Testing & safety
RCDs and arc fault detection devices: what is mandatory and what makes sense?
The residual current device (RCD) saves lives; the arc fault detection device (AFDD) detects dangerous arcs. We explain when each device is required in Germany and where retrofitting is worthwhile.
TOKSEL Elektrotechnik editorial teamPublished on 4 min read
In every modern distribution board they sit side by side: miniature circuit breakers (MCB, Leitungsschutzschalter), residual current devices (RCD, FI-Schutzschalter) and, increasingly, arc fault detection devices (AFDD, Brandschutzschalter). All three provide protection – but against very different hazards. If you know what the devices do, you can decide more easily where retrofitting makes sense.
Three protective devices, three tasks
| Device | Protects against | Trips on |
|---|---|---|
| Miniature circuit breaker (“fuse”) | Overload and short circuit – protects the cable | excessive current |
| Residual current device (FI/RCD) | Dangerous currents through the body – protects people | a fault current to earth, e.g. 30 mA |
| Arc fault detection device (AFDD) | Dangerous arc faults – protects against fires | typical arc patterns in the current waveform |
When is an RCD mandatory?
Residual current devices with a rated residual operating current of no more than 30 mA have been required for numerous circuits in new installations for many years. Under the current DIN VDE 0100-410, this applies in particular to:
- sockets up to 32 A intended for use by ordinary persons (laypeople),
- outdoor final circuits for portable equipment up to 32 A,
- lighting circuits in dwellings (since the 2018 edition of the standard).
Additional requirements apply to bathrooms, showers and other rooms with increased risk. In practice, almost all final circuits in a home are now protected by RCDs – ideally spread across several devices so that a single fault does not shut down the entire home.
Type A, F or B?
Not every RCD detects every kind of residual current:
- Type A detects AC residual currents and pulsating DC residual currents – the standard in households.
- Type F is additionally suitable for appliances with frequency converters, such as some washing machines or pumps.
- Type B also detects smooth DC residual currents. It is required, for example, for wallboxes without integrated DC residual current detection, for frequency converters in industry or, depending on the manufacturer's specifications, for photovoltaic inverters.
Which type is required is determined by the standard and the manufacturer's specifications for the connected equipment.
Arc fault detection devices (AFDDs): mandatory or recommended?
Brandschutzschalter (literally “fire protection switches”) – technical term arc fault detection device (AFDD, Fehlerlichtbogen-Schutzeinrichtung) – detect arcs such as those that occur on crushed cables, loose terminals or damaged plugs. Such arcs do not reliably trip either MCBs or RCDs, but they can cause fires.
Under the current DIN VDE 0100-420, AFDDs are only required in certain areas – in particular in sleeping rooms of facilities such as children's day-care centres and nursing and retirement homes. In ordinary residential buildings they are currently not a general requirement. However, they are recommended for bedrooms and children's rooms, for older buildings with timber-beam ceilings and for rooms in which combustible materials are stored.
What applies in older buildings?
Existing installations enjoy existing-installation protection (Bestandsschutz): there is no automatic obligation to retrofit RCDs. However, if you modify or extend the installation – for example by adding new circuits in the kitchen or bathroom – the new parts must comply with current standards.
Regardless of this, retrofitting an RCD is one of the most effective and inexpensive safety measures of all. The prerequisite is an installation with a protective conductor (Schutzleiter). With old two-core installations, the wiring must first be checked – find out more in our guide Rewiring older buildings.
Test RCDs regularly
Every RCD has a test button. Press it regularly – many manufacturers recommend doing so every six months. If the device does not trip, it must be checked immediately by a qualified electrician (Elektrofachkraft). In businesses, the functional test is part of the requirements of DGUV Regulation 3.
We retrofit RCDs and AFDDs and inspect existing installations in Cologne, Hürth and the Rhein-Erft district – find out more under Electrical installation and Testing & maintenance.
Frequently asked questions
Why does my RCD keep tripping?
Common causes are faulty appliances, moisture in outdoor sockets or junction boxes, or too many loads on a single RCD, whose small leakage currents add up. A qualified electrician can systematically narrow down the cause.
Do I need a Type B RCD for my wallbox?
Only if the wallbox does not have integrated DC residual current detection. Many current models already have this detection built in; in that case, a Type A RCD is usually sufficient. The manufacturer's specifications are decisive.
Can an AFDD be retrofitted?
Yes, AFDDs can be retrofitted in most distribution boards. They need a little more space than an MCB; we check on site whether there is enough room.
Related service
Electrical installation
Electrical installation for commercial and residential buildings – new builds and refurbishments, including meter boards and sub-distribution.
This article provides a general overview (as of October 2026) and does not replace an assessment or advice in the individual case. The applicable standards, laws and the requirements of your grid operator are decisive.
