F. Electricity and Magnetism · Electricity and Magnetism

Electrical Safety: Circuits, Homes and High-Voltage Lines

F3.9 F2.1 A1.4About 65 minutes

Learning goals

I can use Ohm's law to explain why current through the body is dangerous and why wet skin increases the danger.
I can explain how fuses, circuit breakers, grounding and ground fault circuit interrupters (GFCIs) protect people.
I can calculate whether appliances will overload a household circuit.
I can describe safety precautions around overhead and buried power lines, downed lines and transformer stations.

Words to know

fuse
A thin metal strip that melts and opens the circuit if the current is too large. It must then be replaced.

circuit breaker
An automatic switch that opens the circuit if the current is too large. It can be reset.

ground (earth) wire
A wire that connects metal parts of an appliance to the ground, giving fault current a low-resistance path so the breaker trips.

GFCI (ground fault circuit interrupter)
A device in an outlet or breaker that cuts the power in a fraction of a second if a small amount of current leaks out of the circuit, for example through a person or water.

overload
Drawing more current than a circuit is designed for, which can overheat its wires.

short circuit
An accidental very-low-resistance path that allows a very large current.

Ontario One Call
Ontario's free service that arranges for buried lines to be located and marked before anyone digs.

Respect the current

Throughout this unit you have used batteries of a few volts. A household outlet supplies 120 V, and the lines along your street and across the countryside carry thousands to hundreds of thousands of volts. The physics you have learned explains why electricity can harm people, and how engineers and electricians design systems that keep us safe. This lesson is about using that physics to stay safe for life.

It is the current that harms

Current through the body interferes with nerves and muscles, including the heart, and can cause deep burns. How much current flows depends on Ohm's law: I = V ÷ R.

Dry skin has a fairly high resistance. Wet skin, sweaty skin or a break in the skin has a much lower resistance, so the same voltage drives a much bigger current. That is why electricity and water are such a dangerous mix.
The path matters: current from hand to hand or hand to foot passes through the chest and heart.
The values in the diagram below are approximate, but they show that a tiny fraction of an ampere can be deadly. A household circuit can supply 15 A or more.

Rough effects of current through the body. Notice the scale: each mark is 10 times the one before. Even 0.1 A can disrupt the heart, and a household circuit can supply much more.

Worked example

Example 1: Why wet hands are dangerous

Suppose a person's body resistance is about 100 000 Ω with dry skin and about 1000 Ω with wet skin. Estimate the current if they touch 120 V in each case.

Dry: I = V ÷ R = 120 ÷ 100 000 = 0.0012 A = 1.2 mA: a tingle.
Wet: I = 120 ÷ 1000 = 0.12 A = 120 mA: enough to disrupt the heart's rhythm.
The voltage is the same; the lower resistance of wet skin multiplies the current by 100. (Real body resistance varies a lot, which is exactly why no shock should ever be considered "safe".)

About 1.2 mA dry versus 120 mA wet.

Protection in your home

Fuses and circuit breakers protect the wiring. Each branch circuit in a Canadian home is commonly rated 15 A or 20 A. If too many appliances are on (an overload) or there is a short circuit, the current rises, the wires would overheat and could start a fire, so the breaker trips or the fuse melts. Never replace a fuse with a higher-rated one or bypass a breaker.
Grounding: the round third prong on a plug connects the appliance's metal case to the ground wire. If a loose live wire touches the case, current rushes to ground through this low-resistance path and trips the breaker, instead of passing through you.
Polarized plugs (one blade wider) make sure switches cut the live side. Double-insulated tools (marked with a square inside a square) have no ground prong because no exposed metal can become live.
GFCIs protect people. A GFCI compares the current going out on the live (hot) wire with the current coming back on the neutral. Normally they are equal. If even a few milliamperes leak out, for example through water or a person, the GFCI cuts the power in a fraction of a second. That is why electrical codes require them for outlets near sinks and bathtubs, outdoors and in other wet locations. You can spot a GFCI outlet by its TEST and RESET buttons.

How a GFCI works. The live and neutral wires both pass through a sensing coil. Equal and opposite currents make magnetic fields that cancel. If some current leaks to ground, the currents differ, the changing field induces a signal in the coil (Faraday's law), and the switch opens.

Worked example

Example 2: Will the breaker trip?

A 120 V kitchen circuit has a 15 A breaker. A 1500 W kettle and a 900 W toaster are switched on at the same time. Will the breaker trip?

Kettle: I = P ÷ V = 1500 ÷ 120 = 12.5 A. Toaster: 900 ÷ 120 = 7.5 A.
The appliances are in parallel, so the currents add: 12.5 + 7.5 = 20 A.
20 A > 15 A, so yes, the breaker trips. Run them one at a time or on different circuits.

Yes: 20 A exceeds the 15 A rating.

Worked example

Example 3: Birds on a wire

A bird sits on a single high-voltage line and is not harmed. A person on a metal ladder who touches the same line can be killed. Explain using potential difference.

Current flows only if there is a potential difference across a path.
Both of the bird's feet are on the same wire, at the same potential, so the potential difference across its body is almost zero, and almost no current flows through it.
A person touching the line while standing on the ground (or on a ladder on the ground) bridges the line and the ground: a potential difference of thousands of volts. A large current flows through the body.
That is why you must never let any object (a ladder, a pole, a kite string, a drone) come near a power line.

