Electrical Safety and Shock

It’s the current that hurts

A shock happens when your body becomes part of a circuit. How bad it is depends on how much current flows through you, the path it takes, how long it lasts, and its frequency. The body is most sensitive to the 50- and 60-hertz frequencies used for power. OSHA’s table, for current flowing from hand to foot for one second, shows how little it takes:

Why water makes it worse

Ohm’s law says current equals voltage divided by resistance. Most of your body’s resistance is in your skin, and water changes it enormously. NIOSH says dry skin may have a resistance of 100,000 ohms or more, but wet skin may have only 1,000 ohms. At 120 volts:

• Dry skin: 120 ÷ 100,000 = 1.2 milliamps. A faint tingle.
• Wet skin: 120 ÷ 1,000 = 120 milliamps. In OSHA’s 50-to-150 milliamp range, where breathing can stop and death is possible.

The OpenStax textbook uses different estimates, 200,000 ohms dry and 10,000 ohms soaking wet, and reaches the same lesson: getting wet can raise the current by a factor of ten or more, past the point where you can let go. OSHA adds that pure water is a poor conductor, but the small amounts of salt and other impurities in real water make it conduct, and wet skin acts as a conductor. That’s why outlets near sinks, tubs and outdoors need GFCIs (Lesson 6).

Never work live

Electricians plan their work so they never touch anything energized. OSHA’s rules say parts that have been switched off but not locked out or tagged must be treated as if they’re still energized, and only qualified people may work on energized parts. NIOSH puts it simply: always test a circuit to make sure it is de-energized before working on it.

On the job, that means lockout/tagout. OSHA says the first step before any inspection or repair is to turn the current off at the switch box and padlock the switch in the OFF position, so no one can switch it back on by mistake, and a tag warns others. Then a qualified person tests to prove the parts are dead (Lesson 9).

Overhead and downed power lines

Overhead power lines are usually not insulated. OSHA tells workers to stay at least 10 feet away from overhead power lines and to assume they are energized. Its rule for workers who aren’t qualified for power-line work sets a 10-foot clearance for lines up to 50,000 volts, plus 4 inches for every 10,000 volts above that, and the same for ladders, poles and equipment they’re holding or using. Ladders used near live parts must have nonconductive side rails.

Never go near a downed or fallen power line. It can electrocute you if you touch it or the ground nearby, and it can energize fences and other objects it touches. Call the utility. If a power line falls on your car, OSHA says to stay inside unless the vehicle is on fire.

Extension cords and power strips

CPSC has estimated that about 2,000 home fires a year are associated with electric cords or plugs, killing 60 people. Its advice and the U.S. Fire Administration’s:

• Use extension cords only when necessary and only temporarily. They aren’t as safe as permanent wiring.
• Don’t overload a cord or power strip. Use power strips with internal overload protection.
• Never use an extension cord with a major appliance; plug those straight into the wall.
• Use three-wire cords for appliances with three-prong plugs, and cords labeled for outdoor use outdoors.
• Don’t run cords under rugs or furniture, and don’t nail or staple them in place.
• Replace cracked, worn or damaged cords. Don’t repair them.

Cord size matters as much as wire size in the walls. CPSC warns that thin 18-gauge cords will overheat if they carry 15 amps.

Electrical fires

In 2021, U.S. fire departments responded to an estimated 24,200 home electrical fires, which caused about 295 deaths, 900 injuries and over $1.2 billion in property loss, according to the U.S. Fire Administration. In its study of 2014 to 2016, electrical fires most often started in bedrooms and in attics or crawl spaces, and wiring and outlets were the most common equipment involved.

Warning signs CPSC lists include breakers or fuses that trip often, lights that dim or flicker, sparks, sizzling or buzzing sounds, switch plates or cords that are hot or discolored, a smell of hot insulation, and any shock, even a mild tingle. Stop using that outlet, switch or appliance and have an electrician check it.

