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.
Continue your circuit →No account needed · Computers, tablets, and phones · 230/400 V, 50 Hz; 127/220 V, 60 Hz in Brazil
L 0.00 A · N 0.00 A · ΔI 0.00 A
The RCD trips at a 30 mA difference.
Switch open. Nothing flows.
The lamp's neutral is on the N bar, after the RCD.
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.
Devices, terminals, wires, and the state of every switch and breaker.
Supply TN-C-S 230 V
RCD 30 mA → F1 B10
Switch closedSteady state by modified nodal analysis with complex phasors, at the supply frequency and at every frequency the electronics add.
V at every terminal
I in every wireThe 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 msCurrents in the wires, the trip, and a finding in plain words, with the current's path marked wire by wire.
Finding: critical
wires 1 → 5Open 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
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
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
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
Plug a washing machine with a frequency converter into a socket behind a type AC RCD and put an earth fault inside it. The RCD stays closed: it cannot see current at a mixed frequency. Set it to type F and it trips.
RCD 30 mA type AC: stays closed mixed frequency: type F needed
Ask about your circuit in your own words. An AI assistant answers from the simulation, names your devices, and each answer is checked before you see it.
Why did RCD 30 mA trip? checked against the simulation
Low-voltage installations as an apprentice meets them: house boards, lighting, sockets, motor control, and the tests that prove them.
| Supplies | 230/400 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 |
|---|---|
| Countries and rules | Poland (PN-HD 60364) · Germany (DIN VDE 0100) · Austria (OVE E 8101) · Switzerland (NIN 2025) · Portugal (RTIEBT, TT by default) · Brazil (NBR 5410) · United States (NEC 2023) · United Kingdom (BS 7671) · elsewhere (IEC 60364) |
| Earthing | TN-C-S, TN-S, TT · the service connection's PEN, split on a PEN bar in the board · the earth electrode with its RA · realistic TT: L and N from the network, PE from the installation's own electrode, an unearthed TT flagged · Ze of the network · a broken PEN · touch voltage on exposed metal |
| Protection | MCB curves B, C, D, one to three poles · RCD 10–500 mA, two and four poles, selective · RCD types AC, A, F, B: a type too weak stays closed on a DC or mixed-frequency fault · RCBO · AFDD · fuses · the service fuse · selectivity · GFCI breakers and receptacles on 120/240 V |
| Board | main switch · energy meter · surge protectors, one to four poles · N, PE, and PEN bars · DIN rails |
| Cables | cross-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 |
| Switching | one-way, two-way, intermediate, in one, two, or three gangs · push-buttons |
| Control | contactors 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 |
| Loads | lamps · heaters · sockets, single- and three-phase: type E, Schuko, Swiss, NEMA · appliances with electronics: power supplies, a washing machine or heat pump on a frequency converter, an EV charger on a DC bus · their harmonics loading the neutral · their standing leakage adding up behind one RCD · motors, single- and three-phase, with a terminal box for star or delta · indicator lamps · phase indicators |
| Faults | earth 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 · a TT installation not earthed · an RCD type too weak for the load |
| Instruments | multimeter 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 |
| Drawings | wiring diagram · single-line diagram · board layout · terminal plan · breaker labels |
A course from the first lamp to motor control, a lab for every device, fault boards and endless fault practice, and exam practice with a timed mock exam. Exercises check your wiring, however you wire it.
Start the course →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.
Check what a customer's odd wiring really does before it goes into a wall: what trips, and what stays live.
And with WireWhy Pro: describe the house or list the small tasks, and send a quote from your own prices that the customer accepts by link.
See WireWhy Pro →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
Learning is free, without an account. Plans add devices, exam practice, saved circuits, and tools for running a class.
€0no account
The canvas and the simulation with every explanation, the home installation devices, the realistic view, the multimeter, exercises and a first fault board, one mock exam with a free account, sharing, and joining a class.
Open the simulator€24.99for 3 months
no renewal, about €8.33 a month
Everything free, plus motors and control devices, every fault board and endless fault practice, every exam board and as many mock exams as you like, each with what you missed, the advanced course units, the installation tester, your circuits saved online, your course progress on your phone and computer (start on the computer at school, carry on on your phone), and short live class sessions (3 followers, 20 minutes).
Open the simulator€11.99per month
Everything in Basic, your progress on phone and computer included, plus the exercise builder with rules for learners, unlimited live class sessions, the single-line diagram export, and professional mode.
Open the simulatorPer schoolper year, by invoice
The teacher plan for every teacher at your school, one invoice, and a pilot semester for the first schools. Optional: Basic for your students at a school price.
Write to usCompare all plans → Prices include VAT; checkout shows them in your currency. Cancel any time; refunds within 14 days. Terms · Refunds
The simulator is free: the canvas, the simulation and its explanations, the home installation devices (three-phase included), exercises, and sharing. Paid plans add motors and control devices, fault practice, exam practice, saving online, your progress on your phone and computer, and tools for teachers.
No. Open the simulator and start; your circuit stays in this browser until you share it. A pass needs no account either: pay now, and sign in later to use it on your other devices. You need an account for a subscription, and to keep circuits and course progress online.
Yes. Devices, wires, and the simulation work by touch; a larger screen is more comfortable for big circuits.
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.