Building an Unterverteilung

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

What is an Unterverteilung?

The Unterverteilung (sub-distribution board; also called Stromkreisverteiler – circuit distribution board) is the “Sicherungskasten” (fuse box) of an apartment or floor. It receives its power via the Zuleitung (incoming supply cable) from the Zählerschrank (meter cabinet) and splits it into the individual Endstromkreise (final circuits): Steckdosen (socket outlets), lighting, cooker, washing machine.

Inside the Zählerschrank sit the Hausanschluss (service connection), the SLS-Schalter (selective main-circuit breaker) and the Zähler (meter). Everything behind it – from the Zuleitung onward – is the customer’s installation. That is exactly where you work most often as an Azubi (apprentice).

The rules for residential buildings are in DIN 18015 (planning and minimum equipment) and in the DIN VDE 0100 series (installation of low-voltage installations).

The components at a glance

Component	Function	Typical selection
Hauptschalter (main switch)	Disconnects the entire distribution board on all poles	4-pole, 63 A
Überspannungsschutz (SPD) (surge protection)	Diverts surges caused by lightning or switching operations	Type 2, directly after the Hauptschalter
RCD / FI-Schutzschalter (residual-current circuit breaker)	Trips on Fehlerstrom (residual current) – protection against electric shock	4-pole, 40 A or 63 A, IΔn = 30 mA, Type A
LS-Schalter (Leitungsschutzschalter) (miniature circuit breaker)	Protects the cable against Überlast (overload) and Kurzschluss (short circuit)	B16 for Steckdosen, B10 for lighting, B16 3-pole for the cooker
Kammschiene / Phasenschiene (comb busbar / phase busbar)	Connects the LS-Schalter of a group to the RCD	3-phase, 10 mm² or 16 mm²
N- und PE-Klemmen (N and PE terminals)	Collect the Neutralleiter (neutral conductors) of each RCD group and all Schutzleiter (protective conductors)	Rail terminal blocks, N disconnect terminals
Hutschiene (DIN rail)	Carries all the Reiheneinbaugeräte (rail-mounted devices)	35 mm; 1 Teilungseinheit (TE, module) = 18 mm
Berührungsschutz-Abdeckung (touch-protection cover)	Covers the terminals so that only the toggle levers remain visible	Matched to the distribution board
Beschriftungsstreifen (label strip)	Stromkreisverzeichnis (circuit directory): which switch controls what	Printed, permanent

Planning: circuits and groups

Before you turn a single screw, you plan on paper:

Count the circuits. DIN 18015-2 specifies a minimum level of equipment. For example, the cooker, washing machine, dishwasher and tumble dryer each need their own circuit. Steckdosen and lighting are split up room by room.
Plan in reserve. Rule of thumb: 20 to 30 % of the Teilungseinheiten stay free. A full distribution board is a problem at the next modification.
Mind the RCD requirement. DIN VDE 0100-410 requires an RCD with IΔn ≤ 30 mA for Steckdosen up to 32 A and for lighting circuits in apartments. In practice today, almost every Endstromkreis sits behind an RCD.
Form groups. Several LS-Schalter share one RCD. Distribute wisely: lighting and Steckdosen of the same room on different RCDs – if one trips, the lights stay on. Six LS-Schalter per RCD is a proven upper limit.
Distribute the Außenleiter (line conductors). Load L1, L2 and L3 as evenly as possible – the Kammschiene does that almost by itself.
Check the Vorsicherung (upstream fuse). The SLS in the Zählerschrank (for example 35 A) must match the RCD (for example 40 A) so that the RCD is never overloaded.

The overview diagram

Read the diagram from top to bottom: Zuleitung → Hauptschalter → SPD → two RCD groups → four LS-Schalter in each. Each RCD group has its own N-Klemme. The Schutzleiter PE goes to a shared PE-Klemme. Number, type and rating always follow the concrete plan.

