Definition
Equipotential bonding is the intentional electrical connection of conductive parts so that they remain at substantially the same potential, preventing dangerous voltage differences between them under normal or fault conditions.
Learning objectives
- Explain how bonding limits dangerous voltage differences between conductive parts.
- Distinguish main protective bonding from supplementary bonding.
- Size and inspect a bonding conductor for fault current and mechanical protection.
- Measure bonding continuity with a low-resistance instrument and interpret the result.
Advertisement
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
Bonding means tying metal parts together electrically so that they cannot develop a dangerous voltage between them. If a metal pipe, a equipment frame and the earthing system are bonded, a fault that raises one part also raises the others, keeping the difference small. What matters for shock is not the absolute voltage of a single part but the voltage difference a person can bridge with their body.
Main protective bonding connects incoming metal services — water, gas, structural steel and similar — to the main earthing terminal. Supplementary bonding adds local connections where the risk of a voltage difference is higher, such as in a bathroom or around a specific machine. Bonding does not replace overcurrent protection or an RCD; it works with them as part of the safety system.
Main protective bonding connects incoming metal services — water, gas, structural steel and similar — to the main earthing terminal. Supplementary bonding adds local connections where the risk of a voltage difference is higher, such as in a bathroom or around a specific machine. Bonding does not replace overcurrent protection or an RCD; it works with them as part of the safety system.
Intermediate How it behaves in a circuit
Bonding conductors must be sized for the fault current and the expected duration, and they must be protected against mechanical damage and corrosion. The resistance of each bond should be low enough that the voltage difference across it stays small under fault current; a bond that looks connected but has a corroded joint can be worse than no bond because it gives a false sense of security.
Main protective bonding connects metallic services at their point of entry to the main earthing terminal. Supplementary bonding connects exposed conductive parts and extraneous conductive parts locally where the disconnection times or the environment make a voltage difference more likely. Bonding must not create an unintended parallel path that carries load current, and it must not be used to bypass a protective device. The installation’s earthing arrangement determines how bonding and protective conductors are connected together.
Main protective bonding connects metallic services at their point of entry to the main earthing terminal. Supplementary bonding connects exposed conductive parts and extraneous conductive parts locally where the disconnection times or the environment make a voltage difference more likely. Bonding must not create an unintended parallel path that carries load current, and it must not be used to bypass a protective device. The installation’s earthing arrangement determines how bonding and protective conductors are connected together.
Advanced Limits, assumptions, engineering detail
At power frequency, a bond is a resistive connection; at high frequency it is an impedance with significant inductance. The geometry and length of the bonding conductor matter for lightning, switching transients and EMC. A long, thin bond can develop a large transient voltage across it even though its DC resistance is tiny. For sensitive electronics, the same bonding network can create ground loops; the solution is careful routing, single-point references where appropriate, and bonding that satisfies both safety and signal-integrity requirements.
Equipotential bonding is also central to lightning protection and static control in hazardous areas. The lightning protection system, structural steel, cable trays, pipes and equipment are bonded to form a controlled current path, and spark gaps or surge protective devices may be used where a direct bond is not permitted. Verification includes visual inspection, low-resistance measurement and, in some installations, periodic re-testing after modification or a lightning event.
Equipotential bonding is also central to lightning protection and static control in hazardous areas. The lightning protection system, structural steel, cable trays, pipes and equipment are bonded to form a controlled current path, and spark gaps or surge protective devices may be used where a direct bond is not permitted. Verification includes visual inspection, low-resistance measurement and, in some installations, periodic re-testing after modification or a lightning event.
Formulas and variables
Resistance of a bonding conductor
R = ρ × L / A
R Conductor resistance · Ω
ρ Resistivity · Ω·m
L Length · m
A Cross-sectional area · m²
20 m of 10 mm² copper with ρ ≈ 1.75 × 10⁻⁸ Ω·m has R ≈ 0.035 Ω.
Voltage difference across a bond
U = I × R
U Voltage difference between bonded parts · V
I Current through the bond · A
R Bond resistance · Ω
100 A through a 0.1 Ω bond produces 10 V between the parts.
Fault current sharing
I1 / I2 = R2 / R1
I1, I2 Currents in two parallel paths · A
R1, R2 Resistances of those paths · Ω
A low-resistance bond takes most of the fault current, which is why a poor parallel path is unreliable.
