Determine the overall SCCR

In accordance with UL 508A SB4.2, the relevant and non-relevant components must be identified to determine the short-circuit current rating (SCCR).

According to Supplement SB4.2, all components in the main circuit must be considered when determining the SCCR, with certain exceptions. The voltage in volts and the short-circuit resistance in amperes or kiloamperes must be specified.

1. Device markings or technical documentation
   Determining the SCCR for all components is based on device markings, technical documentation, and UL certificates (known as "Certification of Compliance," or CoC), which are provided by the component manufacturer. Some SCCRs can also be viewed at Underwriters Laboratories (UL).
2. UL 508A, Table SB4.1: Components without a specified short-circuit resistance
   For components without SCCR specifications, refer to UL 508A, Table SB4.1. In the event that UL-approved components are used that do not have any information stored on the product or in the technical documentation, Table SB4.1 defines the "standard short-circuit resistance" (standard SCCR) for components according to UL 508A.
3. increase the SCCR through use of tested combinations
   If the maximum short circuit occurring at the feed-in point exceeds the specified SCCR of the components, tested combinations can be used.
   If the SCCR of one or more relevant components is lower than the maximum short circuit at the feed-in point, replace the affected active and passive components with tested combinations that have a "high short-circuit resistance" (high capacity short circuit rating). This increases the overall SCCR of the control panel.

In accordance with UL 508A, Supplement SB4.3, the maximum possible short-circuit current for components of the power circuit can be reduced by using current-limiting devices in the feeder circuit. It is imperative here, however, that the current-limiting devices are situated entirely within the feeder circuit. Use as a branch circuit protective device is not permitted.

Determine the overall SCCR for the industrial control panel after the required measures have been established.

The overall SCCR for the control panel is determined by the weakest SCCR value of the component after the required and selected measures have been applied. A review the steps we made showed that in each step we had determined the overall SCCR of the control panel.

Use high SCCR values of tested combinations!

Instead of thinking about how to limit the current or which device can be used, or even thinking about paying more for a device with higher “Standard SCCR”, we recommend to use tested combinations.

When you use the “Standard SCCR” values of individual components, especially motor controllers and overload protection devices, you may end up with an industrial control panel that can only hold up to 5kA, 10kA, or even 15kA. The issue with these low values is that often industrial control panels will need to have an overall SCCR higher than any of these values.

The Siemens portfolio has been tested in accordance with the performance requirements, including short circuit requirements, and this includes the high fault current combination test. You will find also type 1 and type 2 test combinations on the certifications (CoC=Certificate of compliance).

Our TIA Selection Tool tool can be used to select the UL test combinations with a small amount of input. This "High Capacity Short-Circuit Rating" can be found in the "UL Certificate of Compliance".

With the use of the CoC we can increase the critical product’s SSCR through use of tested combinations, so-called "High Capacity Short-Circuit Ratings".

For more information, please refer to the white papers below.