Building it step by step

Apply the five safety rules. Freischalten (disconnect from the supply), gegen Wiedereinschalten sichern (secure against switching back on), Spannungsfreiheit feststellen (verify absence of voltage), erden und kurzschließen (earth and short-circuit), benachbarte unter Spannung stehende Teile abdecken (cover adjacent live parts). Always – even for a “quick” look.
Mount the distribution board. At a comfortable working height (usually between about 1.1 and 1.8 m), freely accessible, dry, not behind doors or furniture.
Feed in the Zuleitung. Remove the sheath, cut the cores to length. Fine-stranded conductors get Aderendhülsen (ferrules). The cross-section depends on current, length and installation method – NYM-J 5×10 mm² is usual with a 35 A Vorsicherung, but you recalculate it every time.
Place the Hauptschalter and SPD. Zuleitung onto the Hauptschalter, behind it the Überspannungsschutz. Its connection leads to L, N and PE as short as possible, under 0.5 m in total.
Place and feed the RCDs. From the Hauptschalter to each RCD: L1, L2, L3 and N. The N goes through the RCD – only that way can it detect Fehlerströme.
Connect the LS-Schalter with the Kammschiene. One Kammschiene per RCD group, from the RCD outgoing terminal across the LS-Schalter. Cut the busbar cleanly to length, no bare ends.
Terminate the outgoing circuits. Außenleiter onto the LS outgoing terminal. Neutralleiter onto the N-Klemme of the same RCD group. Schutzleiter onto the PE-Klemme. Colors: L1 brown, L2 black, L3 grey, N blue, PE green-yellow.
Tighten and check. Torque according to the manufacturer’s specification (for Reiheneinbaugeräte often around 2 Nm), then a Zugprobe (pull test) on every core.
Label everything. Every switch gets its circuit on the Beschriftungsstreifen; the Stromkreisverzeichnis goes on the door of the distribution board.
Carry out the Erstprüfung (initial test) – see the next section. Without a Prüfprotokoll (test report), the installation isn’t finished.

Erstprüfung according to DIN VDE 0100-600

Every new or modified installation is tested before the Inbetriebnahme (commissioning). The Erstprüfung has three parts:

Besichtigen (inspect): correct components, colors, labeling, covers, cable routing, no damage.
Erproben (function-test): function of the protective devices, test button on the RCD, switching functions.
Messen (measure):
- Durchgängigkeit des Schutzleiters (continuity) – every PE is connected with low resistance.
- Isolationswiderstand (insulation resistance) – measured with 500 V DC, at least 1 MΩ for installations up to 500 V.
- Schleifenimpedanz / Kurzschlussstrom (loop impedance / short-circuit current) – the LS-Schalter must disconnect reliably in case of a fault.
- RCD-Auslösung (RCD tripping) – at IΔn within 300 ms at the latest; good devices trip significantly faster. Tripping current and tripping time go into the report.
- Drehfeld (rotating field) – right-rotating (clockwise) at the cooker connection.

The result is a Prüfprotokoll with measured values, date and signature. It is part of the handover.

Common mistakes

N-Leiter of two RCD groups connected. The RCD sees a differential current and trips under load. The most common mistake in the Ausbildung (apprenticeship).
Schutzleiter forgotten or loose. Danger to life – and immediately visible in the measurement.
Wrong characteristic or Nennstrom (rated current). B16 on 1.5 mm² over long distances can be too much; B10 for lighting is the normal case.
Kammschiene cut to the wrong length. Bare ends next to the neighboring device are a short circuit waiting to happen.
Terminals not tightened. Loose terminals get hot – fire hazard. Zugprobe on every core.
No labeling, no report. Without both, the installation is a black box for the next electrician.

Test yourself

🧠 Test yourself

1. Which Fehlerstrom-Schutzschalter is required for Steckdosen circuits in apartments?

2. Why must the N-Leiter of two RCD groups not be connected?

3. Which rule is the first of the five safety rules?

4. Which Leitungsschutzschalter is common for Steckdosen circuits in apartments?

5. What is part of the Erstprüfung per DIN VDE 0100-600?