How to Assemble an Electrical Distribution Board: A Step-by-Step Guide

Assembling a distribution board involves five steps: creating a single-line diagram and selecting protective devices, choosing an enclosure with a 20–30% spare capacity in modules, mounting the devices on DIN rails in the sequence "main switch → selective RCD → group RCDs → circuit breakers", routing the phases using busbars and connecting outgoing lines, neutrals, and PE to their respective terminals, and finally checking terminal tightness, insulation, and RCD operation using the "Test" button.

The most critical aspect of device selection is ensuring the circuit breaker rating matches the conductor cross-section: 10 A for 1.5 mm² (lighting), 16 A for 2.5 mm² (sockets, oven), and 32 A for 6 mm² (hob). 30 mA RCDs are used for socket circuits, 10 mA RCDs for bathrooms, and a selective 100–300 mA RCD is used for the main incoming protection.

Planning

Before assembling the board, you must create a single-line diagram, determine the number of circuits, and select the protective devices. A typical distribution board structure for a two-bedroom apartment includes:

• Main switch (2P, 32–40 A)
• Selective RCD (2P, 63 A, 100–300 mA, Type S) – this prevents it from tripping before the group RCDs
• RCD (2P, 40 A, 30 mA) – for socket groups (1–2 units)
• Circuit breakers (16 A, Type B or C) – for socket circuits (4–8 units); for 2.5 mm² domestic circuits, Type B provides more reliable tripping during short circuits at the far end of the line
• Circuit breakers (10 A, Type B) – for lighting (2–4 units)
• Circuit breaker (32 A) – for the hob (6 mm² cable); the oven should be on a separate 16 A circuit (2.5 mm²)
• RCBO (16 A, 10 mA) – for the washing machine or bathroom
• Voltage protection relay (optional, but recommended)

Matching circuit breaker ratings to copper conductor cross-sections:

Circuit	Circuit Breaker	Copper Cross-section
Lighting	10 A	1.5 mm²
Sockets, oven	16 A	2.5 mm²
High-power appliances (water heater, etc.)	25 A	4 mm²
Hob	32 A	6 mm²

Enclosure Selection

The enclosure is selected based on the number of modules required, including a 20–30% spare capacity. The standard module width is 18 mm (DIN rail). For an apartment, a 24–36 module enclosure is usually sufficient, while a house may require 48–72 modules.

• Surface-mounted – for plasterboard walls or where chasing is not possible
• Flush-mounted – for concrete or brick walls; provides a cleaner aesthetic

Installation Sequence

1. Enclosure installation at a convenient height for maintenance, typically 1.4–1.7 m from the floor (note: standard residential installation heights are generally determined by local accessibility requirements rather than a single fixed value).
2. DIN rail installation and mounting of neutral and PE busbars.
3. Component layout in the following sequence: main switch → meter (if located inside the enclosure) → RCD/RCBO for fire protection/selectivity → group RCDs → MCBs for individual circuits.
4. Busbar installation (comb busbars) to distribute the phase from the RCD to the MCBs.
5. Outgoing line connection – connecting cables from consumers to the bottom terminals of the MCBs.
6. Neutral conductor connection – connecting to the neutral busbars of the corresponding RCD groups.
7. PE (earth) connection – all green-yellow conductors to the common PE busbar.
8. Labeling – each MCB must be clearly labelled (circuit number, purpose).

Important Rules

• Supply is connected to the top terminals of the MCBs; outgoing lines are connected to the bottom terminals.
• Solid core cable is connected directly; stranded cable must be connected using ferrule terminals (e.g., H07V-K).
• Each MCB protects a single circuit (do not combine multiple circuits under one MCB without proper calculation).
• Terminal tightening – use the torque specified in the manufacturer's technical data sheet with a torque screwdriver (re-check after 24 hours).

Attention: older residential buildings with TN-C systems (two wires at the inlet, no separate PE). While an RCD will still function in such a system, the PE busbar must not be connected to the neutral in the consumer's enclosure without a proper system upgrade: makeshift "earthing" (TN-C to TN-S conversion) within a consumer unit is dangerous. Until the system is upgraded to TN-S or TN-C-S, socket-outlet circuits are protected by the RCD, and PE conductors should remain disconnected.

Pre-commissioning Checks

1. Visual inspection: ensure all connections are tight and wires are not pinched.
2. Insulation resistance test using a megohmmeter (must be at least 0.5 MΩ).
3. RCD functional test using the "Test" button.
4. Step-by-step energisation: main switch → RCD → individual circuits one by one.