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IEC 60909_FAULT Study IEC_FAULT 1-3

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1.2 Engineering Methodology

IEC Standard 909 describes a detailed method for calculating three-phase and unbalanced

short circuit duties to compare to electrical apparatus ratings. The Standard contains 14

chapters and an appendix. Individual paragraphs are referred to as articles or clauses, and

sub-paragraphs are referred to as sub-clauses. The Standard is divided into two majorsections: far-from-generator short circuits and near-to-generator short circuits. 

1.2.1 IEC Standard 909Section One of the Standard, “Systems with Short Circuit Currents Having No A.C.

Component Decay (Far-From-Generator Short Circuits),” defines the short circuit currents

that are expected at a fault location, assuming that active sources (machines and network

feeders) have no ac decrement. The Standard calls these machines far-from-the-fault-

location. The Standard defines no ac decrement as a symmetrical short circuit current that

has no time-varying change from peak to peak during the fault. The terms near and far are

defined in Section 1.3.4, “Assumptions of the IEC_FAULT Study.”

Section Two of the Standard, “Systems With Short Circuit Currents Having Decaying

A.C. Components (Near-To-Generator Short Circuits),” examines machines that are

considered near the fault; they exhibit an ac decrement throughout the duration of the fault

condition. Different source types (network feeders, synchronous motors and generators,

and asynchronous motors) are defined differently based on how their ac decrement is

modeled.

Both Sections One and Two discuss the implications of how the short circuit current

arrives at the fault location, and the impact of the dc decay on the short circuit current.

The Standard defines a contribution as coming from a meshed topology if a contribution

current flow splits into two or more currents between the source of supply and the fault

location. The concept of a meshed network is more complex than merely defining the

system as having loops or parallel connections; special procedures are required when

modeling meshed contributions. In addition, careful attention must be paid when

calculating their dc decay currents, regardless of whether the source of the short circuit

contribution is near or far from the fault location.

IEC Standard 909 is a derivative of the German VDE Short Circuit Standard. As such,

 both standards were developed to assist engineers with hand calculations. Some of the

simplifying assumptions necessary for practical hand calculations are not necessarily well-

suited for computerized methods. The computer allows for removal of many of the

limiting assumptions in the hand calculation methods. Whenever PTW identifies a

simplifying assumption in the IEC Standard 909, or if the Standard uses the term “may be

considered,” the IEC_FAULT Study evaluates the assumption and takes the most

conservative implementation approach ⎯ that is, the Study calculates a larger short circuit

current.

1.2.2 Comparing the ANSI and IEC Short Circuit StandardsThere are three significant differences between the IEC methodology and ANSI

methodology.

The first major difference involves calculating the dc decay component. ANSI requires

calculation of a Thevenin equivalent fault point X/R ratio, based on separately derived R