Fault current calculation is the most basic calculation performed on a power distribution system, which is vital for the proper electrical equipment application.
Fault current calculations are performed without current-limiting devices in the system. To determine the maximum “available” fault current, calculations are made as though these devices are replaced with copper bars. This is necessary to project how the system and the current-limiting devices will perform.
Also, multiple current-limiting devices do not operate in series to “compound” a current-limiting effect. The downstream, loadside fuse will operate alone under a fault condition if properly coordinated.
The application of the point-to-point method permits determining available fault currents with a reasonable degree of accuracy at various points for either three-phase or single-phase electrical distribution systems. This method can assume unlimited primary fault current (infinite bus) or it can be used with limited available primary fault current.
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There are several NEC sections with requirements directly pertaining to the proper electrical product application and available fault current. Safe and reliable electrical equipment application, including OCPDs, relies on such power systems analysis study information obtained from fault current and selective coordination studies.
Knowing available fault current throughout the power distribution system or the secondary side of the transformer secondary is important for proper application of overcurrent protective devices operating at full load. The NEC recognizes the importance of fault currents in many areas within its primary voltage requirements, including these important topics and sections:
Available fault current markings
Applying solutions within their ratings
The one-line diagram, often referred to as a single-line, plays an important role in many aspects of power distribution system design, maintenance and construction. A one-line diagram graphically represents the power distribution system. Developing this diagram is the first step in making fault current, selective coordination and incident energy studies. This diagram should show all fault current sources and significant circuit elements. Significant circuit element reactance and resistance values should be included in the diagram. The one-line diagram should be updated any time the power distribution system changes. Changes must be reviewed with attention paid to the impact upon the studies that are based on this diagram’s contents.
Utilities provide power through a transformer or series of transformers depending upon where in the distribution system the facility obtains its power. Most rural locations have a transformer dedicated to a facility or multiple facilities. In some urban areas, for reliability sake, power is derived from utility secondary networks where utility transformers are operated in parallel. Available fault currents on these secondary network systems are very high, in a range greater than 100 kA and upwards of 200 kA.
The fault current that’s typically provided from the utility is an infinite bus calculation based upon the supply transformer’s kVA size and minimum impedance. For applications on a secondary network, consulting the utility is the only way to obtain the available fault current for any given installation.
On-site generation for backup power must be a consideration for the power distribution system equipment. In most cases, the local generation will not provide fault currents greater than what can be seen from a utility. When large systems have multiple generators installed in parallel, such as hospitals or other similar applications, it’s conceivable that available fault currents are greater than that available from utility sources.
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