How do I wire a residual current circuit breaker

Retrofit an FI circuit breaker

Only a residual current circuit breaker protects people from electric shocks

The residual current circuit breaker, or FI circuit breaker for short, ensures that the circuit is interrupted in milliseconds when people come into contact with a defective line or a defective device. As this protection against fatal electric shock often does not yet exist in many old buildings, retrofitting is urgently recommended.

The function of the residual current circuit breaker

Normally functioning electrical devices have an equally strong current flow. The electricity that flows into a consumer (lamp, vacuum cleaner, toaster, etc.) also flows out again. However, if the consumer has an insulation fault (a broken cable) and a person touches it, a more or less strong current is discharged to earth via the body. So there is a difference between the incoming and outgoing current. And this is exactly where the residual current circuit breaker comes into play. It reacts very quickly and switches the device off at a difference of 30mA. Electric shock is still possible, but it is no longer so dangerous. Residual current circuit breakers of normal design switch off faulty electrical devices on all poles within 0.2 or 0.4 seconds.

However, it does not always have to be a person who discharges the fault current. An earth fault when a defective cable touches the earth also triggers the residual current circuit breaker.

IMPORTANT: The use of residual current circuit breakers has been mandatory in Germany since May 1, 1984 in rooms with bathtubs or showers (new buildings). Since February 1, 2009, all socket outlet circuits with a rated current of up to 20A must also be equipped with a residual current circuit breaker in new buildings.

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Retrofitting the residual current circuit breaker

Retrofitting a residual current circuit breaker should definitely be left to a specialist. Especially when it comes to such a sensitive area as protecting life and limb. Basically: Working on the electrical system is taboo for laypeople!

The installation of the FI switch is relatively simple in principle and is carried out in the fuse or distribution box. The device is available in stores for between 30 and 50 euros, depending on the version. Assembly takes about an hour.

FI circuit breakers are available in different sizes: 16 A, 25 A, 40 A, 63 A and 100 A (ampere). The amperage indicates the rated current for which the FI switch is designed. The dimensioning of the FI switch is selected depending on the upstream fuses. As a rule, 16 A is used to protect a house.

A place for the FI circuit breaker should be found in the distribution board after the meter, but in front of the fuses (machines / circuit breakers). In order to get the area behind the meter voltage-free, the fuses in the house connection box of the energy supplier would normally have to be "pulled". Since the house connection box is sealed by the energy supplier, this can only be done by a licensed electrician. If the distribution box is a sub-distribution, screw fuses are usually used in the upstream fuse box. They have to be unscrewed to get the distribution box free of tension.


  1. First the FI switch is attached in the distribution box behind the electricity meter and in front of the fuses (machine / circuit breaker).
  2. In the next step, the connection between terminal L1 and the fuses is disconnected with needle-nose pliers. Then the cable is fed into the connection terminal L1 of the FI circuit breaker and screwed tight. To see whether the cable is stuck, the long-nosed pliers pull the cable briefly and suddenly.
  3. Proceed in the same way with conductors L2, L3 and N.
  4. Then the wire strands in the corresponding colors are fed into the terminals L1, L2, L3 and N of the RCCB and screwed tight. Then the stranded wire is led to the corresponding connection terminals of the fuses (machine / circuit breaker) and clamped.
  5. "Pulled" or unscrewed fuses are reinserted. Before the FI circuit breaker is put into operation, the Duspol at the terminals of the FI switch should be used to determine whether the appropriate voltage is present. There must be 400 volts between the conductors L1, L2 and L3 and 230 volts between L1, L2, L3 and the N.
  6. If this is the case, the residual current circuit breaker is put into operation. To complete the retrofitting of the FI circuit breaker, the test button on the FI circuit breaker is pressed. It has to trigger immediately.

In order to rule out possible errors, the test button of the FI switch should be pressed at regular intervals to guarantee functionality.

Required material

  • FI circuit breaker
  • Wiring strands with wire end sleeves in the corresponding colors

Required tools

  • screwdriver
  • Needle nose pliers
  • Duspol
IMPORTANT: It is not without reason that the installation of electrical systems is one of the master craftsmen in Germany. This means that the electrical installation must be carried out by a master company and is taboo for laypeople! This ban applies to all electrical systems in the house.

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Tags:Electrical installation