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Busways have two ground conductors. One is an actual additional copper or aluminum bus bar in the stack with the phase conductors inside the housing. The other is when the protective metal housing around the bus bars is bonded and used as a ground path.
The housing can be a 50% rated ground, and the ground bus bar can be the other 50% rated ground. Together they make up 100% of the ground path. According to Eaton, in applications with heavy microprocessor-based loads or large computer installations, grounding isolation is essential. In these cases, the housing ground can be insulated from the bus bar ground.
Isolated grounding is used with sensitive electronic equipment. Specifically, this type of equipment grounding configuration is only intended to be used for “reducing common-mode electrical noise on the electronic load equipment circuit.” It keeps ground noise from other equipment from coupling onto the ground used for sensitive equipment. However, in some circumstances, the isolated grounding configuration can actually make your noise worse. It is important to understand all the potential repercussions of isolated ground before designing one into your system.
Keep in mind that you may not associate an external insulated Earth cable with the busway as an isolated ground. The isolated ground (IG) must be inside the busway while using the busway housing as the normal equipment grounding conductor, or in the case of conduit, the isolated ground (IG) must be inside the same the same conduit as the phase conductors and the normal equipment grounding conductor.
An external isolated ground would not follow the exact same path as the busway conductors and create a large loop. In normal conditions, this loop would induce noise on the isolated ground making it worse for electronic equipment. In fault conditions, this loop would be a high-inductance (a loop is a giant inductor) and thus a high-impedance, ground-fault return path. This is a dangerous and counter-productive design.
Design of a system using isolated ground should not be attempted by an engineer without suitable experience because of the many potential problems. A better solution to ensure clean power and ground is to install a grounded isolation transformer very near to your sensitive electronic loads. Again, I recommend IEEE Standard 1100-2005 – IEEE Recommended Practice for Powering and Grounding Electronic Equipment (which is available to purchase online as a PDF). This standard discusses many grounding methods for sensitive electronic equipment that are alternatives to isolation grounding.
An Isolated Ground (IG) conductor can be run parallel to the Protective Earth (also called the equipment grounding conductor) and they are only bonded one point: either at the service entrance or a separately derived source such as an isolation transformer. The electronic equipment and its chassis must be physically isolated from building ground if it is to be plugged into an isolated ground receptacle (IGR). When this equipment grounding configuration is used, “the enclosing metal raceway must still be properly grounded.”
The busway enclosure can be used as a 50% rated ground path. This means that the housing is capable of being an equipment ground conductor at up to 50% of the rated current of the phase bus bars. This is sufficient ampacity for most grounding systems. In the isolated ground configuration, the ground bus bar would be the Isolated Ground, and the busway housing would be used as your normal equipment grounding conductor.
If you want to learn more about applications of isolated grounding, please refer to IEEE Standard 1100-2005 – IEEE Recommended Practice for Powering and Grounding Electronic Equipment. This is part of the Industrial Applications Society’s (IAS) Color Book Series and is often referred to as the “Emerald Book.”
The Engineering Team at E&S Grounding Solutions
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