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Plugging Away - Midwest Electrical Testing

Written By Jason Honick
Sales Representative, Midwest Testing, Switchgear Division

Definition: As the name suggests a Bus Plug is a switched device which plugs into bus duct to extend power from the distribution system. The purpose of a Bus Plug is to provide a localized power feed to electrical devices.

Bus plugs will contain their own source of circuit protection. Circuit protection can be either in the form of a fuse or a circuit breaker.

Bus plug voltage ratings (USA) range from 120/240, 208/120, 240, 277/480, 480, 600 Volts.

Bus Plug Ampere ratings (USA) can range from 15 to 1600 amps. The most common bus plug ampere ratings fall within the range of 30-200 amps. Many 30 amp and 60 amp devices become available on the secondary market so 30 and 60 amps could be the most common size bus plugs in use overall.

Why are bus plugs such a hot item in the used electrical equipment market? They can fail if neglected and so there is always demand for replacements. Why do they fail? The answer can be found in a combination of factors: Bus plugs often operate in environments which are not the most conducive to the long life of electrical devices. The atmosphere in manufacturing facilities can contain quantities and qualities of air contaminants and heat temperatures in great abundance, making for the short life of bus plugs. Oil and various air contaminants accumulate on insulating members of the bus plug causing a decrease in insulation resistance. Low insulation resistance increases the risk of flash over between either phase to phase or phase to ground. High ambient temperatures can lead to "cooking a fuse" where fuse thermal thresholds can be compromised degrading fuse performance. Circuit breakers in an extremely warm bus plug do not fare much better.

Bus plug internal contacts contain silver tabs brazed to copper contact arms. Arcing from normal open and closing operations under load causes the silver contact surface to pit leading to increased degrees of contact deterioration.

Bus plug external contact fingers, which make contact to the bus itself, are often the source of bus plug failure. Finger contact surfaces are vulnerable to environmental contaminants. Heat as a result of a resistance thresholds being breached can lead to failure. It is not uncommon for these fingers to completely melt due to high contact temperatures.

Tap Boxes are unswitched devices with no internal protective device. They act as the ins and outs of a bus duct power distribution system. Tap boxes are devices which can be used to connect power cable feeds to or from Bus Duct. Tap Boxes come in two flavors depending on where the box is installed; "Plug In Tap Box" and "End Tap Box" types. The former is what it sounds like. It connects to the bus duct, mid span, at pre-engineered connection points that are normally covered. End Tap Boxes connect at either end of a particular bus duct run. There are no rules regarding how these boxes are utilized. A bus duct run can be energized from the middle with a tap box or from the end with an end tap box. There are also bus duct caps which cover the end of a bus duct run. Some facilities always terminate their bus duct runs with end tap boxes and never extend that portion of the bus duct further. Using end tap boxes is an expensive alternative over an end cap. All of these devices are items in high demand in the used electrical power distribution market.

One of our product offerings is reconditioned bus plugs. The reconditioning process involves completely disassembling the bus plug. The various component parts are then inspected, cleaned, polished and painted. After reassembly, the reconditioned bus plug is put through a series of tests per Midwest's Quality Control Program, (QCP). The bus plug contacts are tested for contact resistance. The live parts, and fuse holders, are meggered.

Finally the bus plug is load tested. 80% of rated load current is put through the device as a final check before it is boxed and sent to our customer.

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