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causing unnecessary interruption to fault-unrelated services. While it is possible to use primary and secondary injection tests to check and commission the settings of an overcurrents protection scheme, no one has ever put the system online and slammed on it a bolted three-phase fault to prove it’s designed to withstand kA capacity. For a complex HV/LV electrical installation, a complete Fault Current Calculations Study is considered to be one of the most important electrical building services engineering activities that designers and installers must get right in order to meet the life and property safety design requirements of an electrical installation.
Faults In an HV/LV electrical installation, there are fi ve types of electrical fault that can give rise to dangerously high energy fault currents: • A three-phase symmetrical fault; • A three-phase to neutral and/or earth symmetrical fault;
• A double-phase asymmetrical fault; • A double-phase to neutral and/or earth asymmetrical fault; and
• A single-phase to neutral and/or earth asymmetrical fault. Normally1
the three-phase symmetrical fault can
provide the maximum prospective fault current for equipment fault rating selection purposes. It is a legal requirement2
that an electrical installation should be
designed for safety under normal and abnormal (fault) conditions. To obtain the maximum and minimum
www.cibsejournal.com > September 2010 CIBSE Journal 57
NORTH AMERICA · UNITED KINGDOM · EUROPE MIDDLE EAST · LATIN AMERICA · ASIA PACIFIC
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