Consulting-Specifying Engineer - October 2008 - (Page 70)

Codes & Standards Power system circuit breakers An insider looks at amp-interrupting capacity and the calculations to determine values for a power system. BY STEVEN L. PAY, PE, LEED AP, FEA Consulting Engineers, Henderson, Nev. To explain the difference between typical and series amp-interrupting capacity (AIC) ratings, let’s review the typical method. Fault current calculations are needed in an electrical system to ensure that the system can safely handle and protect the specified equipment during a fault. The interrupting capacity of a device is the “the maximum short-circuit current that a protective device can safely clear.” The electrical power system design should provide safe operation under all conditions and allow non-effected buses of the system to continue operation under an isolated fault at a given bus. Asymmetrical fault currents typically are composed of two components, symmetrical (steadystate) and asymmetrical (transient and subtransient currents). Steady-state current is the current supplied in the system during normal operation. Subtransient current is the current supplied in the system from the time the fault occurs to two cycles after the fault occurs. Transient current is the current supplied in the system from two cycles after the fault occurs until the steady-state value is reached. In general, there are two types of power distribution systems, single-phase and three-phase. We’ll discuss three-phase systems in this article. In the three-phase power distribution system, there are two major ways to calculate fault current: the base, or ohmic, method and the perunit method. Ohmic calculation The ohmic calculation method typically is used for simple calculations. This method is from “Engineering Dependable Protection for an Electrical Distribution System – Part I A Simple Approach to Short Circuit Calculations” from Bussmann Mfg. Initially, the utility reactance (the utility resistance is neglected) and the utility transformer resistance and reactance must be determined. This is accomplished using the following equations: Utility Reactance (Xu) = 1,000 x (Transformer Secondary kV)2 Utility Short Circuit kVA Transformer Reactance (Xt) = 10 x (%X) x (Transformer Secondary kV)2 Transformer kVA Transformer Resistance (Rt) = 10 x (%R) x (Transformer Secondary kV)2 Transformer kVA AT A GLANCE National Electric Code 110.9 requires overcurrent protective devices to have an interrupting rating sufficient for the nominal circuit voltage and the available shortcircuit current at the line terminals of the device. The 2005 NEC 240.86 added another type of series-rated system. However, confusion and controversy have surrounded the codes that involve interrupting ratings as related to the types of systems and how to protect buildings, workers, and equipment according to the types of systems selected. To understand the concepts behind fully rated, series-rated, and selectively coordinated systems, a refresher on calculating fault current for each type of system describes the physics behind the codes. The next step is to gather the impedances of the large switches, circuit breakers, and transformers in the system. The manufacturer provides the circuit breaker and transformer values. Basic fault current calculations typically do not consider the minor effects of the impedance of circuit breakers and switches. The feeder impedances are given in tables for the type of conductors and raceway installation. The calculation method next requires that we determine the sum of the resistance and reactance values to the point of the fault. At that point, the total impedance per phase can be determined to the point of the fault by: Total Impedance (Zt) = Rt2 + Xt2 With that established, we must now determine the short circuit symmetrical root mean squared 70 Consulting-Specifying Engineer • OCTOBER 2008

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Consulting-Specifying Engineer - October 2008 Contents Viewpoint Letters News M/E Roundtable Cities Under One Roof Eight Design Tips for Integration Is 'Green' Really New? Down the Drain Product of the Year Awards Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - October 2008

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