Paying attention to certain details of bus duct projects will ensure trouble-free service.
Abus duct system is an effective method of distributing power to your switchgear and various loads. Yet, bus duct problems lead to catastrophic damage and extensive downtime. Let's look at how you properly order and install your system.
Ampacity. You may know the basic bus duct selection requirements already (see sidebar below). One of those requirements is ampacity. Rather than establish ampacity based on a standard chart's temperature assumptions, account for your actual operating environment. Bus typically occupies the high-temperature area near the ceiling.
Once you've determined that area's maximum ambient temperature, factor in the specified temperature rise from no load to full load. For example, it's common for industrial bus duct specifications to limit ampacity to 1000A per sq in. of copper conductor. However, many brands of bus duct have current ratings based on 1800A per sq in. of copper; thus the temperature rise (from no load to full load) increases. If your bus duct run is long and operates at 1800A per sq in., you must accommodate voltage drop within the bus duct.
Location. Many bus ducts installed in industrial and commercial facilities are immediately downstream of the transformers, but upstream of the main overcurrent device. Thus, transformer protection devices often inadequately protect the bus duct zone from fault. So, consider the value of extra protection with service conductor bus ducts. The extra up-front cost is nothing compared to the savings in downtime.
Allow for the space requirements and weight of the bus duct. You may need to change other components or select a different style of bus duct for your installation. One place to pay particular attention is at the transformer. Since torque forces can easily induce transformer-bushing seal leaks, you want to prevent vertical loading on transformer bushings. To do so, ensure proper support for the vertical sections of the bus bars within the bus duct enclosure. Also, the air terminal chamber at the transformer must be of adequate structural design to assume the weight of the vertical bus duct as it terminates at the transformer end. Otherwise, the weight of the bus duct may bend the air terminal chamber - sometimes immediately, sometimes after several weeks. Contact the transformer manufacturer for assistance with determining the suitability of this chamber for the intended bus duct.
If you are installing bus duct outside, select a housing type that sheds rainwater - rather than pooling it on top of the housing. Don't rely on sealants to make up for poor housing selection. Even the best sealants age and crack - especially when exposed to sunlight. Even microscopic cracks can admit sufficient water for a violent electrical explosion. Don't forget those bus duct breather openings. Order them with factory-installed screens to prevent insect penetration.
In humid climates, bus ducts installed outdoors need protection from internal and external rust. You can provide this protection by choosing a housing made of a rust-resistant metal - such as stainless steel. Or you can coat the housing metal to prevent oxidation. You can assure coatings will last a long time by starting with a two-part epoxy primer or chromate vinyl wash primer and finishing with either a two-part polyurethane or catalyst-cured polyester paint. Specify such finishes only if you can't get the rust-resistant metal to begin with. The finishes are expensive and have environmental consequences.
If you are going to install any part of the bus duct outdoors, use factory-installed pre-wired space heaters with integral thermostats to prevent condensation problems. These are not expensive. Where the bus duct enters through an outside wall, use flame-retardant, waterproof, and vapor-resistant wall entrance fittings. It's likely you'll need to support the bus duct system at the wall. In such a case, the wall flange must be able to carry the structural load of the bus duct weight.
Finally, don't forget the assembly hardware. Again, outside service is more demanding - don't settle for anything less than stainless steel. Cost reduction from using cadmium-plated hardware can be economically false due to problems resulting from rust.
Working with the manufacturer. The degree to which you must interact with the factory depends on the size and lead time of your order - as well as your project deadlines. What sets the stage for problems more than anything else is a specification problem. Specifications are often incomplete, incorrect, or poorly communicated. Avoid changes in specification midway through the manufacturing process by carefully planning upfront. Pay special attention to dimensions. This is arguably the biggest problem area in bus duct orders.
When you place your order, work with your sales representative to ensure the manufacturer understands and approves of your specifications. In some cases, the manufacturer will recommend specific changes that will improve performance, reliability, or lead time. Consider such recommendations as advice from an expert, but be sure to review them against the particulars of your installation.
To help assure longevity of your bus duct system, ask the manufacturer to wrap (or power dip) the individual bus bars (within a given bus duct) with non-hygroscopic insulation (it doesn't absorb moisture). When you add insulating boots at joints (during installation), you end up with a second insulation measure.
Always do an acceptance test before beginning the actual installation process. The first step is to make sure the dimensions are correct. Then, check for completeness of your order.
Do you have all the required transitions, wall flanges, transformer flanges, and switchboard flanges? Do you have the copper braids that serve to connect the bus duct bars to the transformer bushings? Are these the correct size and quantity for the ampacity and physical characteristics? Check these braids for the right bar and pad size.
Ensure the bolt-hole sizes, bolt-hole relative locations, and bolt-hole diameters match at each end of each factory-assembled braid. On a large order, it makes sense to visit the factory and do the acceptance testing there. On smaller orders, do this testing as soon as the order arrives - know ahead of time when that will be, and schedule someone to do the testing.
Installation tips. Your bus duct joints and terminations should be silver-plated when made of copper, and tin-plated when made of aluminum. Make every effort to use copper bars when bolting to copper switchgear bars, and to use aluminum bus when bolting to aluminum terminals. When mixing the two metals is unavoidable, liberally apply a manufacturer-approved antioxidant compound to the dry aluminum bars immediately prior to assembly. Ensure no moisture is present in the joint.
Bus manufacturers often provide assembly hardware kits that include bolts, nuts, flat washers, locking devices, and Belleville washers. You'll save yourself considerable design work and project administration headaches by using these kits instead of third-party hardware. If you're not using such a kit, use the bolt material, type, and size the manufacturer recommends for that particular bus duct. If using Belleville washers, use the recommended size and type.
With all assembly hardware, follow instructions precisely so you prevent high-resistance connections or permanent damage to the bus system. While most Belleville washers do not require a torque wrench for proper assembly, nuts and bolts do. Don't use generic torque charts. These assume a bolt composition that probably won't match what you're using. Use torque specifications from the manufacturer of the bolts you are using. Be sure to note the composition (which may include more than the metal content of the bolt), lubrication, hardness, thread pitch, and size when obtaining the torque specification.
Assign someone other than the assembler to test each joint after assembly - because it's easy to miss something. Testing should include visual checks, torque checks (except for Belleville washers), and resistance measurements. Belleville washers are usually at the correct torque, if tightened just until they look flat.
After joints pass the testing, protect them with insulating boots. These should encapsulate the entire array of bolt heads and nuts, since bolts often come very close to other bolts in opposite phases or to the bus duct case.
As a final test, energize the bus with no load. Wait an hour or so, and then give it an infrared scan. You shouldn't have any hot spots - if you do, repair them and scan again. Once you complete this step, you are ready to put the bus into service. Then get ready for a life of trouble-free operation.
Sidebar: Standard Requirements for Bus Duct Selection
• Has the correct voltage rating
• Has sufficient current rating
• Has right quantity of conductors (phase, neutral, and ground)
• Carries UL label or equivalent
• Has adequate short circuit bracing
• Is totally enclosed
• Carries NEMA 1 rating for indoor or NEMA 3R for outdoor use
