What is an RCD and how does it work

An RCD compares the current going to appliances with the current coming back. If some returns another way, it can disconnect the supply. See how with a washing machine.

Does all the current come back?

Electricity supply
Consumer unit with RCD
Washing machine

Simplified TN-S example: PE returns fault current to the supply outside the RCD measurement. The MCB and other protection are omitted. Not a wiring diagram; arrows explain current comparison, not a fixed direction of alternating current.

To appliance via L: 5 A
Back via N: 5 A
Current difference: 0 mA

Normal operation

5 A flows to the machine through live L and 5 A returns through neutral N. Both conductors pass through the RCD sensor. The difference is zero: no residual-current reason to trip.

Contents

40 A and 30 mA mean different things

1: 40 A · In
Rated current it can carry under the manufacturer’s conditions, not an overload trip threshold. A 40 A RCCB need not trip at 45 A: a coordinated MCB or fuse must protect it.
2: 30 mA · IΔn
Rated residual operating current: sensitivity to the difference between outgoing and returning current. 30 mA = 0.03 A. This does not limit the appliance load to 30 mA.
3: Type A
Identifies detectable waveforms. Type A responds to sinusoidal AC and pulsating DC residual currents. Selection depends on equipment and local requirements. It is different from MCB curves B/C/D.
4: T / TEST
Creates an internal imbalance to check operation. It does not measure earthing resistance or replace installation testing by an electrician.

30 mA: when should it trip?

Move the leakage slider. The illustrative outgoing current is 5 A; the model shows currents before possible disconnection. A small load can still produce a detectable imbalance.

Leakage current0 mA

To appliance via L: 5 A
Back via N: 5 A
Current difference: 0 mA

0–15 mA: must hold during the test

At half IΔn, the specified test must not cause tripping. This is a device characteristic, not a safe-current classification for people.

These ranges apply to a standard non-delayed 30 mA RCD with sinusoidal AC residual current under test conditions. Other waveforms and device types have different conditions. 30 mA is not a safe body current, and trip time is not identical for every leakage current.

What an RCD does not protect against

Overload and L–N short circuits

Here RCD means an RCCB without overcurrent protection. Even a large current may return entirely through N with no imbalance. An MCB protects the cable; an RCBO combines both functions.

Not every electric shock

When someone touches L and N together, current through the body may pass out and back through the RCD without an imbalance. An RCD reduces risk in specified conditions; it does not make contact safe.

Not a replacement for PE

PE can carry fault current from a case before anyone touches it. Without PE, the case may remain live until another leakage path exists. An RCD does not fix a missing or broken PE. TT uses a different return path; requirements depend on the earthing system.

What the TEST button does

1.
Expect connected appliances to lose power. Follow the specific device instructions and local requirements; no single test interval applies to every model and environment.
2.
Use the built-in T/TEST button as instructed. With the required supply conditions, the device should trip. Never create leakage with wires, water or bodily contact.
3.
If it does not trip, do not assume the protection works: contact an electrician. A successful TEST does not verify PE continuity, insulation or the installation’s disconnection time.

Common questions

Can an RCD work without an earth conductor?

It can detect current returning outside the monitored conductors without PE. But with an unearthed faulty case, leakage might only start when someone touches it. It does not replace PE.

Can I replace 30 mA with 100 mA to stop trips?

That does not resolve the cause. A 100 or 300 mA device is not equivalent to 30 mA for additional personal protection. Find the fault, connection error or accumulated leakage.

Why do several rooms lose power together?

One shared RCCB may protect several circuits with individual MCBs. Leakage on one disconnects the whole group. Individual RCBOs can reduce the affected area.

Are RCD, RCCB and RCBO the same?

RCD is the umbrella term. RCCB has no overcurrent protection; RCBO combines residual-current, overload and short-circuit protection. This guide illustrates an RCCB.

How often should I press TEST?

As often as the device instructions say: many manufacturers recommend testing regularly, for example every few months. Pressing TEST cuts power to every circuit behind the RCD, so save your work first. If it does not trip, do not rely on it: call an electrician.

What is an RCD, and how does it detect leakage?