Busbar trunking systems
Busbar trunking systems are stationary enclosed assemblies used to distribute and control electrical energy for all types of loads, intended for industrial, commercial and similar applications, in the form of a conductor system comprising busbars which are spaced and supported by insulating material in a duct, trough or similar enclosure. The technical specifications are defined in IEC 61439 (Low-voltage switchgear and controlgear assemblies), while part 6 (IEC 61439-6) specifically includes definitions and states the service conditions, construction requirements, technical characteristics and verification requirements for Busbar trunking systems. Due to this strict regulation in the standard and the related verification tests, busbar trunking systems are considered as reliable products for energy transport and distribution. Busbar trunking systems are also known in the American market as busways (bus way) or busduct (bus duct)-systems. In North American markets these systems are often in accordance to UL 857.
History
One of the first uses of busbar trunking systems is known in the US American automotive industry in the 1930s: For the industrial production, high amounts of electrical energy were required from the electrical machines. This was transmitted by busbar trunking systems. A known manufacturer is the company Bulldog, which introduced its busbar trunking system to the market in 1932. After the Second World War, this technology was used also in the European market: The first busbar trunking systems were introduced by Hein Moeller in 1950 in Europe and produced since 1954 by company Klöckner-Moeller in Germany.[1] In the same period, company Telemecanique established the system Canalis in France. The standard IEC 60439-2 defined busbar trunking systems since 1993; since 2013, the successional IEC 61439-6 is valid. This standard needs to be read in context with IEC 61439-1 (Low-voltage switchgear and controlgear assemblies – Part 1: General rules). Field of application: Busbar trunking systems can be used for the transport as well as for the distribution of electrical energy: Transport means the connection between two electrical systems (e.g. transformer to switchboard).
Distribution is the supply of electrical energy from one source (e.g. electrical switchboard) to several decentral consumers (e.g. motors). For this, removable tap-off units are plugged onto the system. These units might contain circuit-breakers, fuses and optional switching devices, which protect the consumers.
The current rating of busbar trunking systems is in the range of 25 A to 6,300 A or even more. The nominal voltage is limited by IEC 61439-1/-6 to 1,000 V AC / 1,500 V DC. The use of busbar trunking systems in direct-current circuits is not often but generally possible.
Supplier
Beside global players like Schneider-Electric (France) and Siemens (Germany), also various medium-size companies like EAE (Turkey), Pogliano BusBar S.r.l. (Italy), E&I (Ireland) are offering busbar trunking systems in accordance to IEC 61439-1/-6. Several other local suppliers are based in Asia. In the US market, various companies offer systems according to UL standards.
Construction of busbar trunking systems:
A busbar trunking system contains an arrangement of conductors (mostly aluminum or copper), which are in a channel, a tray or another housing, divided from each other with different insulation material.[2] Principally, a classification into three designs is possible:
Air-insulated systems: The conductors (with or without insulation material) are placed with sufficient distance to each other inside an enclosure. This design is common for low and medium current systems (e.g. Schneider Electric: System KN; EAE: System KO) but also used for some high-current systems (e.g. Siemens: System LD). Depending on the degree of protection, the heat dissipation will be either via convection or radiation from the enclosure.
Sandwich systems: In this design, the conductors are electrically insulated by thin polyester foils, but mechanically close to each other. The housing presses the bars to a compact block, and the internal air gaps are minimized. The heat dissipation works via radiation; the housing is often made in the shape of a heat sink.
Solid insulated systems: The conductors are embedded in a solid insulation material: This is a heat-conductive insulation material e.g. epoxy. Air bubbles should be minimized. The heat dissipation is the radiation of the outer surface, which is often made from the same insulation material.
Functional units of a busbar trunking system
Conductor:
The conductors transport the electrical energy. They are made of metallic material (aluminum or copper). The conductor surface could be coated with tin (or in seldom cases silver) to improve the electrical contact. In the case of copper conductors, this surface treatment is not essential. If the copper has no surface treatment, further measures to reduce the contact resistance (like cleaning the surface) are necessary. For aluminum conductors, surface treatment is mandatory because of its oxidation. In most cases, this will be a tin surface with a nickel or copper interlayer. In every case, it is important that the layers do not cause functional-critical whiskers. (This phenomenon is known especially for tinned copper bars.)
