Consulting-Specifying Engineer - October 2007 - (Page 49)

What consulting engineers need to know about overcurrent protection of low-voltage electrical systems. BY JACK SMITH, Managing Editor, Plant Engineering magazine hen designing non-residential electrical systems, engineers must consider electrical protection from many perspectives. Although the entire electrical distribution system of a facility is important—from switchyard to light bulbs—“the overcurrent protective system is the very heart of the electrical distribution system,” wrote George Farrell and Frank Valvoda, PE, in a 1965 series of articles in Actual Specifying Engineer, predecessor to Consulting-Specifying Engineer. Although written more than four decades ago, that statement is still true. “Faulty or inadequate overcurrent protection can bring about the loss of the entire facility served by the system—or, worse, it can cause needless death or injury to personnel.” (Editor’s note: The series is being republished in monthly installments at www.csemag.com.) What has changed since the first publication of the popular article series, “The Art of Protecting Electrical Systems,” is the introduction of more electrically intuitive devices, which have changed electrical design practices significantly. W E Overcurrent Overcurrent is current that exceeds the ampere rating of conductors, equipment or devices under conditions of use, and includes both short circuits and overloads. During a short circuit, current flows outside its normal path. Insulation breakdown or faulty equipment connections can cause short circuits. The load determines circuit current during normal fault-free conditions. However, during a short circuit, electrical current bypasses the load, taking the path of least resistance. System impedance—or AC resistance—determines short circuit or fault current magnitude, which ranges from fractions of an amp to 200 kA or more. An overload is an overcurrent condition within normal current paths—there is no insulation breakdown. However, if an overload is allowed to persist, it causes equipment or wiring damage. Temporary overloads can be harmless; sustained overloads can cause damage. Temporary overloads may be caused by momentarily pushing equipment past its limit, such as cutting too deeply with a milling machine or from starting large motors or other inductive loads. Temporary overloads occur frequently, are typically harmless and should be allowed to subside. An overcurrent protective device (OCPD) should not open the circuit, allowing motors to start and loads to stabilize. Sustained overloads can be caused by continually overloading electrically-driven mechanical equipment, failed bearings or other equipment malfunctions. They also are caused by installing loads such as equipment or additional lighting circuits that increase power demand beyond planned capacity. If sustained overloads are not disconnected within appropriate time limits, they eventually will overheat circuit components and cause thermal damage to insulation and equipment. When starting up a new facility, after system modification or equipment installation, it’s possible to encounter crossed phase wiring, or perhaps even bolted faults. “Bolted faults are characterized by a solidly connected fault path causing high levels of current to flow through this solid connection,” said Todd Lottman, product manager of services at St. Louis-based Cooper Bussmann. “This type of fault is commonly used when testing electrical equipment for short-circuit current ratings and overcurrent protective devices for interrupting ratings.” Consulting-Specifying Engineer • OCTOBER, 2007 49 http://www.csemag.com

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - October 2007

Consulting-Specifying Engineer - October 2007 Contents On the Web In the News M/E Roundtable How To Beat the Energy Codes Professional Practices Codes & Standards A Capital Job on Engineered Buiding Systems Going Green in Data Centers Overcurrent Protection: Fuses or Breakers? Third-Party Liability Management Report New Products Product Spotlight Jobs/Classifieds Specifier's Notebook

Consulting-Specifying Engineer - October 2007

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