Short Circuit Current Ratings for Industrial Control Panels

Short circuit current ratings should not be overlooked during panel design. Negative impacts can occur to production, equipment and, worst case, plant personnel.

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Per 2014 NEC Article 100, short circuit current rating (SCCR) is defined as: “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.” To simplify, SCCR basically specifies the highest fault current that a piece of equipment can withstand without causing extensive equipment damage.

Safety is the main reason for correctly determining the SCCR of an industrial control panel. If a control panel is connected to an electrical system with an available fault current higher than the SCCR of the panel, a catastrophic event could occur that could result in extensive damage and/or personnel injury.

A properly designed panel with an SCCR higher than the available fault current will be capable of clearing the fault with minimal to no damage. For example, an industrial control panel with an SCCR of 10 kA is connected to an electrical system that is capable of providing 40 kA of fault current. In normal conditions (no fault), the control panel would function with no issues. However, if a fault were present, extensive damage would occur, resulting in fire, personnel injury or worse.

How to determine SCCR

Two methods are available for determining the SCCR of an industrial control panel. The first is testing of a fully assembled panel. This method is the least likely to be used because of cost, unless a large quantity of the same control panel will be constructed. The second is using the device analysis method described within UL508A. Per Section SB, the overall panel SCCR can be determined using the following steps:

  1. Establish the SCCR of individual power circuit components as specified in SB4.2.
  2. Modify the available SCCR within a portion of a circuit in the panel due to the presence of current limiting components as specified in SB4.3.
  3. Determine the overall panel SCCR as specified in SB4.4.

Here is more explanation about each of those steps:

Step 1: Ratings of individual devices can be found either printed on the device itself or within the device’s specification sheet. If a device is unmarked, UL508A provides Table SB4.1 for determining the SCCR. Also, manufacturers will test their load controller, motor overload relay or combination motor controller with a specific overcurrent protective device to achieve a higher combination rating. (As an example, a motor controller with a rating of 5 kA is tested with a 30 A Class J fuse and given a combination rating of 100 kA. If that same motor controller is protected by a 30 A circuit breaker, the rating would only be 5 kA). Most major controller manufacturers do combination tests on an annual basis, and the results are available through the UL website.

Step 2: Per UL508A, Section SB4.3, there are three ways the branch circuit rating could be reduced. If the branch circuit is supplied by a power transformer, the requirements are provided in Section SB4.3.1. The other two ways are similar in that they use the let-through current characteristics of the protective device. A circuit breaker must be labeled as “current limiting” and provided a peak let-through current value in order to modify the SCCR. For fuses, the type (CC, G, J, etc.) specifies the peak let-through current characteristics. The peak let-through current values can be determined by the manufacturer’s supplied let-through current curves or by UL508A Table SB4.2.

Step 3: The last step is to gather all the information determined in the first two steps and apply the rules per UL508A, Section SB4.4. If the calculated SCCR of the panel is greater than the available fault current, your panel is adequately designed. However, if the SCCR of the panel is too low, the devices will need to be reevaluated. Possible ways to increase the SCCR are to use higher-rated devices, use protective devices with higher current-limiting capabilities, or determine if combination tested components can be used.

Bryan Reinecke is a project engineer at Bachelor Controls Inc., a certified member of the Control System Integrators Association (CSIA). For more information about Bachelor Controls, visit its profile on the Industrial Automation Exchange.

 

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