What is the difference between a circuit breaker and an RCD?

A breaker responds to the size of the current: when too many appliances are on (overload) or there is a short circuit, the main current exceeds its rating and it opens, protecting the wiring. An RCD compares the live and neutral currents. Normally they are equal; if someone touches live while standing on the floor, or a metal case leaks, some current returns through the ground, and a difference above 30 mA trips it within milliseconds, protecting people. A shock between live and neutral trips neither, which is why it is the most dangerous case.

Why does the test pen light at both holes of a socket?

Usually the incoming neutral is broken. The house neutral is no longer tied to earth but is still connected to live through any appliance that is switched on, so the neutral hole also lights the pen while the appliances do not work. Tick “Incoming neutral broken” in the tool and test both holes to see it.

How do I measure power with a 3000 r/(kW·h) meter?

The disc turns 3000 times per kW·h. Run only the appliance, count n turns in time t, then W = n/3000 kW·h = n/3000 × 3.6×10⁶ J and P = W/t. For example 50 turns in one minute gives W = 6×10⁴ J and P = 1000 W.

How is the actual power found at a lower voltage?

Treat the resistance as constant: R = U_rated²/P_rated from the rating plate, then P = U²/R, i.e. P = (U/U_rated)²·P_rated. A “220 V 100 W” lamp on 110 V gets a quarter of the power, 25 W. For two lamps with the same rated voltage in series, the one with the lower rated power has the larger resistance and therefore the larger actual power, so it is brighter.

Why do metal-cased appliances need 3-pin plugs?

The long pin connects the metal case to the earth hole. If the insulation fails and the case becomes live, the fault current flows to earth through a wire of only a few ohms, so the case stays near earth potential and very little current would pass through a person; the large leakage also trips the RCD at once. Without the earth, touching the case is a one-wire shock.

🏠

🏠Household Circuits & Electrical Safety - breakers, meter, test pen, shocks

The household-electricity chapter of middle-school physics as a hands-on wiring diagram: service cable, energy meter, main switch, circuit breaker, RCD, a lamp with its switch on the live wire, and a 3-pin socket (left neutral, right live, top earth). Plug in a TV, fridge, kettle and more and watch the total power and current change; overload the circuit and the wires glow before the breaker trips. Switch on test-pen mode and click wires or socket holes to see whether the neon lights, including the classic broken-neutral and switch-on-neutral faults. Further tabs cover timing the meter disc to measure power, tiered tariffs, actual power at reduced voltage, Joule heating, motors, one- and two-wire shocks, and a random exam-style quiz.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

Loading tool...

How to use Household Circuits & Electrical Safety

1On the House wiring tab, add appliances and pick a breaker rating; watch the total power, current and tripping
2Turn on test-pen mode and click live, neutral, earth or a socket hole; add a fault and test again
3On Meter & bill, start timing, count the disc turns to get the power, and enter readings and tariffs to get the bill
4Use Power & heat and Shock & safety to vary voltage and resistance and see why things happen
5Try the Quiz tab for random calculations and multiple-choice questions with instant marking

FAQ

What is the difference between a circuit breaker and an RCD?: A breaker responds to the size of the current: when too many appliances are on (overload) or there is a short circuit, the main current exceeds its rating and it opens, protecting the wiring. An RCD compares the live and neutral currents. Normally they are equal; if someone touches live while standing on the floor, or a metal case leaks, some current returns through the ground, and a difference above 30 mA trips it within milliseconds, protecting people. A shock between live and neutral trips neither, which is why it is the most dangerous case.
Why does the test pen light at both holes of a socket?: Usually the incoming neutral is broken. The house neutral is no longer tied to earth but is still connected to live through any appliance that is switched on, so the neutral hole also lights the pen while the appliances do not work. Tick “Incoming neutral broken” in the tool and test both holes to see it.
How do I measure power with a 3000 r/(kW·h) meter?: The disc turns 3000 times per kW·h. Run only the appliance, count n turns in time t, then W = n/3000 kW·h = n/3000 × 3.6×10⁶ J and P = W/t. For example 50 turns in one minute gives W = 6×10⁴ J and P = 1000 W.
How is the actual power found at a lower voltage?: Treat the resistance as constant: R = U_rated²/P_rated from the rating plate, then P = U²/R, i.e. P = (U/U_rated)²·P_rated. A “220 V 100 W” lamp on 110 V gets a quarter of the power, 25 W. For two lamps with the same rated voltage in series, the one with the lower rated power has the larger resistance and therefore the larger actual power, so it is brighter.
Why do metal-cased appliances need 3-pin plugs?: The long pin connects the metal case to the earth hole. If the insulation fails and the case becomes live, the fault current flows to earth through a wire of only a few ohms, so the case stays near earth potential and very little current would pass through a person; the large leakage also trips the RCD at once. Without the earth, touching the case is a one-wire shock.

Features

Full house wiring diagram with meter, main switch, breaker, RCD, lamp switch and 3-pin socket, plus current-flow animation
12 common appliances and a custom load: total power, main current and spare capacity, with overheating and tripping
Test-pen simulation: correct and wrong grip, both socket holes live when the neutral breaks, the hidden danger of a switch on neutral
Energy meter disc timing (N r/kW·h), reading differences and tiered electricity bills
Actual power vs voltage, Joule's law, kettle boiling time, motor efficiency and which of two series lamps is brighter
Six shock scenarios showing whether the RCD and breaker act, safety rules and a random quiz

Related tools

View all 196 Physics Tools →