Busbar trunking systems and tap-off units — IEC 61439-6

The guide on busbar trunking systems introduces the principle of a closed, prefabricated busway conductor system as an alternative to cable wiring over long runs, and the guide on electrodynamic forces during busbar short-circuits (IEC 60865-1) covers the mechanical loading of the busbars themselves during a short-circuit. This article goes deeper into the specific fabrication standard for this system type — IEC 61439-6 — and into the component that actually makes the system usable: the tap-off unit.

Scope of IEC 61439-6

IEC 61439-6 is part of the IEC 61439 family of assembly fabrication standards (see also the guide on IEC 61439 panel building versus installation standard for the distinction between fabrication and installation standards in general) and applies specifically to busbar trunking systems (busways) with a rated voltage up to 1,000 V for AC or 1,500 V for DC. The standard covers both systems intended for the generation, transmission, distribution and conversion of electric energy, and systems for special applications (for example on board ships, in rail vehicles, or for machine electrical equipment).

Straight busway elements versus the tap-off unit

A busbar trunking system consists of two functionally distinct components, both covered by IEC 61439-6 but each with its own verification requirements:

The straight busway elements (plus bends, tee-pieces, expansion units) form the conducting run itself — the standard sets requirements for, among other things, mechanical strength, short-circuit withstand, temperature rise at rated current and the degree of protection (IP rating) along the entire run.
The tap-off unit is the point where a functional unit (a circuit breaker, a socket outlet, a switching group) is electrically connected to the continuous busbars, typically without the rest of the system needing to be de-energised for that purpose. The standard distinguishes, among other things, between fixed tap-off units and trolley-type tap-off units with a movable mechanism, which can be fitted or moved at pre-provided tap-off openings along the run.

Why the tap-off unit is the practical bottleneck

The straight busway elements are relatively easy to assess: their current-carrying capacity and short-circuit withstand are typically identical along the whole run. In practice, the tap-off unit is more often the bottleneck, for two reasons:

Connecting while energised: on a system with provided tap-off openings, a tap-off unit can be fitted or removed while the busbars remain energised — this requires an internal design of the tap-off unit that prevents unintended contact with live parts during fitting, and is one of the core subjects of verification under IEC 61439-6.
Contact resistance at the tap-off point: the contact connection between the tap-off unit and the continuous busbar is mechanically a separate joint (comparable to the clamp connections covered in the guide on tightening torque and the Belleville washer) and therefore a potential location for increased contact resistance and local heat build-up if the contact pressure of the tap-off mechanism decreases over time.

Practical relevance

When assessing an existing busway run (for example during a periodic NEN 3140 inspection or a thermographic survey, see the guide on thermographic inspection) each tap-off unit deserves individual attention: a well-performing busway run with one overheated tap-off unit often indicates reduced contact pressure at that specific tap-off point, not a general problem with the busway system. When extending an existing run with a new tap-off unit, it is also important to verify that the new tap-off unit is of the same manufacturer and type as the busway system it is connected to — mechanical and electrical compatibility between tap-off unit and busway is typically system-specific and not universally interchangeable between brands.

Common mistakes

Attempting to fit a tap-off unit of a different brand or type to an existing busway run, assuming tap-off openings are universal — mechanical fit and electrical contact pressure are typically system-specific.
Fitting or removing tap-off units while energised without following the manufacturer's procedure, even though the system is designed for this — an incorrectly performed action can still cause unintended contact with live parts.
Only looking at the straight busway elements during a thermographic survey and skipping the tap-off units — precisely because of their separate mechanical contact connection, tap-off units are more often the source of local overheating than the continuous busway run itself.
Overlooking the IP rating of the busway run when installed in a dusty or humid environment — not every busbar trunking system is suitable for every environmental class, even if it is amply sufficient electrically.