Busbar Trunking & Powertrack Systems

ASTA BEAB Accredited test facilities have been pre-eminent in proving the performance of busbar 'trunking systems' for almost 70 years. Before national standards existed ASTA, its member labs, manufacturers and users created and adopted their own common instructions for tests on busbar and powertrack systems. These persisted until the application of BS5486-2 and then BSEN60439-2 became practical and accepted. BSEN61534 was then subsequently released. Busbar trunking expert, Colin Stephens from ASTA BEAB takes a technical look at the testing of these systems.

It was the French who coined the word omnibus in 1828 with reference to vehicles. Later it was immortalised as a Flanders and Swann's six wheeler, 97 HP vehicle used by London Transport. As spin-offs the terms bus and busbar have been used since the early days of electrical development, to identify conductors for power and communication distribution.

Whether used by themselves or within switchgear assemblies, busbars are fundamental to the security of electrical systems and must be capable of withstanding the thermal and mechanical forces experienced during normal operation and fault situations. They live a quiet, hidden, existence for the main part but it is important to remember that by their nature they are exposed to the consequences of any and all abuse that arises through problems at the outlets of any sub-circuit. Though protection may be provided, the clearing times used for discrimination means that buses need to be proofed against foreseeable faults to ensure reliable, maintainable, supplies to all users.

In the heavy current field, there are two distinct categories of busbar. Busbar trunking systems or 'busways', providing the basic infrastructure of the power system, which may be high or low voltage and at the delivery end of the supply system, what has become known as 'powertrack'. This is now commonly used for underfloor distribution to socket outlets and in ceilings for connection of luminaires, allowing flexibility in final circuit design for redesign of office layouts for example.

Standards for low voltage busbars for power distribution are split at 63A. Up to 63A there is a new standard, IEC61534 issued in 2003, which has a set of subparts still under development, to cover different installation arrangements. Above 63A, IEC60439?2 is well established and applied for systems of 63A to 5000A and beyond. For HV busbar systems, there are a suite of national standards, typified by BS159:1992, and the principles of IEC62271 are sometimes used.

A key focus of IEC61534 is on the wiring accessory aspect of powertrack. The standard incorporates the usual dielectric, temperature rise, mechanical strength and material tests. It also includes defined classifications for tap-offs, and what is necessary for permitted removal under different load conditions. There are clauses that address other constructional requirements as well as performance factors like normal operation, and the effects of temperature on the stability of the assembly. Where national variations of plugs and socket systems are relevant, the relevant standards can be used to evaluate safety with regard to dimensions, withdrawal forces and the like, as applied in the country of use. This reflects the usage which can be expected and is a significant difference from 60439-2. At present, 61534 does not require short circuit tests, even though a distribution system will almost inevitably see such faults in its lifetime, such as system damage, or misbehaviour of a connected load. Paragraph 512-02-01 of the UK wiring regulations states that "Every item of equipment shall be suitable for . the current likely to flow in abnormal conditions for such periods of time as are determined by the . protective devices concerned". Verification of performance under such conditions is therefore most prudent. Levels of 16000A prospective current have commonly been used for testing low power 3 phase systems.

IEC60439-2 is part of the switchgear suite of standards adding special requirements to 60439-1. The philosophy follows the bespoke nature of "Type Tested Assemblies" (TTA) and has to allow a wide degree of flexibility to cater for the paradox of a standard for non-standard products. The products can be subject to major stresses and the standard gives opportunities to rate bars for prospective and short time short circuit currents. Typical levels can be 50000A for 1 second, however 150000A is not unknown. In the 2000 edition, tests were incorporated to evaluate the performance of busbar systems in fire situations. These address the situations of flame propagation, where a section is exposed to a burner for a set duration, and resistance to fire in the circumstances of building penetrations. The latter is a stern examination, with the busbar exposed to temperatures over 10000C. Performance is measured as the time to continue compliance however, so comfort can be taken that outright failure is unlikely. As major elements of building structures, these are now recognised as key performance parameters, especially as structures and electrical loads climb ever higher.

A further change in the 2005 amendment is the incorporation of thermal cycling tests for busbar plug-in tap-off units in clause 8.2.1.8. This may pose new challenges for the manufacturer and indeed for laboratories. Significant current switching is required, where equipment is not normally available with automatic sequencing. The amended standard takes full effect in October 2008.

So now omnibus bars are transmitting far in excess of 97HP. The introduction of BSEN61534 enhances the relevant standards, allowing the different constructional principles to be better addressed. Until wireless power transmission is perfected, perhaps as a variation of Tesla's resonating tower, the role of busbar systems will remain key to the distribution of power through plants and buildings of all sizes. We would therefore be wise to maintain and extend the thoroughness of testing and certification to ensure compatibiliy is maintained throughout the evermore complex systems.