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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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Ohm’s Law

One volt will cause a current of one ampere to flow through a resistance of one ohm

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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)

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Electrical Shock Injuries

• Contraction of chest muscles• Temporary paralysis of nerve

centers• Ventricular fibrillation• Heart muscle stops contractions• Hemorrhage and destruction of

tissues• Burns

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• 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)

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HUMAN RESISTANCE TO ELECTRICAL CURRENT

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Electrical Shock Injury ExampleIn 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?

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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.

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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.

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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

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Power Generation, Transmission, & Distribution

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Service Entrance & Premises Wiring

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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

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Portable Power Tool Wiring

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Protection & Prevention

• Insulation• Overcurrent devices• Grounding• Polarity• Ground-fault circuit interrupter• Special wiring and equipment

in hazardous locations• Work practices

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Insulation• The insulation on the conductors in

cords and cables is intended to prevent contact with live parts

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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

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•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

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Plug Fuse Panel

Plug Fuse

Plug Fuses

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Cartridge Fuses

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Circuit Breaker Panel

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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

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Equipment Grounding

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Equipment Grounding

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• 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

System Grounding

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System Grounding

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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

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Correct Polarity

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Reversed Polarity

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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

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Hazardous Locations

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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

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• 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

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Explosion-Proof Equipment

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Explosion-Proof Fitting

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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

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• 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

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GFCI Circuit Breaker

GFCI Receptacle

Portable GFCI

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• 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

Work Practices

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• 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

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• 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