Busbar Trunking has been around in industry for a good period of time, so why does cable maintain its first choice position with so many engineers? As in most issues there are two sides. There are some applications where cable is the best product for the job, but busbar can compete with cable in most of them and clearly becomes the industry standard method in others. Busbar Trunking performs the function of cable by transporting the current from A to B, but then goes further by becoming the product for holding protection devices, typically in tap-off boxes. Therefore, Busbar Trunking is also the replacement for distribution and panel boards. Typical layout of Busbar Trunking system -a stretched power distribution panel! Now we can clearly see that we are not looking at a direct comparison, but an alternative method of providing a distribution scheme. By breaking the product into sections, we look to see if busbar trunking is an alternative that engineers should investigate prior to committing themselves to cables.
Busbar Trunking is a simple product, solely made up from enclosures, conductors and insulation - no different from cable? But due to the design and associated standard EN 60 439-2, its electrical characteristics are clearly defined by manufacturers and is not affected by factors associated with cable such as grouping factors, installation method. The compact sandwich construction of busbar trunking, particularly high power trunking, ensures very low impedance values, which gives good voltage drop figures and a reduced power loss in the system. Short-circuit withstand figures are also clearly stated and notified by a third part test laboratories (Barduct would use ASTA by choice). With the information at hand at the start of the design, the engineer can produce first draft calculations that are meaningful, as the end result is unlikely to change.
Other than the transporting of energy from A to B as in transformer to switchboard "goal posts" which is the direct cable replacement, busbar then give the distribution board some competition."Decentralized distribution" is the result of a busbar scheme. Why centralise all the production devices in one place when the function of the device is to protect a machine or load, which should be upto 50-100 m away? By loading the busbar in the environment where the power is to be used, each tap-off unit, which contains the protective device, is positioned close to the load, ideally situated to be operated if the load require disconnection for maintenance or due to breakdown. As only one load is supplied from each tap-off unit, the busbar remains live and functioning correctly while the maintenance or breakdown is dealt with. This relates to reduced downtime and loss of production for the end client. Each busbar can give electrical distribution capability to cover a 6-8 m wide band along the run. Therefore, 1 m of busbar equals 6-8 sqm of distribution coverage, an ideal budgetary tool and design concept for the end client to hear.
In commercial buildings, the thought of cables being used to feed each floor independently from a main switchboard in the basement would be unthinkable and this area is now the domain of rising mains. So now, by laying the rising main horizontally across the building, we now see the applications being switched to industry, retail, hospital, hotels and virtually any other building type. As the busbar is taking the power of various loads along the length of the run standard voltage drop calculations no longer apply. The manufacturer gives voltage drop figures, normally in the expression of millivolts per amp per metre allowing a simple calculation be made on a given length. The voltage drop figures will vary depending on the power factor of the system and is based on the full rated load flowing and take into account the resistance across the joints. More accurate results are achieved by considering the actual current flowing if it is less than the rating of the bar. In this instance the resistance of the conductors will be lower due to the reduced temperature and thus the resultant voltage drop will be reduced from the catalogue information. The formula is RX r20 (1 + 0.004 (Tc-20)) ohm/metre. Tc is approximately Ta + Tr where RX is the actual conductor resistance. This is the ambient temperature and Tr is the full load temperature rise in °C (assume 55 °C).
On site, nowadays, the pressures are such that the products should be made to ensure that they can be installed quickly and perform to the required level with the minimum of technical expertise. We are fully aware of the shortage of skilled labour in the industry that will hit us shortly. As a busbar installation is a system which has a number of trunking elements to form the run and all the elements are complete in their own right, the system becomes a simple set of products which are easy to handle and install. At the same time, the electrical and mechanical joints are designed to be fool-proof and the IP rating is inherent in the busbar product. This gives a safe system by reducing the human error factors. Busbar trunking has a number of physical attributes, which can improve the project. Amp for amp high power busbar trunking is smaller, more compact than cables installed on tray or ladder. Also, the weight of busbar per metre is often less than cables an tray combined. The actual routing of the busbar is improved as trunking can be manufactured to right angles thus reducing the space lost due to cable bending radius. All these things make the co-ordination of a number of runs very much simplified. Finally, as the busbar is in component form, the run can be started in areas, which are available, and areas, which may not be, weatherproof or not handed over, can be left until later. This approach allows the installation team to be as efficient and flexible as possible in the process to maintain the tight programmes which is now being set.
In addition to the above, busbar trunking systems offer very low combustive energy as compared to equivalent cable runs. This leads to increased safety in the event of fire.
Satisfying the client
To ensure that the end client is satisfied and becomes a regular client, we need to give him not just his requirements from day one, but provide the design, which allows him to grow and develop. Busbar trunking is the product that gives the flexibility to change the production flow by moving machines, the assurance that shutdowns are kept to minimum by the single isolation of a load and that the protection devices is only meters away from the machine it is protecting. With these attributes the system will still be satisfying the clinet 20 years later. In conclusion, is busbar a replacement for cable? No, busbar is the solution that ends clients want that replaces cables and distribution boards. A bold statement but in todays competitive market engineers need to move forward and access the options and not just rest on traditional methods for solutions.