon Synapse K+K+ Na + The Axon maintains a chemical balance with more potassium ions inside the cell and sodiom ions outside the cell.

HU151_ Lect7_ Electrical5 How Human Nerve Cells Transmit Signals. Dendrites Axon Synapse K+K+ Na + K+K+ When signal is transmitted the myelin sheet changes so that the sodium and potassium ions change places. This results in an electrical change in the cell and this in turn causes the next section of myelin to change.

HU151_ Lect7_ Electrical6 External Electrical Stimulation of Human Nerve Cells Electricity flowing through the human body can cause enough of a change in the electrical environment around a nerve cell to stimulate it.

HU151_ Lect7_ Electrical7 Conductivity of Human Body Electric current seeks the path of least resistance to the ground. Human tissues and body fluids are relatively good conductors because of high aqueous-electrolyte content. If a person touches an energized bare wire or faulty equipment, electricity will instantly pass through the body to the ground, causing potentially fatal shock.

HU151_ Lect7_ Electrical8 Effects of Mains Derived Current on the Human Body. Perception Can not let go Current Interruption of Normal Cardiac Function As current increases the effects get more severe. Tingly feeling Stimulates muscles And you cant let go

HU151_ Lect7_ Electrical9 Effects of Mains Derived Current on the Human Body. Ventricular Fibrillation:I > 50 mA Can’t let go:I > 5 mA Tingling SensationI > 0.5 mA For comparison two 60 Watt light bulbs draw a total of 1 ampere of current.

HU151_ Lect7_ Electrical10 Electrical Hazards I.Shock II.Burns III.Falls IV.Fire V.Explosions

HU151_ Lect7_ Electrical11 ELECTRIC SHOCK Electric shock occurs when the human body becomes part of the path through which current flows. The direct result can be electrocution.

HU151_ Lect7_ Electrical12 Electrical Burns Most common nonfatal electrical injury. Types: – Internal: “deep tissue”. – Skin: “entry” and “exit” points. – Arc: “flash” burns from heat and radiant energy. Common sites of visible skin burns are the hands and feet.

HU151_ Lect7_ Electrical13 Arc Flash and Arc Blasts Arc Flash: –80%-Burns due to ignition of clothing –Temperature-35,000 F –Fatal Burns-10 ft. –Molten metal Arc Blast: –Pressure Wave –Heat –Molten metal –Destruction of structures and life

HU151_ Lect7_ Electrical14 Arc Blast Cause –Short Circuit caused by working on energized equipment (Dropped Tool) –Occurs in milliseconds –Temp: 30,000 degrees –Air expands very violently (Excessive pressure)

HU151_ Lect7_ Electrical15 Falls Involuntary muscle contractions can “throw” workers and cause falls. If working at elevation, the fall may cause serious injury or death.

HU151_ Lect7_ Electrical16 Wiring Fires Wiring often fails due to: – faulty installation, – overloading, –physical damage, –aging and –deterioration by chemical action, heat, moisture and weather. Such wiring should be replaced and new circuits installed.

HU151_ Lect7_ Electrical17 EXPLOSIONS Explosions occur when electricity provides a source of ignition for an explosive mixture in the atmosphere.

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HU151_ Lect7_ Electrical19 1.Grounding 2.Fuses and Circuit Breakers 3.Ground-Fault-Circuit-Interrupters 4.PPEs 5.Insulated Tools 6.Signs and Tags Electrical Hazard Control

HU151_ Lect7_ Electrical20 Grounding Grounding is a method of protecting from electric shock. It offers low resistance path that has sufficient current-carrying capacity to prevent the build- up of hazardous voltages. Two Types –System Grounding –Equipment Grounding

HU151_ Lect7_ Electrical21 Electrical System Grounding One conductor of the circuit is intentionally grounded to earth Protects circuit from lightning, or other high voltage contact

HU151_ Lect7_ Electrical22 Equipment Grounding All metal frames & enclosures of equipment are grounded by a permanent connection or bond The equipment grounding conductor provides a path for dangerous fault current to return to the system ground at the supply source should a fault occur

HU151_ Lect7_ Electrical23 Fuses and Circuit Breakers –Each circuit must be protected by a fuse or circuit breaker that will blow or “trip” when its safe carrying capacity is surpassed. –If a fuse blows or circuit breaker trips repeatedly while in normal use (not overloaded), check for shorts and other faults in the line or devices. –Do not resume use until the trouble is fixed.

HU151_ Lect7_ Electrical24 Ground-Fault Circuit-Interrupters (GFCI’s) GFCI’s are designed to detect any leakage of current in an electrical circuit. GFCI’s turn off or “trip” the circuit whenever the leakage is greater than 5 mA. For comparison two 60 Watt light bulbs draw a total of 1 ampere of current.

HU151_ Lect7_ Electrical25 Types of GFCI’s 1. A GFCI receptacle used in place of standard receptacle. 2. A portable GFCI plugs into a standard receptacle. 3. A GFCI circuit breaker combines leakage current detection with the function of a circuit breaker. Whenever working in a wet area, or outdoors, employees should use one of these types of GFCI’s.

HU151_ Lect7_ Electrical26 Live parts of electric equipment operating at 50 volts or more guarded against accidental contact by approved cabinets Guarding of live parts

HU151_ Lect7_ Electrical27 Extension Chords Use extension cords only when necessary and make sure they are heavy enough for the job. Avoid creating an “octopus” by inserting several plugs into a multi-plug outlet connected to a single wall outlet. (CDC)

HU151_ Lect7_ Electrical28 Extension Chords (2) Extension cords should only be used on a temporary basis in situations where fixed wiring is not feasible. If it is necessary to use an extension cord, never run it across walkways or aisles. –It causes a potential tripping hazard. –It wears down the insulation.

HU151_ Lect7_ Electrical29 Electrical Protective Equipment Employees working in areas where there are potential electrical hazards shall be provided with, and shall use, electrical protective equipment that is appropriate for the specific parts of the body to be protected and for the work to be performed

HU151_ Lect7_ Electrical30 Insulated Tools When working near exposed energized conductors or circuit parts, each employee shall use insulated tools or handling equipment if the tools or handling equipment might make contact with such conductors or parts

HU151_ Lect7_ Electrical31 Safeguards for personnel protection The following alerting techniques shall be used to warn and protect employees from hazards which could cause injury due to electric shock, burns, or failure of electric equipment parts: –Safety signs and tags –Barricades –Attendants

HU151_ Lect7_ Electrical32 Lockout/Tagout  Your personal lock and personal danger tag is what protects you from systems being re-energized while you are working on them.  You are the only person authorized to remove them except under specially controlled conditions.  If you don’t install them, you are not protected!

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