High Fault Currents
Introduction
The impedance of mains, submains and final subcircuit is a major factor in reducing high fault currents in the installation
Guidelines for selecting protective devices
Overcurrent protection is the main type of protection used in most electrical installations. In some circumstances, a Safety Switch or R.C.D. protection may also be mandated or selected
Factors limiting fault current
The increased generating capacity of modern supply systems has led to an increase in the prospective current available at a customer’s electrical installation under short circuit conditions
Determining prospective fault currents
When calculating fault levels it is essential to know the kV.A rating and impedance of the transformer supplying the installation
Protecting customer’s installation against damage
Fault currents in a customer’s installation are subject to limiting due to circuit impedance and the selection and installation of a suitable fault current limiter
Determining when fault current limiting is required
When necessary, the customer should provide and install fault current limiting equipment to protect his installation
Appendix A
Cable impedances for various lengths of copper conductors
Appendix B
A graph which shows how prospective fault currents decrease as the conductor impedance increases
Last updated July 16, 2009
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