Communication SignalsCommunication Signals

Electrical SafetyElectrical Safety

Fox Valley Technical CollegeAppleton, WI, USA

Current Paths Through the BodyCurrent Paths Through the Body

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It's not the It's not the voltagevoltage,,but the but the currentcurrent that kills. that kills.

• People have been killed by as little as 100 volts AC in the home and with as little as 42 volts DC.

• The real measure of a shock's intensity lies in the amount of current (in milliamperes) forced through the body.

• Any electrical device used on a house wiring circuit can, under the right conditions, transmit a fatal amount of current.

• Currents between 100 and 200 milliamperes (0.1 ampere and 0.2 ampere) are fatal.

• Anything in the neighborhood of 10 milliamperes (0.01) is capable of producing painful to severe shock.

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Safe Current ValuesSafe Current Values

1 mA or less:• Causes no sensation - not felt.

1 mA to 8 mA:• Sensation of shock• Not painful• Individual can let go at will, since

muscular control is not lost.

8 mA to 15 mA:• Painful shock• Individual can let go at will since

muscular control is not lost.

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Unsafe Current ValuesUnsafe Current Values15 mA to 20 mA:• Painful shock• Control of adjacent muscles lost• Victim can not let go

50 mA to 100 mA:• Ventricular fibrillation - a heart condition that can

result in death - is possible

100 mA to 200 mA:• Ventricular fibrillation occurs

200 mA and over:• Servere burns• Severe muscular contractions - so severe that chest

muscles clamp the heart and stop it for the duration of the shock. (This prevents ventricular fibrillation).

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As the current rises,As the current rises,the shock becomes more severe.the shock becomes more severe.•Below 20 milliamperes, breathing becomes labored

•Breathing ceases completely even at values below 75 milliamperes.

•As the current approaches 100 milli-amperes ventricular fibrillation occurs.

– This is an uncoordinated twitching of the walls of the heart's ventricles.

– Since you don't know how much current went through the body, it is necessary to perform artificial respiration to try to get the person breathing again

– If the heart is not beating, cardio pulmonary resuscitation (CPR) is necessary.

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Electric ShockElectric Shock

Electrical shock occurs when a person comes in contact with two conductors of a circuit or when the body becomes part of the electrical circuit.

• A severe shock can cause the heart and lungs to stop functioning.

• Severe burns may occur where current enters and exits the body.

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PreventionPreventionis the best medicine for electrical shock.is the best medicine for electrical shock.

•Respect all voltages– have a knowledge of the principles of electricity– follow safe work procedures

•Do not take chances– take a basic course in CPR (cardiopulmonary

resuscitation) so you can aid a coworker in emergency situations.

– Always make sure portable electric tools are in safe operating condition.

– Make sure there is a third wire on the plug for grounding in case of shorts.

• Any fault current should flow through the third wire to ground instead of through the operator's body to ground if electric power tools are grounded and if an insulation breakdown occurs.

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Typical Fuses & Circuit Breakers & Their Typical Fuses & Circuit Breakers & Their SymbolsSymbols

Fuse Symbol

Circuit Breaker Symbol

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Important Fuse Concepts

• Ampere Rating:The maximum current the fuse will handle without “blowing”

• Voltage Rating: [“burn- back voltage rating”]The maximum voltage the fuse can withstand after it becomes an “open circuit”

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Important Fuse Concepts

• “Over” Current: Any current in excess of the rated current.

– Over Current vs. Time CurvesAt 125% of rated current, many fuses can take up to 4 hours to open circuit!

• “Fault” Current: Current due to a fault; short circuit current.

– AIC: Amps Interrupt Current rating:The current the fuse can tolerate without exploding, during a fault condition, until the fuse becomes an open circuit.

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What does a fuse protect?

• NOT people!

• Not necessarily the equipment either!

• It prevents overheated wires and components, and thereby prevents fires.

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First Aid for Electric ShockFirst Aid for Electric Shock

Free the victim carefully from contact with the electricity:• Turn off the supply switch or remove the electrical device’s plug from its receptacle.

• If the switch or receptacle cannot be quickly located, the suspected electrical device may be carefully pulled free of the victim.

• If the equipment cannot be moved, the injured person should be pulled free of contact with stationary equipment.

– Protect yourself with dry insulating material– Use a dry board, belt, clothing, or other available non-conductive

material to free the victim from electrical contact.– Do NOT touch the victim until the source of electricity has been

removed.

• Warn other persons arriving on the scene not to touch the suspected equipment until it is de-energized.

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First Aid for Electric ShockFirst Aid for Electric Shock • A person who has stopped breathing is not necessarily dead but is in immediate danger. Life is dependent on oxygen. Death will surely result from continued lack of breathing.

• The heart may continue to beat for some time after breathing has stopped.

• Artificial ventilation (respiration) may be used to save a person who has stopped breathing.

• Records show that seven out of ten victims of electric shock were revived when artificial respiration was started in less than three minutes. After three minutes, the chances of revival decrease rapidly.

• Do not give artificial ventilation to any person who is breathing naturally.

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First Aid for Electric ShockFirst Aid for Electric Shock

• Victims of electrical shock may suffer heart stoppage (in addition to loss of breathing).

• Artificial ventilation alone is not enough when the heart has stopped.

• A technique known as CPR is needed to sustain life until help arrives.

• You should take a course available through the American Red Cross to learn the latest techniques used in CPR.

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First Aid for Electric ShockFirst Aid for Electric Shock

• Note: A heart that is in fibrillation cannot be restarted by closed chest cardiac massage. A special device called a defibrillator is available in some medical facilities and ambulance services.

• Muscular contractions are so severe with 200 milli-amperes and over that the heart is forcibly clamped during the shock. This clamping prevents the heart from going into ventricular fibrillation, making the victim's chances for survival better.

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Copper wire resistance table

 AWG Feet/Ohm Ohms/100ft Ampacity*   10 490.2 .204 30

12 308.7 .324 20

14 193.8 .516 15

16 122.3 .818 10

18 76.8 1.30 5

20 48.1 2.08 3.3

22 30.3 3.30 2.1

24 19.1 5.24 1.3

26 12.0 8.32 0.8

28 7.55 13.20 0.5

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ReferencesReferences

• http://www.elec-toolbox.com/Safety/safety.htm#general • http://www.cdc.gov/niosh/topics/electrical/ • http://www.cpsc.gov/cpscpub/pubs/elec_sfy.html• http://www.nssl.noaa.gov/edu/safety/lightning.html• http://www.usfa.dhs.gov/citizens/all_citizens/

home_fire_prev/electrical.shtm• http://www.smud.org/safety/world/html/parents2.html • http://www.osha.gov/SLTC/electrical/index.html

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