If there’s fire or smoke, get everyone out and call 911 from outside, on a mobile phone or from a neighbor’s. Fire extinguishers rated Class C are made for fires in plugged-in electrical equipment, because their agent doesn’t conduct electricity. The U.S. Fire Administration says to use an extinguisher only if the fire is small and contained, you’re safe from the smoke, and you have a clear way out; otherwise, leave.

Check yourself

Tap an answer to check it, or open “Show the answer.”

1. According to OSHA, about what current can cause respiratory arrest, with death possible?

   • 1 milliamp
   • 5 milliamps
   • 50 to 150 milliamps
   • 15 amps
   Show the answer

   50 to 150 milliamps. OSHA lists 50 to 150 milliamps as causing extreme pain, respiratory arrest and severe muscle contractions.

2. With NIOSH’s wet-skin figure of 1,000 ohms, how much current would 120 volts drive?

   • 1.2 milliamps
   • 12 milliamps
   • 120 milliamps
   • 120 amps
   Show the answer

   120 milliamps. I = V ÷ R = 120 ÷ 1,000 = 0.12 A = 120 milliamps.

3. What does lockout/tagout prevent?

   • Theft of tools
   • Someone switching the power back on while you work
   • Surges
   • Low voltage
   Show the answer

   Someone switching the power back on while you work. A padlock and tag on the switch keep it OFF until the person who placed them removes them.

4. How far does OSHA tell workers to stay from overhead power lines?

   • 1 foot
   • At least 10 feet
   • 100 feet
   • Any distance if wearing gloves
   Show the answer

   At least 10 feet. Stay at least 10 feet away and assume the lines are energized; more for higher voltages.

5. A cord feels hot when a heater is plugged into it. What’s the best move?

   • Cover it with a rug
   • Unplug and use a properly rated cord, or plug the heater straight into the wall
   • Add another cord
   • Wrap it in tape
   Show the answer

   Unplug and use a properly rated cord, or plug the heater straight into the wall. A hot cord is overloaded or too small. Heaters and major appliances belong in a wall outlet.

Go deeper

• OSHA 3075: Controlling Electrical Hazards (2002, revised) OSHA’s full guide to electrical hazards.
• NIOSH: Electrical Safety, Safety and Health for Electrical Trades, Student Manual (Publication 2009-113) NIOSH’s student manual for the electrical trades.
• OpenStax College Physics 2e, 20.6 Electric Hazards and the Human Body The physics of shock and thermal hazards, with worked examples.
• U.S. Fire Administration: Appliance and Electrical Fire Safety U.S. Fire Administration electrical fire safety tips.
• U.S. Consumer Product Safety Commission: CPSC Guide to Home Wiring Hazards (Publication 518) CPSC’s guide to warning signs in home wiring.
• Being an Adult 101: Test Smoke and CO Alarms Test smoke and CO alarms.

Sources

• OSHA 3075: Controlling Electrical Hazards (2002, revised)
• OpenStax College Physics 2e, 20.6 Electric Hazards and the Human Body
• NIOSH: Electrical Safety, Safety and Health for Electrical Trades, Student Manual (Publication 2009-113)
• OSHA 29 CFR 1910.333: Selection and Use of Work Practices
• OSHA Fact Sheet: Working Safely with Electricity (FS-3942, 2018)
• OSHA Fact Sheet: Working Safely Around Downed Electrical Wires (FS-3941, 2018)
• U.S. Consumer Product Safety Commission: Extension Cords and Power Strips safety alert (NSN-09-2)
• U.S. Fire Administration: Appliance and Electrical Fire Safety
• U.S. Consumer Product Safety Commission: CPSC Guide to Home Wiring Hazards (Publication 518)
• U.S. Fire Administration: Residential Building Electrical Fires (2014–2016), Topical Fire Report Series Vol. 19, Issue 8
• U.S. Fire Administration: Fire Extinguishers
• U.S. Consumer Product Safety Commission: Home Electrical Safety Checklist (Publication 513)