Protective Devices – LS, RCD, SLS and Surge Protection

🌐 Machine-translated page. The German version is authoritative.

Two Kinds of Protection

Schutzorgane (protective devices) protect either the installation (wiring, devices) or people – some protect both.

Hazard	Protection by	Component
Überlast (overload) – too many loads, the cable gets warm	Tripping when the current is too high for a longer time	LS-Schalter (miniature circuit breaker), Schmelzsicherung (fuse)
Kurzschluss (short circuit) – L touches N or PE, huge current	Tripping within milliseconds	LS-Schalter, Schmelzsicherung, SLS
Fehlerstrom (residual current) / electric shock – current flows via the enclosure or a person to earth	Tripping at a small difference between outgoing and returning current	RCD (FI-Schutzschalter)
Überspannung (surge) – lightning, switching operations in the grid	Diverting the voltage peak to earth	SPD (Überspannungsschutzgerät, surge protective device)

Leitungsschutzschalter (LS-Schalter)

The LS-Schalter (colloquially “Sicherungsautomat”) protects the Leitung (cable) against Überlast and Kurzschluss. It has two tripping mechanisms:

Thermischer Auslöser (thermal trip) (bimetal): reacts to Überlast – slowly, depending on how far the current exceeds the rated value. 1.45 times the rated current must trip it within one hour.
Magnetischer Auslöser (magnetic trip): reacts to Kurzschluss – instantly, as soon as the current reaches a multiple of the rated current.

The label, for example B16, means:

Part	Meaning
B	Tripping characteristic: magnetic tripping at 3 to 5 times the rated current. Standard for dwellings.
C	Tripping at 5 to 10 times the rated current – for motors, devices with high inrush current.
D	10 to 20 times – transformers, welding equipment.
16	Rated current 16 A

Assignment in the flat: B10 for lighting (1.5 mm²), B16 for Steckdose (socket outlets) (2.5 mm² or 1.5 mm² for short runs), B16 3-pole for the cooker (400 V, 5×2.5 mm²), B16 or B20 for the Durchlauferhitzer (instantaneous water heater) depending on power and cross-section.

The rated current must match the Leitungsquerschnitt (cable cross-section) and the Verlegeart (installation method) – the cable must be able to carry continuously the current the LS lets through. That is a calculation, not a rule of thumb.

Schmelzsicherung

The Schmelzsicherung (Neozed D01/D02 in the distribution board, NH-Sicherung in the Hausanschlusskasten (service entrance box)) is the older brother of the LS-Schalter: a wire melts on overcurrent. It is cheap, robust and can interrupt large short-circuit currents, but it must be replaced. In the Hausanschlusskasten and as a backup fuse it is still standard; in flat distribution boards the LS has displaced it.

RCD – the Fehlerstrom-Schutzschalter (FI)

The RCD (Residual Current Device, in German FI-Schutzschalter) protects people. It compares the current flowing out through the Außenleiter (line conductors) with the current returning through the N (Neutralleiter, neutral conductor). Normally the sum is zero. If current flows back via another path – via the PE (Schutzleiter, protective conductor), via a defective device, via a person – a Differenz (difference) arises. From the Bemessungsdifferenzstrom IΔn (rated residual operating current) onwards, the RCD trips.

Specification	Meaning
IΔn = 30 mA	Personal protection. Mandatory for socket outlets up to 32 A and lighting in dwellings (DIN VDE 0100-410). Tripping at the latest after 300 ms, in practice 20–40 ms.
IΔn = 300 mA	Fire protection, not personal protection. In agriculture, for entire installations.
Typ AC	detects only sinusoidal AC residual currents – not permitted in Germany
Typ A	additionally pulsating DC residual currents – standard for dwellings
Typ F	additionally mixed frequencies – washing machines, Wärmepumpe (heat pump) with Frequenzumrichter (frequency converter)
Typ B	additionally smooth DC residual currents – Wallbox (EV charger) (if there is no DC protection in the Wallbox), PV inverters without transformer
4-pole 40 A	switches L1, L2, L3, N; the rated current (40 A) is the load capacity – the RCD does not protect against Überlast, it needs a Vorsicherung (upstream fuse)

The Prüftaste T (test button T) generates an artificial Fehlerstrom. It must be pressed monthly (it says so on every device) – the only test the customer is allowed to do themselves.

RCBO (FI/LS combination) is an RCD and an LS in one device for a single circuit. More expensive, but if a fault occurs only one circuit goes down.

SLS – the selective main breaker

The SLS-Schalter sits ahead of the meter in the Zählerschrank (meter cabinet) (VDE-AR-N 4100). It is the Vorsicherung for the entire Kundenanlage (customer installation) and has the E characteristic: it is selektiv (selective) with respect to the LS-Schalter behind it. In the event of a Kurzschluss in the flat, the B16 trips, not the SLS – otherwise the whole house would go dark. Typically 3×35 A or 3×50 A for a single-family house, 3×63 A with a Wärmepumpe and Wallbox.

The SLS belongs to the Zählerschrank equipment and, although it sits ahead of the meter, may be operated by the electrical contractor – it is the Freischaltpunkt (isolation point) for work on the Unterverteilung (sub-distribution board).

SPD – Überspannungsschutz

Überspannungsschutzgeräte (SPD, Surge Protective Device) divert voltage peaks before they destroy devices. Three types, three locations:

Type	Location	Against
Typ 1 (lightning current arrester)	Hausanschluss (service entrance) / Zählerschrank	direct lightning strike (only with a lightning protection system)
Typ 2 (surge arrester)	Zählerschrank or Unterverteilung	Überspannung from the grid, remote effects
Typ 3 (device protection)	Steckdose, device	residual voltage for sensitive electronics

Since DIN VDE 0100-443 and -534 (2016), Überspannungsschutz (surge protection) has been mandatory in new residential buildings. The Typ 2 arrester sits in the Unterverteilung directly behind the Hauptschalter (main switch) with cables as short as possible (under 0.5 m in total) to L, N and PE.

How They Work Together in the Distribution Board

The order from the supply side: SLS (ahead of the meter) → Hauptschalter → SPD → RCD → LS → circuit. Each component has its job; none replaces another. An RCD without an LS lets the cable burn up, an LS without an RCD lets 20 mA flow undisturbed through a person. How this is actually wired up is shown in the guide Building an Unterverteilung.

Test Yourself

🧠 Test yourself

1. What does an LS-Schalter primarily protect?

2. What does the "B" in B16 mean?

3. Which RCD type is required as a minimum for dwellings?

4. Where is the SLS-Schalter located?

5. Does an RCD protect against Überlast?