Bonding

Part of Electrical Wiring and Installation

Connecting all metal components in a building into one continuous electrical network to eliminate dangerous voltage differences between surfaces.

Why This Matters

Bonding is the underappreciated sibling of grounding. While grounding connects the electrical system’s neutral to earth, bonding connects all metal objects in and around a building to each other — water pipes, gas pipes, metal conduit, equipment frames, structural steel. The goal is to ensure no two touchable metal surfaces can ever be at different voltages.

This matters because people touch multiple metal surfaces simultaneously. If a water pipe is at 0V and a nearby appliance casing is at 50V (due to a fault), a person touching both creates a 50V circuit through their body. If both surfaces are bonded together, both are at the same potential — no current flows through the person regardless.

In rebuilding scenarios, where improvised wiring, aging equipment, and damp conditions create frequent fault hazards, comprehensive bonding is often more practically important than elaborate grounding. It’s the difference between a safe system that fails gracefully and one that silently energizes metal surfaces waiting for a victim.

The Bonding Concept: Equipotential Zone

The goal of bonding is to create an equipotential zone — a region where all conductive surfaces are at the same electrical potential. In such a zone, a person cannot experience a shock between two surfaces because there’s no voltage difference, hence no current driving force.

Contrast with grounding: Grounding establishes the system neutral at earth potential and provides a fault current return path. Bonding ensures all metal surfaces in the environment are at the same potential, whether that potential is earth or slightly above. Together, they ensure both that fault currents flow quickly to earth (tripping fuses) and that no dangerous gradients exist between surfaces while faults develop.

Think of it as: grounding protects the circuit; bonding protects the person.

What Needs to Be Bonded

Main bonding conductor connects:

• Electrical service entrance neutral to earth ground
• Water main pipe (metal) to the bonding bar
• Gas service pipe (metal) to the bonding bar
• Structural steel (if present)
• Central heating/cooling system metal components

Supplementary bonding in specific rooms:

• Bathrooms/wet rooms: all metal in the room (towel rails, bath frame, faucets, waste pipes, radiators) must be bonded together, even if not part of the electrical circuit
• Kitchens: gas cooker frame, metal sink, water and gas supplies
• Boiler rooms: all pipes, equipment frames, boiler body

Equipment bonding (covered under grounding systems):

• Individual appliance frames to protective ground terminal
• Motor frames to equipment grounding conductor
• Generator frames to local ground

Bonding Conductor Sizing

The bonding conductor must be large enough to carry any fault current long enough for the protective device to operate.

Main bonding conductor: Typically minimum 6mm² copper for small systems; 10mm² or larger for larger installations. The key: must be the same size as or larger than the neutral conductor, since fault current equals line current.

Supplementary bonding (within a room, short runs):

• Minimum 2.5mm² if mechanically protected (in conduit)
• Minimum 4mm² if not mechanically protected
• Between two exposed parts: minimum √(combined cross-sections) / 2

Conductors should be: Copper or copper-clad steel, green/yellow striped insulation (standard color for protective conductors in most countries).

Making Bonding Connections

To metal pipes: Use pipe clamps specifically designed for bonding — not plumbing-grade pipe straps. The connection must be:

• Metal-to-metal contact over significant area
• Clean of corrosion, paint, and oxidation at contact point
• Mechanically secure and not removable without a tool
• Located away from isolation joints (plastic pipe sections that would break the bonding)

Connection procedure:

1. Select a location on the pipe that won’t be disturbed (not near flexible joints)
2. Clean the pipe surface to bare metal (file or sandpaper)
3. Install bonding clamp — purpose-made bronze or brass pipe clamps
4. Use tinned copper or bare copper conductor
5. Label “SAFETY ELECTRICAL CONNECTION — DO NOT REMOVE”

To equipment frames: Screw or bolt to a tapped hole or grounding lug on the frame. Clean the metal surface. Ensure star washers bite into metal. Alternatively, use a cable lug crimped to the conductor and bolted to frame.

Common mistakes:

• Bonding to plastic sections of pipes (breaks the bond)
• Relying on mechanical pipe connections for electrical bonding (plumbing fittings have poor electrical conductivity)
• Not labeling bonding conductors (someone may later disconnect them thinking they’re unnecessary)
• Using undersized clamps that work loose from pipe vibration

Bonding Across Isolation Points

Plastic pipes and plastic-lined pipe sections break continuity. If a metal pipe has a plastic section, bond both metal sides separately:

[Metal pipe A] --[bond wire]--> [Bond bar]
[Metal pipe B] --[bond wire]--> [Bond bar]

Both sides are now bonded to the common bar, so they’re at the same potential even though there’s no direct metal path between them through the pipe.

Testing Bonding

Continuity test:

1. Disconnect the bonding conductor at one end
2. Use a low-resistance meter or battery-lamp tester
3. Measure resistance between the bonding conductor and the item it should bond to (via the bonding bar)
4. Acceptable result: below 1 ohm, ideally below 0.5 ohm
5. High resistance indicates corroded joint, wrong contact point, or broken conductor

Voltage test (in-service): With system powered:

1. Touch voltmeter probes to two different metal surfaces that should be bonded
2. Expected reading: 0V (they’re at the same potential)
3. Any reading above 1V: bonding is inadequate or a fault exists somewhere

Bonding in Practice: Building a Bonding Network

For a small building being wired from scratch:

1. Install a bonding bar near the electrical panel (copper bus bar, 25mm × 3mm or larger)
2. Run main bonding conductors to: electrical earth, cold water intake, hot water, gas pipe
3. Run supplementary bonding in each wet room: connect to bathroom, kitchen, boiler room metal
4. Bring all conductors to the bonding bar using labeled connectors
5. Document the bonding network — note every connection made and where

A well-labeled bonding network can be inspected and tested years later. Without documentation, future workers may disconnect bonding conductors they don’t understand — creating hazards that won’t be discovered until an accident.

Bonding costs almost nothing relative to the safety benefit it provides. In a rebuilding community, where the consequences of an electrical accident may be far worse than in modern times (no ambulances, no hospitals), thorough bonding is one of the highest-value safety investments you can make.