What is bonding?
Joining the exposed metal parts of AGL equipment to each other, and then to the earthing system, so that if a fault energises one part they all rise together and no dangerous voltage appears between parts that can be touched at once.
Bonding joins the exposed metal parts of AGL equipment to each other with a low-resistance jumper or strap, for example a fixture body to its base can, a can cover to the can, or a sign frame to its housing. The bonded parts are then tied into the earthing system.
Its job is to stop a voltage appearing between two metal parts that can be touched at the same time. If a fault energises one part, the bond carries the rest with it, so the parts rise and fall together and nothing dangerous exists between them. A bond is only as good as its contact, so jumpers are fixed to clean metal, tightened properly and tested.
Metal tied to metal. Keeps parts at one potential.
Metal tied to the ground. Gives fault current a path to earth.
- Identify. Find metal parts that can be touched together or that sit close enough to differ in potential.
- Prepare. Clean the contact surfaces down to bare metal, free of paint, dirt and corrosion.
- Fit. Attach a jumper or strap with suitable lugs and fixings, leaving enough slack for any movement.
- Tighten. Secure to the specified torque and protect the joint against corrosion.
- Test. Measure the resistance across the bond with a low-resistance ohmmeter and record it.
Frequently Asked Questions
Q1What is bonding in airfield lighting?
Joining exposed metal parts together with a low-resistance conductor, a bonding jumper or strap, so that they are held at the same electrical potential.
Q2What is the difference between bonding and earthing?
Bonding connects metal parts to each other so no voltage can appear between them. Earthing connects them to the ground. In practice a bonded group is also tied to the earthing system, so the two work together.
Q3Why bond parts that are already earthed?
Each earth connection has some resistance, and one can corrode or loosen. A direct bond between neighbouring parts keeps them at the same potential even when one earth path is poor, and it covers parts that have no earth lead of their own.
Q4What is typically bonded in an AGL installation?
Fixture body to base can, base-can cover to the can, sign frame to its housing and base, metal pit covers to their frames, and metal ducts or glands, as the project design specifies.
Q5Is the primary cable conductor ever bonded?
No. The series-circuit conductors stay insulated from earth and from the metalwork. Connecting one to a bonded part would create an earth fault.
Q6How is a bond checked?
By visual inspection of the jumper, lug and fixing, then a low-resistance measurement across the joint. A reading that is high or unstable points to a loose, corroded or painted contact.
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