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Bond resistance | R | ohm (Ω) |
| Bond current | I | ampere (A) |
| Voltage difference | U | volt (V) |
| Conductor cross-section | A | square millimetre (mm²) |
Worked examples
Worked example
Resistance of a bonding conductor
Problem. A 10 mm² copper bonding conductor is 20 m long. Using ρ = 1.75 × 10⁻⁸ Ω·m, estimate its resistance.
Solution. A = 10 × 10⁻⁶ m². R = ρL/A = 1.75 × 10⁻⁸ × 20 / (10 × 10⁻⁶) = 0.035 Ω.
About 0.035 Ω; the bond is electrically short at power frequency but must still be inspected and tested.
Worked example
Voltage difference during a fault
Problem. A fault current of 100 A flows through a bonding path with 0.1 Ω resistance. What voltage difference appears across the bond?
Solution. U = I × R = 100 × 0.1 = 10 V.
10 V across the bonded parts; low enough to be much safer than an unbonded exposed part, but the design must still limit touch voltage and disconnection time.
Practical applications
- Preventing dangerous voltage differences between metal services and equipment
- Supporting protective earthing and fault disconnection in buildings
- Controlling static and lightning current paths in structures and plants
- Reducing ground loops and noise in sensitive electronic installations
Automotive applications
Automotive bonding connects engine, transmission, body, exhaust and control modules to the battery negative and chassis. Engine-to-body straps carry starter return current, sensor reference grounds must be clean and stable, and audio or communication systems can suffer from ground loops if bonding is poor. In electric vehicles, the high-voltage system is isolated and monitored; low-voltage bonding and high-voltage isolation must be designed and tested separately.
Industrial applications
Industrial bonding connects equipment frames, cable trays, pipes, tanks and structures to the earthing system. In hazardous areas it controls static discharge, and in lightning-protected plants it provides a controlled path for lightning current. Bonding conductors are sized for fault and lightning currents, protected mechanically and inspected periodically. In EMC-sensitive plants, bonding layout and cable routing are part of the noise-control design.
Measurement procedure
- De-energise and isolate the installation before inspecting or testing bonding conductors.
- Inspect each bond for corrosion, loose terminals, mechanical damage and unauthorized disconnection.
- Measure bonding resistance with a low-resistance ohmmeter using a test current high enough to reveal a poor joint; a basic multimeter may not resolve fractions of an ohm accurately.
- Use a four-wire method for very low resistances so lead and contact resistance do not distort the result.
- Record the measurement points and values, and re-test after any modification or lightning event.
Common mistakes
- Assuming a bond is good because the conductor is present, without checking the joints and resistance.
- Using bonding to carry normal load current or as a substitute for a neutral conductor.
- Disconnecting a bond to fix noise instead of correcting the routing or the source of interference.
- Fitting an undersized bonding conductor that cannot carry the fault or lightning current.
Safety considerations
- Bonding is a safety function: never remove or bypass a protective bond without a documented design change.
- Before working on bonding, isolate and prove dead; bonded metalwork may be connected to other live systems.
- Treat lightning protection bonding with particular care; a direct or nearby strike can impose enormous currents and voltages.
- In hazardous areas, bonding and static control are part of the explosion-protection concept; only trained personnel should modify them.
Frequently asked questions
What is the difference between bonding and earthing?
Earthing connects parts to the general mass of earth. Bonding connects conductive parts to each other so they stay at substantially the same potential. Bonding is often connected to the earthing system, but its purpose is to prevent voltage differences between parts, not only to provide an earth path.
Why is bonding important in a bathroom?
Bathrooms combine water, earthed metalwork and a person with reduced body resistance, so voltage differences between pipes, taps and equipment are especially dangerous. Supplementary bonding keeps those parts at nearly the same potential. Local rules define exactly where it is required.
Can a poor bond cause electrical noise?
Yes. A high-resistance or inductive bond can let different parts of a system sit at different high-frequency potentials, creating ground loops and noise. The fix is proper bonding geometry and routing, not simply disconnecting the bond.
How is bonding tested?
By visual inspection and a low-resistance measurement between the bonded parts, using a dedicated tester and a test current suitable for the joint. The measured value is compared with the design or the local requirements, and results are recorded.