Equipotential Bonding and Lightning Protection: The Panel Side Interface
equipotential bonding: Equipotential bonding brings every simultaneously accessible conductive part together through low impedance connections so that the potential difference between them approaches zero. In a lightning protection scheme this bonding is designed together with SPD coordination and separation distance.
Contents
During a lightning discharge or a short circuit, the hazard often comes not from the current itself but from the potential difference appearing between two metal parts. Equipotential bonding exists precisely to remove that difference, and on the panel side it takes shape as the earth bar, the protective conductors and the SPD connections. Without properly executed bonding, even the best surge protective device cannot deliver its rated protection level.
What equipotential bonding is and why it is required
An installation contains many conductive parts: water pipes, ventilation ducts, structural steel, cable armour and panel enclosures. If they are not connected together, hundreds of volts can appear between them during a fault or a lightning strike, and that difference is applied as touch voltage to a person or to the input circuits of electronic equipment. Bonding brings all these parts to the main bonding bar, unifies the reference point and gives the current a controlled path to follow.
Main bonding bar and supplementary bonding
The main equipotential bonding bar is normally located at the building entry point, close to the foundation earth electrode and the main panel; on a copper bar every connection is made with an individual bolt and must be removable. Conductor cross-sections are determined by the protective conductor size and the share of lightning current expected. In wet areas, medical locations and sections with dense electronics, supplementary bonding is applied, aiming to reduce the impedance locally between simultaneously accessible parts as well.
Interface with the lightning protection system
An external lightning protection system consists of air terminations, down conductors and earthing. The critical point on the panel side is the separation distance between the down conductor and the metal parts of the installation; where that distance cannot be achieved, a direct connection or an isolating spark gap is used. Part of the lightning current enters the installation through the soil and the bonding network, so the main panel must carry a surge protective device capable of withstanding lightning current. Earthing systems are not kept apart, a single earthing network is the target.
Lightning protection zones and SPD coordination
| Zone boundary | Purpose | Typical SPD | Connection rule |
|---|---|---|---|
| Exterior to building entry | Share of direct lightning current | Type 1, 10/350 waveform | Shortest route to the main bonding bar |
| Building entry to main board | Switching and induced surges | Type 2, 8/20 waveform | Total connection length kept short |
| Main board to terminal device | Fine electronic protection | Type 3 or combined | Positioned close to the equipment |
| Data and signal lines | Common mode surges | Signal line SPD | Bonded to the same earth reference |
The zone concept divides an installation into graded regions from the exposed exterior towards sensitive equipment. At every zone boundary both bonding and a suitable class of surge protective device are applied: a lightning current type at the entry, a type for switching and induced surges at the main board, and fine protection close to the terminal device. The SPD connection conductor is kept as short as possible; as total connection length grows, the residual voltage rises and protection effectiveness drops. The table below summarises zone and SPD matching.
Measurement, verification and maintenance
Whether bonding works is proven on site by measurement, not only on the drawing. Continuity is tested with a low resistance instrument and it is recorded that the expected value is not exceeded at connection points. Earth resistance measurement is interpreted with the electrode type and seasonal effects in mind. Maintenance checks bolt torques, corrosion and SPD status indicators; where a lightning event counter is installed, its records feed the maintenance plan.
Key takeaways: equipotential bonding
- Basic principle — Every simultaneously accessible conductive part meets at a single reference point.
- Bar design — On the main bonding bar each connection uses an individual, removable bolt.
- Separation distance — Where it cannot be met, direct bonding or an isolating spark gap is used.
- SPD connection — The shorter the connecting conductor, the lower the residual voltage.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Are earthing and equipotential bonding the same thing?
No. Earthing references the system to earth, while bonding aims to reduce the potential difference between conductive parts. They work together, but their purposes and sizing rules differ.
Should separate earthing systems be built for different systems?
No. Separate, unconnected earth systems create dangerous potential differences between them during a fault. Modern practice builds a single earthing network to which all systems are bonded.
Why must the SPD connecting cable be short?
Because surge current rises very quickly, an additional voltage drop appears across the inductance of the cable. That drop adds to the protection level of the SPD, so a longer cable means a higher voltage reaching the protected device.
Related articles
- Surge Protective Device (SPD) Selection: Type 1, Type 2 and Type 3
- Earthing System Types: TN-S, TN-C-S, TT and IT
- Sizing the Neutral and Earth Bars in a Switchboard
- Earthing Grid Design: Step and Touch Voltage Calculation
- Wind Turbine Electrical Panel: Tower Distribution, Protection and Lightning
- All technical articles
Let us review your bonding plan, SPD stages and panel earth bar design together; Devpan engineers the protection interface on the switchboard side. Related Devpan solution: medium voltage switchgear.
