Equipotential Bonding Done Right: Main and Supplementary Bonding
Equipotential bonding is one of the most important protective measures of the LV installation — and one of the most common defects found in E-checks. This overview shows when main and supplementary bonding is needed, which cross-sections apply, and how to avoid common mistakes.
MEB and supplementary bonding: the differences
There are two types of equipotential bonding with different purposes.
Main equipotential bonding (MEB)
Connects all conductive parts entering the building to the main earthing terminal. Mandatory for every building.
Supplementary bonding
In bath and special-purpose rooms (sauna, swimming pool). Connects all local conductive parts additionally.
What belongs on the main earthing terminal
These components must be connected to the MET.
Water pipe
Metallic water inlet pipe directly after entry.
Gas pipe
For metallic gas lines — connection just after entry.
Heating
Central heating flow and return, if metallic.
Antenna / SAT
Antenna system via separate earthing.
PV system
DC main equipotential bonding, separated from AC bonding.
Steelwork / rebar
For steel-frame buildings or steel rebar in the foundation.
Cross-sections for bonding conductors
VDE 0100-540 prescribes minimum cross-sections.
| Application | Minimum cross-section | Note |
|---|---|---|
| MEB main conductor | 6 mm² Cu | Half the cross-section of the largest PE, min. 6 mm² |
| Supplementary bonding | 2.5 mm² Cu (protected) / 4 mm² (unprotected) | Bath, sauna |
| Foundation earth electrode | Steel 90 mm² or 30×3.5 mm flat steel | Per DIN 18014 |
Most common mistakes in practice
Water pipe forgotten
The absolute classic — during renovation, the water inlet pipe is often not connected.
Plastic water pipe ignored
Even with plastic piping, the downstream metal part (e.g. water meter fitting) must be connected.
Wrong cross-section
MEB with 2.5 mm² instead of 6 mm² is a clear violation — VDE 0100-540 is unambiguous here.
Bath supplementary bonding missing
In the bath, tub, shower, heating, pipes must be connected via supplementary bonding — often forgotten.
Verification of equipotential bonding
Low-ohm measurement
Resistance between earthing points must be < 0.5 Ω.
Visual inspection
Check terminal points, correct cross-sections, labeling.
Documentation
All bonding connections in the test report per VDE 0100-600.
Conclusion: bonding is mandatory detail work
Equipotential bonding saves lives but is often given short shrift in practice. Anyone with the key connections and cross-sections in mind avoids the most common defects — and passes every E-check.
💡 Tip: During every renovation, open the MET and check whether all inlets really are connected. In old buildings, this is almost always work to do.
Document earthing and bonding completely
myElectricPlan documents the MET and supplementary bonding directly in the system schematic — including minimum cross-sections per VDE 0100-540.
Plan earthing concept now