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Circuit breakers

Content objective:

What are we learning and why are we learning this? Content, procedures, or skills.

Vocabulary objective

Tier II: High frequency words used across content areas. Key to understanding directions & relationships, and for making inferences.

Tier III: Low frequency, domain specific terms.

Building on what we already know

Make connections to prior knowledge. This is where we build from.


You can use adapters to turn one outlet into two…and then two outlets into four, and so on. What happens if you turn on all the devices connected to all these cords at the same time?

They draw a lot of current through the wires to that outlet – and those wires can overheat, and start an electrical fire.

Electrical fire

Electrical fire outlet

This is why we need something which can detect abnormally high electrical currents – and cut them off.

These devices are circuit breakers and fuse boxes.

Here we see what could be a potentially fatal accident: a wet electrical appliance could conduct enough electricity to kill a person.  How can we avoid this?

hair dryer in water safety

Hair dryers are made safer nowadays because they include a ground fault circuit interrupter (GFCI) or Residual Current Device (RCD)

These shut off a circuit when it detects that current is flowing along an unintended path, such as through water.

ground fault circuit interrupters

A GFCI on a hair dryer.

ground fault circuit interrupter hair dryer

How does a Residual Current Circuit Breaker Work?

It works by comparing the current going in to an appliance with the current coming out.

When an appliance is working correctly, all of the current entering the appliance through the live wire is returned to the power supply through the neutral wire.

In the picture below the strength of the magnetic field is the same in both coils because they both have the same current.

{ from gcsescienc e.com  }

circuit breaker

If something goes wrong with the appliance then some of the electric current will flow through the earth wire. The amount of current flowing through the neutral wire decreases and now there is a difference between the current entering the appliance through the live wire and the current returned to the power supply through the neutral wire. This difference is called the residual current.

The coil connected to the neutral wire now has a weaker magnetic field than the coil connected to the live wire. The iron rocker turns about the pivot and the contacts are disconnected which switches off the appliance and makes it safe. See the picture below.

{ from gcsescienc e.com  }

circuit breaker open

Here’s a great infographic showing a mechanism inside a modern circuit breaker.

Circuit breakers GIF infographic Southland Electrical

Learning Standards

Massachusetts 2016 Science and Technology/Engineering (STE) Standards

HS-PS3-3. Design and evaluate a device that works within given constraints to convert one form of energy into another form of energy. {e.g. chemical energy in battery used to create KE of electrons flowing in a circuit, used to create light and heat from a bulb, or charging a capacitor.}

HS-ETS4-1(MA). Research and describe various ways that humans use energy and power systems to harness resources to accomplish tasks effectively and efficiently.
Clarification Statement:
• Examples of energy and power systems can include fluid systems such as hydraulics and pneumatics, thermal systems such as heating and cooling, and electrical systems such as electronic devices and residential wiring.

College Board Standards for College Success: Science

Objective PS.4.2 Electric Circuit Interactions
Students understand that during electric circuit interactions, electrical energy is transferred from the source of electric current to the electrical device, or devices, in the circuit

PS-PE.4.2.2 Investigate the variables (e.g., the number of series batteries, the length of a wire, the diameter of a wire, the metal composition of a wire) that affect the current flow through a wire.

PS-PE.4.2.4 Given a real-world situation involving simple electric circuits, with the electrical energy source and device(s) identified as the system:
PS-PE.4.2.4a Observe changes in the objects and the surroundings during the interaction.

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