Output short-circuit current bearable by the UPS

June 01, 2016

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The output short-circuit current bearable by the UPS  has become the hot topic since 14th February 2016, when amendment A1:2013 to the international standard IEC 62040-1:2008, which regards the general and safety requirements for UPS, came into force.

This amendment introduces specific requirements to the UPS output short circuit withstand current. In particular, it forces UPS manufacturers to guarantee a minimum value depending on the UPS rating in order for the UPS to be sold. Aside from the practical implications introduced with this amendment, let’s try to understand what a short circuit current is and why it is so significant.

The prospective short circuit current is a fundamental data to design electrical systems capable of guaranteeing security even in the worst fault conditions. Therefore, in order to guarantee the safety of the whole system, designers will have to make sure that the UPS short circuit current is higher than the fault current level of the site. In order to achieve the required security level it’s necessary to take into account all of the components.

Solely considering the prospective short circuit current of the source can be penalizing because the softening action carried out by the impedance of the connection between source and bypass input would be left out. Throughout the current path, between the bypass input and the UPS output, there are various components and devices which are run through by the short circuit current. Their design indirectly sets the limit of the maximum source perspective short circuit current.

Among these we can mention the cables, the sectioning devices, the static devices (named SCR after the acronym of “Silicon Controlled Rectifiers”) and possible fuses.

UPS block diagram

Figure.1. UPS block diagram.

The short circuit current effects can be electro-dynamic or thermal. The thermal effects consist of heat dissipation over the conductors by Joule effect and, as short-circuit currents are much higher than rated current, the component overheats and degrades.

On the other hand, the electro-dynamical effects are attractant or repulsive mechanical actions between the components and are functions of the square of the current peak. When there is the need for maximum device protection, the use of high-speed fuses is necessary. This type of fuse is capable of limiting the prospective short circuit current peak whenever it overcomes a certain level.

A valid solution to achieve a higher short circuit current level is to increase the fuse size at the expense of the SCR protection. In this scenario, in the rare case of a short circuit occurring right at the UPS output, both the SCR and the fuses will need to be replaced.

At this point, we’ve considered all the components that in a scenario with a single UPS. When the electrical system is instead composed of more than one UPS connected in parallel, it remains necessary to consider the short-circuit current bearable on the UPS output scenario, and in this case the maximum prospective short circuit current is equal to 80% of the single UPS value multiplied by the number of UPS in the parallel system (without considering any redundancy). 80% is a safety margin which takes into consideration that there can be a short circuit current imperfect distribution due to asymmetries in the equipment connections.

Just by reading the brief explanation above it’s clear that the short circuit current bearable by a UPS is a complex theme, but it’s also delicate at the same time.

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