The bird has no potential difference across it; the person is between the line and the ground.

High-voltage safety outside

Overhead lines: they are usually not insulated. Keep yourself and anything you hold, such as ladders, pool skimmers, kites and drones, well away. Ontario's Electrical Safety Authority advises staying at least 3 m from overhead lines. Electricity at high voltage can arc across a gap without contact.
Downed lines: always assume a fallen line is live. Stay at least 10 m away (about the length of a school bus) and call 911 and the local utility. The ground near a downed line can be energized; if you are close, shuffle away with small steps, keeping your feet close together, so there is little potential difference between your feet.
In a vehicle touching a downed line: stay inside and call for help. Leave only if there is a fire, by jumping clear with your feet together, without touching the vehicle and the ground at the same time, then shuffle away.
Buried lines: before digging (fence posts, gardens, decks), contact Ontario One Call, a free service that arranges for underground utility lines to be located and marked: "call or click before you dig".
Transformer stations and green boxes: never climb fences around transformer stations, and do not sit on, play on or open pad-mounted transformer boxes. If you see damage, report it to the utility.

Safe practice with circuits

In the lab and at home: disconnect power before working on a circuit; never touch circuits with wet hands; use only cords and appliances in good condition; do not run cords under carpets; and never put anything but a plug into an outlet.

Hands-on activity

Home electrical safety check

You need: a notebook, and a grown-up to walk around with you. Grown-up note: this is a look-only activity. Do not open the electrical panel cover, touch wiring, or unplug anything while your hands are wet. Pressing a GFCI's TEST and RESET buttons is safe and is recommended monthly by manufacturers, but do it together.

Find the outlets near the kitchen and bathroom sinks and any outdoor outlets. Do they have TEST and RESET buttons (GFCIs)? With your grown-up, press TEST (the power to that outlet should go off), then RESET.
With your grown-up, look at the labels on the electrical panel door (do not remove the cover). How many circuits are there? What current ratings do you see (often 15 A and 20 A)?
Pick one circuit, such as the kitchen counter. List the appliances often used on it and their wattages. Calculate the total current if they ran at once. Would the breaker trip?
Check for overloaded power bars, damaged cords or cords under rugs.
Outside, look for overhead lines near trees, decks or play areas. Where would it be unsafe to use a ladder or fly a kite?

What to look for: GFCIs near water; total currents compared with breaker ratings (for example, a 1500 W kettle alone draws 12.5 A, most of a 15 A circuit); a short list of fixes, such as moving a heater to another circuit or replacing a frayed cord.

Your turn

Use Ohm's law to explain why touching a live wire with wet hands is more dangerous than with dry hands.

What is the difference between what a circuit breaker protects and what a GFCI protects?

A 120 V circuit has a 15 A breaker. A 1200 W microwave and a 600 W coffee maker run at once. Will it trip?

What does the round third prong on a plug do?

How far should you stay from a downed power line, and what should you do?

Why must you contact Ontario One Call before digging a garden or setting fence posts?

More practice: warm-up

What happens to a fuse when the current is too large?

Where are GFCI outlets usually required?

What is an overload?

Why is a bird on one power line not harmed?

Name two objects you should keep away from overhead power lines.

More practice: on your own

A GFCI trips when the outgoing and returning currents differ by about 5 mA. Explain, using Faraday's law, how it detects this.

Someone replaces a 15 A fuse with a 30 A fuse because it "keeps blowing". Why is this dangerous?

Explain why you should shuffle with small steps away from a downed power line.

A 15 A, 120 V circuit already powers a 600 W heater and 200 W of lights. What is the most extra power that can be added?

Why are double-insulated tools allowed to have only two prongs?

Challenge

A worker's body resistance is 2000 Ω. Accidentally, one hand touches 120 V while the other touches a grounded pipe. (a) Estimate the current. (b) Would a 15 A breaker protect him? (c) Would a GFCI?

Explain why high-voltage lines can be dangerous even without touching them, and why safety distances increase for higher voltages.

A family uses an extension cord rated 10 A to run a 1500 W space heater. Is this safe? Explain.

Check your understanding

Quiz

Which device protects people by detecting current leaking out of a circuit?

Why do fuses and breakers exist?

A 120 V, 15 A circuit runs a 1200 W hair dryer and a 900 W heater. What is the total current?

What should you do if you see a fallen power line?

Why is wet skin more dangerous than dry skin around electricity?

What should you do before digging holes for a fence in Ontario?

For parents and teachers

Big idea: Expectation F3.9 asks students to describe safety precautions for electrical circuits and high-voltage transmission (for example "call before you dig", GFCI outlets in bathrooms, transformer stations, buried cables and overhead lines). The physics from this unit (Ohm's law, current paths, induction) explains each precaution.

How to help: The home safety check is worth doing together: test your GFCIs (TEST then RESET), look at your panel's labels without removing the cover, and talk about where it would be unsafe to use a ladder or fly a kite. Agree on what your family would do if a line came down near your home or car.

Please note: this lesson does not include a "Just for fun" joke, because it deals with serious injury risks. If your teen has experienced a shock or an electrical fire, check in about how they feel discussing it.

Common difficulties:

Thinking voltage alone is what harms. It is the current, which depends on voltage and resistance.
Thinking breakers protect people from shock. They protect wiring; GFCIs protect people.
Underestimating overhead lines: they are usually bare, and high voltage can arc across a gap.