Insulation
To isolate the conductors from each other and to the housing, insulation is necessary: This could be made by sleeving material (e.g. polyester foils like Mylar, insulation surface like Epoxy) or adequate air distances. In some cases, it is a combination of different measures. The standard IEC 61439-1/-6 defines the technical details for it. The insulation must withstand also in the case of a short circuit (see IEC 61439-1, chap. 5.3.4): the conductors must be segregated safely and constantly, for which tests must be conducted. This short-circuit level is one of the important technical data for a busbar trunking system.
Housing
The housing of a busbar trunking system is the protection against touching as well as against mechanical damage. A metallic enclosure must be integrated into the protective measures (connection to PE), so that a connection to the body of the busbar trunking system will lead to the trip of the feeding protection device.
Connection unit
The junction of busbar trunking systems is done by a connection unit. This connects two adjacent busbar trunking units mechanically and electrically. Hereby, two different principles are common:
- Junction via joint block: Two open ends of busbar trunking systems are joined by a separate component (‘fish-plate’)
- Direct connection: Two open ends of the busbar trunking system are joined directly to each other
To reach a good electrical connection, a uniform contact pressure is essential: This works with the help of screw or clamp connections. To keep the contact pressure also during operation, spring elements are usually integrated in connection units. In the case of screwed connections, a torque must be reached; this could be secured by a tear-off screw or torque instructions.
Major components of a busbar-trunking system
IEC 61439-6 defines the following units with these terms:
Feeder busbar trunking unit (feeder BTU)
Feeder units are components on which the electrical energy is inserted into a busbar trunking system. Common components are:
- Transformer connection pieces to connect the low-voltage side of a transformer, usually via flexibles (braided or laminated)
- Switchboard connection piece for a direct connection to a low-voltage switchboard. In most cases, this is made by solid (copper) connectors
- Cable feeder to connect cables, the cables are often fed by switchboards or busbar trunking systems
Busbar trunking unit (BTU)
Busbar trunking units are connected to feeder units: These busbar trunking units can have several shapes: Apart from straight pieces (usually up to 3 m), these could be angles or other changes of direction to follow the structure of the building or to bypass obstacles. Straight busbar trunking units may contain tap-off points: On this element, tap-off units can be connected to tap energy from the busbar trunking system (IEC 61439-6: busbar trunking unit with tap-off facilities). A special element of a busbar trunking unit is the expansion element (IEC 61439-6: busbar trunking thermal expansion unit): These can compensate for the thermal expansion of the conductor due to the current flow. As an alternative, the expansion compensation could also be integrated into the joint of the busbar trunking system (e.g. KN of company Schneider Electric).
Tap-off unit
With tap-off units, energy can be taken from the busbar trunking system. The tap-off unit must be inserted into a tap-off point. During this process, the tap-off unit is mechanically and electrically connected to the busbar trunking unit. This could be supported by integrated or separate sliding mechanisms. In accordance with the standard[3], it is necessary that the PE conductor is closed before and breaks after the phase conductors (‘first make, last break’).
A tap-off unit usually contains protection devices: In a fuseless design, this could be Miniature circuit breakers (MCB) or molded case circuit breakers (MCCB). With fuses, it could be fuse disconnectors or disconnectors with fuses. It is important that the primary wiring of a tap-off unit must be short-circuit proof. Additional components (e.g. control devices) could also be integrated into a tap-off unit. According to the definition of IEC 61439-6[4], tap-off units are only for tapping from and not for entering energy into the busbar trunking. This is important in relation to the protection against electric shock as well as the risk of an arc if an MCCB blows its hot ionized gases to live parts.
Miscellaneous:
Several accessories are offered for busbar trunking systems: Beside system-specific support systems and covers, also safety related components are available: With proven fire barriers, a busbar trunking system can pass through firewalls. Country specific tests for these fire barriers are often necessary.
Trends for busbar trunking systems:
Modern busbar trunking systems are using the current conductors to transmit electrical data of the system. This is done with the help of power-line communication (PLC). For some applications, the use of specifically designed busbar trunking systems is common (e.g. wind energy: System LDM of Siemens, datacenter: system databar of Anord Mardix; system LData of Siemens)
References
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