Short answer
The RCD sits between the main switch and the circuits. Line and neutral go in at the top and out again at the bottom to the breakers and the neutral bar. The critical mistake is the neutral: every circuit must return its neutral through the same RCD that supplies its line. If you do not, it trips straight away or, worse, it does not protect.
Who this article is for This is a technical explanation for people who want to understand how it works, and for fellow electricians. It is not an instruction to do it yourself. In a consumer unit the supply from the main fuse stays live, even when all breakers are off. Only work here if you are demonstrably competent under NEN 3140.
Where it sits in the chain
The order in a domestic consumer unit is always:
- Service connection and main fuse (from the grid operator, sealed)
- Kilowatt-hour meter
- Main switch
- RCD or RCDs
- Circuit breakers, one per circuit
- The fixed wiring into the house
Each RCD monitors the circuits behind it. A maximum of four final circuits per RCD, which follows from NEN 1010. See also separate circuits.
How it works, briefly
Inside is a current transformer that both line and neutral pass through. In a normal circuit the current going out is exactly equal to the current coming back, so the magnetic fields cancel each other out.
If current leaks away to earth, less comes back through the neutral than went in through the line. That difference induces a voltage in a detection coil, which releases the relay. With a 30 milliamp difference it switches off within a few hundredths of a second.
This is exactly why the neutral is so critical: if the neutral of a circuit returns through a different RCD, the device sees a difference that is not there and trips. Or the other way round, and then it misses a real leak.
The connection step by step
- Isolate the consumer unit and check it is dead. Note: only the part after the main switch is isolated. Before the main switch it stays live. Measure, do not assume.
- Clip it onto the DIN rail. Usually on the left of the row, with its breakers to the right of it.
- Connect the supply to the top terminals. Line and neutral come from the main switch. The cross-section follows the rated supply, usually 10 or 16 mm². Ferrules on every conductor, see wiring in the consumer unit.
- Connect the outputs to the bottom terminals. The line goes to the busbar feeding the breakers of this RCD. The neutral goes to a separate neutral bar that belongs to this RCD only.
- A separate neutral bar per RCD. This is the key point. Each RCD gets its own neutral terminal block and the circuits behind that RCD use only that neutral bar.
- Tightening torques as specified by the manufacturer. With a torque screwdriver, not by feel.
- Test and measure. Insulation resistance, continuity of the protective conductor, and the RCD itself tested for tripping current and tripping time with an RCD tester.
- Update the circuit chart.
The four mistakes made most often
1. Neutrals mixed up. By far the most common. Circuit 5 takes its line from RCD B but its neutral lands on the neutral bar of RCD A. Symptom: the RCD trips as soon as that circuit is loaded, or it stays on even though there is a leak. This almost always happens when an existing consumer unit is extended and someone grabs the first free neutral terminal they see.
2. More than four circuits behind one RCD. Four is the limit. More means that a fault takes out a large part of the house at once, and that the cause is harder to find.
3. Undersized supply. The RCD must be able to handle at least the rated current of the main switch. A 40A RCD behind a 63A main switch is wrong.
4. Wrong type. Type AC is no longer allowed in new installations. Type A is the basis. For EV chargers and some inverters you need type B or a charger with its own DC detection, see EV charger at home: single-phase or three-phase.
RCD or RCBO
| RCD | RCBO | |
|---|---|---|
| Protects against earth leakage | yes | yes |
| Protects against overload and short circuit | no | yes |
| Modules | 2 each, protects up to 4 circuits | 1 per circuit |
| In the event of a fault, what goes off | all circuits behind it | only that one circuit |
| Material costs | lower | higher |
| Wiring mistakes with the neutral | risk present | not possible |
RCBOs cost more but are better in practice, and the classic neutral fault cannot happen with them. They are my standard unless the customer wants something else.
From my own work
At a house in Sittard the RCD tripped at random, sometimes nothing for weeks, then three times in one evening. The previous electrician had fitted a new RCD, with no improvement. When I mapped out the consumer unit it turned out that during an extension two years earlier the neutral of the loft circuit had been put on the wrong neutral bar. As soon as something heavy was switched on in the loft, RCD A saw a difference that was not really there. Five minutes of reconnecting, problem gone.
When to call an electrician
For the connection itself, and for checking an existing consumer unit that several people have worked on. RCDs that trip at random are a wiring fault in nine out of ten cases, not a faulty component.
If you are stuck right now, go to the fault service and choose a time slot yourself. If the consumer unit needs renewing, see consumer unit replacement or request a quote.
Frequently asked questions
Can I connect an RCD myself?
There is no legal ban on installation work by private individuals, but in practice: do not. The part before the main switch stays live and without test equipment you cannot prove it is right.
How many circuits can go behind one RCD?
A maximum of four final circuits.
Why does each RCD need its own neutral bar?
Because the device compares the current in the line with the current returning in the neutral. If the neutral runs through a different RCD, it measures a difference that is not there.
What is the difference between an RCD and an RCBO?
An RCD only protects against earth leakage. An RCBO does that and at the same time protects against overload and short circuit, per circuit.
Which cross-section for the RCD supply?
That follows the main switch. Usually 10 mm² at 40A, usually 16 mm² at 63A.
My RCD trips at random, is it broken?
Usually not. In nine out of ten cases it is a wiring fault with the neutral or an appliance that leaks. First work through the step-by-step plan for an RCD that keeps tripping
Further reading
Which consumer unit brand should you choose? Hager, Schneider, ABB and the rest
All brands are equally safe. The difference is whether you can still add a circuit in eight years' time, and that is exactly what nobody tells you.
Meter cupboard dimensions: what fits and what are the requirements?
The standard sizes, how many modules really fit and the three things you need to measure before you order a bigger consumer unit.
Wiring in the consumer unit: which wire, which colour, which cross-section
Wire type, colours and cross-section per breaker, plus the mistake that most often causes a hot terminal. Background, not DIY instructions.
Questions and comments
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