Art of protecting electrical systems

From 1965 through 1970, Consulting-Specifying Engineer’s predecessor, Actual Specifying Engineer, ran a series of articles on overcurrent protection. Due to the immense popularity of the 31 installments in the series, the authors, George Farrell and Frank Valvoda, PE, reprised the series in an updated version beginning in the Feb. 1989 issue of CSE.

Over the years since the last installment ran in the late ’90s, we have received many requests to re-run this series. Consequently, we are making the series available as monthly installments at www.csemag.com. In September, we posted Part 12, “Approximating Short-Circuit Calculations for Conductors.”

“Large or complex radial systems, closed-loop systems ,and networks require analysis by the per-unit method,” explained the authors. “However, for many straightforward radial systems, short-cut methods provide a quick, safe approach to determining fault currents for equipment application.”

These estimates are valid even for final calculations, depending on the project’s size and the fault current available from the utility. This article describes a short-cut method for calculating the decrease in fault current caused by circuit conductors.

The October installment in “The art of protecting electrical systems” series is about calculating short-circuit currents at secondary of transformers. While Part 12 presents a point-to-point method of calculating short-circuit currents, in Part 13 offers similar tables but ones that were updated to provide values of available fault currents at the secondaries of standard and low-impedance three-phase transformers.

In November, look for Part 14 of this series in the Electrical community at www.csemag.com, which offers a step-by-step guide for calculating four types of single-phase faults.