Busbar trunking systems (IEC 61439-6) — when to use them instead of cable

For power distribution over long distances at high current, or in spaces where the layout of machines and workstations changes regularly, a busbar trunking system (BTS) is often a more practical alternative to a traditional cable route. IEC 61439-6 is the product standard that sets requirements for this type of system, as part of the broader IEC 61439 family for low-voltage switchgear and controlgear assemblies.

What a busbar trunking system is

A busbar trunking system consists of prefabricated straight sections, bends and plug-in tap-off units that are coupled together into a continuous distribution run. Instead of running a new cable for every machine or workstation, a tap-off unit is plugged in at any desired point along the busbar to feed the local distribution board or machine.

Key requirements from IEC 61439-6

The standard applies to systems up to 1000V AC or 1500V DC.
The rated current (Inc) of each circuit — including the incoming unit, the busbar unit, the tap-off unit and the outgoing circuit — must be equal to or greater than the assumed load of that circuit.
Where a tap-off unit has multiple circuits, those circuits must be able to carry their rated current continuously and simultaneously multiplied by the RDF (Rated Diversity Factor), within the limit of the tap-off unit's own rated current.
Compliance of a busbar must be verified for: temperature rise, short-circuit withstand, dielectric strength, IP protection rating and mechanical strength.

Advantage over cable

Where a cable route often has to be fully replaced or extended when the layout changes, a busbar trunking system allows a tap-off unit to simply be relocated, added or removed without affecting the main busbar itself — relevant for factory floors with regularly changing machine layouts.

Note: the RDF factor is system-specific and is specified by the manufacturer for the particular busbar configuration — this article gives the principle, not a universally applicable calculation example.

Practical relevance

In a factory hall with high-power machines that periodically change location (for example, a production hall with a flexible layout), a busbar trunking system is often more cost- and time-efficient than laying new cable routes each time — the initial investment is usually higher, but the flexibility for later changes often outweighs this.

Common mistakes

Ignoring the RDF factor of the tap-off units when simultaneously loading multiple circuits on a single tap-off unit — this can lead to overload even if each individual circuit stays within its own rated current.
Installing a busbar trunking system without verifying the required IP protection rating for the environment — particularly relevant in dusty or damp industrial halls.
Mounting tap-off units from a different manufacturer on an existing busbar without checking compatibility — busbar trunking systems are generally not universally interchangeable between manufacturers.