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The SCCR is defined by the National Electrical Code (NEC) as the maximum short-circuit current that a component or system must be able to safely withstand. In addition to the NEC, the U.S. Occupational Safety and Health Administration (OSHA) and Underwriters Laboratories (UL) have also issued guidelines regarding SCCR. Before a machine or system can be put into operation in North America, it must meet the SCCR values set out by these standards.
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SCCR standards serve the primary purpose of preventing the installation of devices with an SCCR that is less than the available short-circuit current at the point of installation because these conditions could cause an electric shock or fire. However, about 50 percent of machine and plant manufacturers only comply with the typical minimum SCCR of 5 kA when manufacturing switchgear systems.
This can become a problem if the short-circuit current of the feeder exceeds 5 kA. Importers face the risk of these machines not meeting NEC requirements and therefore being able to be accepted. However, the short-circuit strength and thus the SCCR value of a machine can be improved, making the system suitable for a wide range of connection values. This makes installation easier for the end customer and ensures acceptance of the machines, systems or control panels in line with the UL standard and NEC guidelines.
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The first step, of course, is to calculate the short-circuit strength and determine the SCCR value. If it is below the short-circuit current at the point of installation, there are generally two ways to increase the SCCR:
1. Replace the parts or components with the lowest SCCR in order to increase the overall SCCR.
2. The short-circuit strength of electrical circuits with low SCCR values can be improved by connecting a current-limiting device upstream.
According to UL 508A, only the SCCR value in the branch circuit may be increased using a current-limiting device. These can be fuses complying with UL 248, circuit breakers complying with UL 489 as well as manual and combination motor controllers of type E/F complying with UL 508. It is not permissible to increase the values of the components in the feeder circuit. Furthermore, a current-limiting overcurrent protective device in the feeder circuit must not be used to increase the switching capacity of a downstream overcurrent protective device in the branch circuit or improve the SCCR of a downstream combination motor controller in the branch circuit.
In simple terms, this means that the SCCR of a contactor, overload relay, variable frequency drive or other components in the branch circuit can be improved. On the other hand, it is not possible to improve the switching capacity for circuit breakers and fuses or the SCCR for type F combination motor controllers by means of an upstream current-limiting device, even if they are located in the branch circuit.
For current-limiting fuses, the peak let-through current value must be determined using Table SB4.2 from the UL 508A standard (see Figure 1). The use of peak let-through current data published by the manufacturer for specific fuses is not permitted because fuses can be replaced and there is no guarantee that a fuse will be replaced with the same fuse from the same manufacturer after the device has been installed. However, for circuit breakers marked as current-limiting, the manufacturers' peak let-through current data is used.
Figure 1: Value for peak let-through current in current-limiting fuses. You find the total tabel for different fuses classes with more explanations is our white paper about the SCCR.
The branch circuit SCCR can also be increased using power transformers, provided there are no space constraints. For example, it is possible to use a transformer outside a control system because it reduces the possible short-circuit current at the location where the system is installed. Depending on the transformer and the network parameters, the possible short-circuit current in the transformer secondary circuit may be less than 5 kA, making devices with a standard SCCR below 5 kA acceptable.
For systems with a control system that do not achieve an SCCR suitable for the short-circuit current possible at the particular installation point, the possible short-circuit current can be strengthened by an upstream isolation transformer (same primary/secondary voltage), provided that the secondary short-circuit current available does not exceed the switchgear system's SCCR. Reducing the available short-circuit current outside the control system is normally a last resort.
To ensure electrical safety, devices in North America must comply with the SCCR requirements of the relevant standards (further information can be found in the whitepaper "Current-limiting components: Improving the short circuit current rating (SCCR)" from Eaton). Current limiting devices can be useful in achieving the desired SCCR for the switchgear system. They can extend the permissible range of possible short-circuit currents for which a switchgear system is designed, often with minimal effect on standard designs. The most successful time to incorporate these components in terms of cost efficiency and effectiveness is during the design phase of a project.
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