Earthing and bonding for busbar trunking systems

Everyone scrutinises a busduct's current rating. Far fewer give the earth path the same attention — yet on a fault, that earth path is what carries the fault current home, operates the protective device, and keeps the housing from becoming live. This article explains how earthing and bonding work in busbar trunking and what to check on site.

Earthing vs bonding

The two terms are often used loosely, but they do different jobs. Earthing provides a low-impedance return path so fault current flows back to the source and trips the protective device fast. Bonding ties metal parts together so they stay at the same potential — nobody touching two bonded surfaces during a fault sees a dangerous voltage between them.

A busduct system relies on both: a proven earth path along its length, and bonding to the building's earthing system and to adjacent metalwork.

The earth path inside the busduct

Sandwich-type busbar trunking handles the protective earth in one of two ways: the metal housing itself is rated as the PE conductor, or a dedicated earth bar runs alongside the phase bars — frequently both.

What matters is that the manufacturer declares the earth path's effective cross-section and its short-time withstand rating, verified by type test to IEC 61439-6. An earth path that is adequate for a small feeder may be undersized for a 4000 A rising main with a high prospective fault current.

Joints — the weak link

A busduct earth path is not continuous metal; it is a chain of bolted joints. Every joint is a potential weak point. An under-torqued bolt, a missing washer, paint left under a contact face or corrosion over time all raise the joint's resistance.

Raise enough joint resistances and the earth-loop impedance climbs until the protective device no longer disconnects within the required time. That is why joint torque and earth continuity are inspection items, not afterthoughts.

What to verify on site

• Joint bolts tightened to the manufacturer's specified torque
• Contact faces clean — no paint, oxide or debris
• Earth continuity measured across every joint and end to end
• Busduct bonded to the building earth at the declared points
• Measured fault-loop impedance compared against the design figure

Fault-loop impedance

The proof of a sound earthing arrangement is the measured fault-loop impedance. It must be low enough that, on an earth fault, the protective device disconnects within the time the wiring regulations and IEC 60364 require. Measuring it at the far end of the run — and comparing it with the design calculation — confirms the whole earth path, joints included, is doing its job.

How Al Sanaya can help

We engineer and install Linkk and Megaduct busduct across the GCC and MENA, and our scope includes verified joint torque, earth continuity testing at every joint, and documented fault-loop impedance measurements as part of handover.