Busbars explained

Q. What is the history?
Busbar trunking has been available in the UK for over 50 years. It has progressed from derivations of busbar chambers with heavy metal gauge; metal closures with bolt-on covers and clamp connections for tap-offs; to totally enclosed sheet steel or aluminium casings with copper or aluminium conductors and plug-in, tap-off outlets that are fully shuttered and finger-protected for increased safety.

The latest generation incorporates a five-pole system which has the advantage of a high integrity, 100% separate earth bar while retaining a 100% neutral bar. 200% neutral systems are available for applications with high harmonic currents.

Q. What does a busbar trunking system consist of?
It is manufactured into totally enclosed, prefabricated sections consisting of copper or aluminium busbars with current ratings ranging from 25-7500 A. Power is simply tapped-off by plugs or tap-offs positioned at regular intervals.

Typically, a busbar system will consist of 3 or 1·5 m-lengths, end-feed/live-end boxes, end covers and various tap-off units at different ratings and hanging brackets.

Q. What are the advantages of using busbar trunking?
It has several key advantages over conventional forms of power distribution, including:

• reduced on-site installation times when compared to hard-wired systems, thus leading to cost savings;
  
• increased flexibility in design and versatility with regards to future modifications;
  
• increased safety features brought about by the use of high quality, manufactured components, which provide greater safety and peace of mind for specifiers, contractors and end-users.

Q. In which applications are busbars being used?
Busbar trunking systems are now being used in a variety of high and low voltage applications including production plants, workshops, assembly lines, warehouses, distribution centres and retail outlets.

Q. Is it difficult to estimate a busbar installation?
No. The beauty of busbar is its simplicity – from the design stage through to installation on site. This is because the technical characteristics and price of each component are always known.

Installations require no special tools. The joints, lengths and accessories are simply bolted or screwed together. The medium and high power systems have double-headed torque bolts, which on tightening will sheer when the correct torque has been achieved.

In addition, as busbar is a factory manufactured and tested product, a high quality, safer installation is ensured.

Most manufacturers provide a range of technical support services to electrical contractors, who can buy fully designed and engineered systems. The design and estimating stages are also supported by the manufacturers, who provide quotations for components and in most cases can advise and assist in the design.

Q. How do such installations compare in terms of cost?
Most favourably. Historically, the material cost of busbar has been a point of concern for electrical contractors. However, it is short-sighted to compare the cost of busbar against that of a length of cable and not the real cost of a cable installation, which includes multiple runs of cable, tray and fixing, plus the protracted time and effort of the cable pulling operation.

The cost of busbar systems has fallen in real terms over recent years. Add to this, the saving in installation time – up to 50% less than cable. The reality is that contractors can offer a more competitive bid and state-of-the-art power distribution installation that is infinitely more flexible in use. The installation time for busbar is virtually the same as for fitting cable tray – before laying the cables.

Q. How does it operate?
Busbar trunking falls into two distinct categories – distribution and feeder.

Distribution busbar provides power through tap-off points along the busbar length at typically 0·5 or 1 m centres. Tap-off units are simply plugged in along the busbar to supply a load. Each individual tap-off can normally be added to or removed with the busbar live, thus eliminating production down time.

Fitted vertically, the same systems can be used for rising mains applications, with tap-offs feeding individual floors. As protection devices (fuses, switch fuses or circuit-breakers) are located along the busbar run, the need for large distribution boards and quantities of cables running to and from installed equipment is reduced.

High power, feeder busbar takes power directly from A to B, usually from a power transformer to switchboard or switchboard to switchboard. This basic configuration is often referred to as a goalpost installation. If a power source must be tapped off then it is possible with most manufacturers' ranges to do this at a joint along the run.

Feeder and distribution busbar systems in the medium and high power ranges are available in two forms: the flat, spaced configuration, where the conductors are either air-insulated or have pvc-insulated conductors within an earthed metal enclosure; or the sandwich configuration, which is becoming the norm for most manufacturers at these ratings.

With a sandwich construction the conductors are individually insulated and mounted to form a sandwich of conductors and insulation within the busbar casing. Such busbars have very good mechanical strength, give higher fault level withstand characteristics and lower volt drop characteristics. It is a very compact product, making it ideal for installing into the ever-reducing available space for building services. In rising main applications this configuration negates the need for fire barriers because there is no air gap within the casing to give a chimney effect, which would allow the passage of smoke and fire.

Q. Do busbars have IP ratings?
Most busbar systems have IP ratings up to IP55, which allows them to be used in difficult environments. However, for extreme conditions such as petrochemical or outdoor applications, resin-encapsulated busbar is available, which gives a rating up to IP68. This, as the name suggests, is a system where the conductors are totally encapsulated in an insulating casing of flame retardant, self-extinguishing epoxy resin mix, which at IP68 classes it as submersible in water to defined limits.

Q. What are the aesthetic advantages?
Where aesthetics must be considered, busbar trunking can be installed with natural galvanised, aluminium or painted finish. Special colours to a specific colour scheme are also available on request.

Another point to remember is that when comparing with cable, busbars are more compact when installed because allowances for large bending radii are not necessary – an important consideration in view of the smaller switchboard rooms now being made available.

There has never been a better time for electrical contractors to consider using busbar. Manufacturers offer a full technical back-up – from design to installation support. The competitive price levels combined with the time saved on site and the other benefits discussed, make busbar trunking systems a must for today's power distribution installation.