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Short Circuit Current Bearable By the UPS

January 01, 2017

According to the standards, an “overcurrent” is a current which exceeds its rated value. Overcurrents can arise in different modalities, depending on whether they are the result of an overload or a short-circuit.

While the former is a normal situation for an electrical circuit, the latter occurs after a negligible impedance fault between two points of the circuit where there is a voltage in ordinary operating conditions.

One of the features that must be carefully evaluated when choosing a UPS is its capability to properly withstand a short circuit current on its output for a certain amount of time.

This capability depends on whether the output short circuit current is withstood solely by the inverter or by the source through the static bypass.

In the first case the capability strictly depends on the UPS design, while in the second it also depends on the source prospective short circuit current (named Icp). The Icp is independent of the UPS and, because of its electrodynamic and thermal effects on the components, in some cases it is appropriate to define its limits and/or to implement particular solutions which allow limiting its extent within the UPS or the entire site.

However, in other cases limiting the prospective short circuit current is necessary in order to stay within the limits of the protective devices downstream the UPS.

All the choices mentioned above risk negatively affecting the short circuit current that the electrical distribution downstream the UPS requires to guarantee the correct coordination between the various site protective devices.

Therefore you need to take into consideration how the maximum source-Icp can be mitigated by the UPS while operating in double conversion mode.

The aims of this white paper are to clarify the concept of prospective short circuit current bearable by the UPS, to explain the role of fuses and to indicate the requirements of the international standard IEC 62040-1.

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