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What does it mean when a 50% earthing bar is listed?

Hi Medhat,

Thank you for your question regarding 50% earthing bars, it is our pleasure to help.

Busways have two earth 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 earth path.

The housing can be a 50% rated earth, and the earth bus bar can be the other 50% rated earth.  Together they make up 100% of the earth path.  According to Eaton, in applications with heavy microprocessor-based loads or large computer installations, earthing isolation is essential.  In these cases, the housing earth can be insulated from the bus bar earth.

Isolated earthing is used with sensitive electronic equipment.  Specifically, this type of equipment earthing configuration is only intended to be used for “reducing common-mode electrical noise on the electronic load equipment circuit.”   It keeps earth noise from other equipment from coupling onto the earth used for sensitive equipment.  However, in some circumstances, the isolated earthing configuration can actually make your noise worse.  It is important to understand all the potential repercussions of isolated earth 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 earth.  The isolated earth (IG) must be inside the busway while using the busway housing as the normal equipment earthing conductor, or in the case of conduit, the isolated earth (IG) must be inside the same the same conduit as the phase conductors and the normal equipment earthing conductor.

An external isolated earth 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 earth 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, earth-fault return path.  This is a dangerous and counter-productive design.

Design of a system using isolated earth should not be attempted by an engineer without suitable experience because of the many potential problems.  A better solution to ensure clean power and earth is to install a earthed 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 earthing methods for sensitive electronic equipment that are alternatives to isolation earthing.

An Isolated Earth (IE) conductor can be run parallel to the Protective Earth (also called the equipment earthing 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 earth if it is to be plugged into an isolated earth receptacle (IGR).  When this equipment earthing configuration is used, “the enclosing metal raceway must still be properly earthed.”

The busway enclosure can be used as a 50% rated earth path.  This means that the housing is capable of being an equipment earth conductor at up to 50% of the rated current of the phase bus bars.  This is sufficient ampacity for most earthing systems.  In the isolated earth configuration, the earth bus bar would be the Isolated Ground, and the busway housing would be used as your normal equipment earthing conductor.

If you want to learn more about applications of isolated earthing, 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.”

Best regards,

The Engineering Team at E&S Earthing Solutions

 

Photo credit: E&S Earthing Solutions

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