Open the simulator

Wired it wrong? Find out why.

Build a circuit, switch it, break it. WireWhy calculates the current in every wire, so you see why the RCD trips, the breaker holds, or the lamp stays dark.

Open the simulator or start the course →

No account needed · Computers, tablets, and phones · 230/400 V, 50 Hz; 127/220 V, 60 Hz in Brazil

Bathroom light · demo board
Supply TN-C-S LNPE RCD 30 mA CLOSED F1 B10 Switch N bar Lamp 1 A Supply TN-C-S LNPE RCD 30 mA CLOSED F1 B10 Switch N bar Lamp 1 A
Through the RCD

L 0.00 A · N 0.00 A · ΔI 0.00 A

The RCD trips at a 30 mA difference.

What happened

Switch open. Nothing flows.

The lamp's neutral is on the N bar, after the RCD.

Readings and the finding are the simulator's results for this board. Open this board in the simulator →

The circuit is solved, not just drawn.

A wiring diagram shows what is connected. WireWhy works out what that wiring does: every time you add a wire, flip a switch, or put in a fault, the whole circuit is calculated again.

  1. Your circuit

    Devices, terminals, wires, and the state of every switch and breaker.

    Supply TN-C-S 230 V RCD 30 mA → F1 B10 Switch closed
  2. Solving

    Steady-state AC, by modified nodal analysis with complex phasors.

    V at every terminal I in every wire
  3. Protection

    The fastest protective device that should trip, trips. Then the circuit is solved again, until nothing changes.

    ΔI 1.00 A ≥ 30 mA RCD trips after 20 ms
  4. What you see

    Currents in the wires, the trip, and a finding in plain words, with the current's path marked wire by wire.

    Finding: critical wires 1 → 5
Water heater on the demo board
load
39 A
cable
1.5 mm²
breaker
F2 B32
instant trip
no: that needs 5 × 32 A = 160 A
cable
overheats until its insulation fails
found by
the installation tester, insulation resistance
Metal light fitting, PE left off
fault
line to the fitting's metal
PE terminal
230 V against earth
protection
nothing trips
with PE wired
the breaker or the RCD clears it

Four things to try in the first five minutes

Follow the current that trips the RCD

Open the demo board and switch the bathroom light on. The RCD trips, and the wires that carry the stray current are numbered from the supply.

RCD 30 mA tripped after 20 ms at 1.00 A cause: the lamp's neutral bypasses the RCD

A traced RCD trip: numbered wires show the current that bypasses the RCD's neutral.

Put a fault on a device

An earth fault, a short circuit, or an overload, on any device. Leave the earth off a metal fitting and it shows the voltage you would touch.

Metal fitting: PE terminal at 230 V nothing trips

An earth fault on a metal light fitting with its earth left off: the fitting shows 230 V in red, and the finding explains why touching it is dangerous.

Measure it before you are told the answer

Clip the multimeter to any two terminals, or take the installation tester for continuity and insulation. Each reading goes into a results table against its IEC 60364-6 limit.

Lamp L–N: 229.2 V PE continuity, supply to lamp: 0.01 Ω

The multimeter clipped to a lamp's screws, reading 229.2 V, with the reading recorded in the results table.

Find the hidden fault

One fault is hidden in a working circuit: a failed lamp, a loose screw, a leak to earth. Switch, measure, name the device or wire at fault, and get a score for the answer, the reading that proves it, and safe isolation.

Hall light dark: lamp L–N 230 V E1, failed device: 85 / 100

A fault board answered: 230 V across the hall light that stays dark is recorded, the lamp is named as failed, and the answer scores 85 out of 100.

What WireWhy models

Low-voltage installations as an apprentice meets them: house boards, lighting, sockets, motor control, and the tests that prove them.

Supplies230/400 V 50 Hz · 220/380 V 50 and 60 Hz · 127/220 V 60 Hz · 120/240 V 60 Hz split-phase with 120/208 V three-phase · one, two, or three phases
EarthingTN-C-S, TN-S, TT · Ze of the network and RA of the earth electrode · a broken PEN · touch voltage on exposed metal
ProtectionMCB curves B, C, D, one to three poles · RCD 10–500 mA, two and four poles, selective · RCBO · AFDD · fuses · the service fuse · selectivity · GFCI breakers and receptacles on 120/240 V
Boardmain switch · energy meter · surge protectors, one to four poles · N and PE bars · DIN rails
Cablescross-section, length, copper or aluminium · rated by how they are installed · their resistance in the fault loop · overheating when the breaker does not cover the cable · junction boxes
Switchingone-way, two-way, intermediate, in one, two, or three gangs · push-buttons
Controlcontactors that hold themselves in · interlocked reversing starters · star-delta starters · overload relays · motor protection breakers · phase monitoring relays · time relays · impulse relays · staircase timers · time on the canvas: watch it run, or skip to the next event
Loadslamps · heaters · sockets, single- and three-phase · motors, single- and three-phase, with a terminal box for star or delta · indicator lamps · phase indicators
Faultsearth fault · short circuit · overload · open or poor contact · failed device · a neutral bypassing an RCD · a neutral touching earth · a lost PEN · unearthed metal · a surge from the supply
Instrumentsmultimeter V, Ω · voltage tester and safe isolation with a lock-off · socket tester · installation tester: continuity, insulation resistance, loop impedance, RCD trip time and current, phase sequence · a results table and a test report
Drawingswiring diagram · single-line diagram · board layout · terminal plan · breaker labels

What it is not

  • Not SPICE: no transistors and no transients, only installation wiring at 50 or 60 Hz.
  • Not a certified design package: the drawings are for training and planning.
  • Not a substitute for testing a real installation, or for a qualified electrician.
  • No arcs and no surges are solved: an AFDD trips as an RCBO would, and a surge from the supply reaches the surge protector by a rule.

Which rules it follows

  • Generic training assumptions based on IEC 60364. The currents themselves are solved from the circuit, not looked up.
  • Verdicts and the test report follow the country chosen in Preferences: the ⅔ correction of the loop test in Poland (PN-HD 60364-6) and Germany (DIN VDE 0100-600), Brazil's NBR 5410, and on 120/240 V the NEC's GFCI protection, small-conductor sizing, and multiwire branch circuits. WireWhy Pro's installation planner follows the full national rules: PN-HD 60364 and N SEP-E-002 in Poland, DIN VDE 0100 and DIN 18015 in Germany, NBR 5410 in Brazil.
Everything WireWhy does →

Who uses it

Learning

A course from the first lamp to motor control, a lab for every device, fault boards and endless fault practice, and exam-style boards. Exercises check your wiring, however you wire it.

Start the course →

Teaching

Show a borrowed neutral or a lost PEN as often as the class needs. Build exercises that mark themselves, and let a class follow your canvas with a short code.

Working electricians

Check what a customer's odd wiring really does before it goes into a wall: what trips, and what stays live.

Why I built it

Circuit simulators either teach electronics at component level, or let you draw wiring without saying what the installation does. I wanted one where you could wire a bathroom light wrong, switch it on, and see why the RCD trips.

Mateusz Hinca, WireWhy

Questions

Is it free?

Yes. The canvas, the simulation, exercises, sharing, and picture export are free to use.

Do I need an account?

No. Open the canvas and start. Your circuit is kept in your browser until you share it.

Does it work on a phone or tablet?

Yes. Devices, wires, and the simulation work by touch; a larger screen is more comfortable for big circuits.

Can I use it with my class?

Yes. Share an exercise as a link, or start a follow-along session: the class joins with a short code and sees your canvas live.

Build a circuit. Break it. See what happens.

Open the simulator