Busbar Trunking System

Overview

A busbar trunking system, also called bus duct, distributes power along a building using rigid metal conductors held inside a metal housing rather than cables in tray. It runs as a continuous spine from the supply, often a transformer or main panel, to loads spread across floors and bays. Power is drawn at any point through a Tap-off unit that plugs into openings in the housing. The system covered here is a sandwich type, where the bars are pressed close together with thin insulation between them, giving low reactance and a compact cross section.

A typical 30 m run is built from a End feed unit at the source, a series of straight and elbow lengths, and a closing End cover. Lengths are connected by bolted joints, supported by hangers and brackets, and broken at fire and movement points by barriers and an expansion joint.

Conductors and housing

Each Straight trunking length carries a Busbar Set of three phase bars and a neutral, plus a separate Earth bar. The bars are flat copper, sized for the rated current, and stacked with Bar insulation wrap between them: an epoxy powder coat on the bar followed by a Mylar film wrap. This sandwich arrangement keeps the loop area small, which lowers the voltage drop and the magnetic forces during a fault.

The bars sit inside a Trunking housing of folded steel that doubles as a low-impedance earth return and as mechanical protection. A Housing cover closes the assembly, and Insulating clamp block spacers along the length hold the bars on centre and resist the short-circuit forces quoted as the Icw rating. Openings in the side, each fitted with a Tap-off window, let tap-off boxes engage the bars without an electrician opening the live housing. An End plate closes each end of the length before it is jointed to the next. Where the route turns, a pre-formed Elbow length makes the bend with the bars already shaped.

Joints

Lengths are joined by a Bolted joint block. The sandwich design allows a single-bolt joint: one central Torque-indicating joint bolt draws two Joint pressure plate members together, clamping the silver-plated Silver-plated contact plate faces of both lengths into firm contact. The bolt is a torque-indicating shear head, so the installer tightens it until the head snaps at the correct torque, giving a visible record that the joint is sound.

Contact pressure is held over time by a stack of Belleville washer disc springs, which take up any relaxation as the bars heat and cool. A Joint insulation pad keeps the phases apart inside the joint, and a snap-on Joint cover shroud shrouds the assembly to restore the housing rating once the joint is made.

Tap-off units

Loads are fed from the run through tap-off units. A Tap-off unit is a sheet steel box that hangs on the housing and plugs into a tap-off window. Inside, a set of Sprung tap contact sprung contacts grip the phase and neutral bars, while a leading Tap-off earth contact makes earth first and breaks it last for safety. A Auto contact shutter covers the bars when the box is removed, so the open window is not live to the touch.

Each box contains an Moulded-case circuit breaker moulded-case circuit breaker to protect the outgoing circuit, mounted on a DIN rail and wired through a Terminal block to the outgoing cables. Cables leave through a Cable gland in the gland plate. A Door interlock stops the Tap-off door opening while the breaker is closed, and prevents the box being unplugged under load.

Installation

The run is hung from the structure with a Support hanger set at the spacing given by the maker, usually every 1.5 to 2 m, and fixed to walls with a Wall bracket where needed. On long or vertical runs a Expansion joint with flexible copper links absorbs the movement caused by thermal cycling, so the joints are not stressed.

Where the duct passes through a fire-rated floor or wall it is fitted with a Fire barrier, an intumescent assembly that seals the penetration and stops fire and smoke tracking along the housing. After assembly the run is labelled with a Phase and earth marker set for phase rotation, neutral and earth, then insulation-resistance and continuity tested before energising. Because the bars are factory built and the joints are bolted to a set torque, commissioning is faster than terminating an equivalent run of large single-core cables, and adding a load later is a matter of plugging in a new tap-off box at the nearest window.