# Using current-limiting devices to improve Short-Circuit Current Rating (SCCR): Your questions answered

## More answers about using current-limiting devices to improve Short-Circuit Current Rating (SCCR), the topic of a May 18 Webcast, are provided by speaker Erik Barnes. Topics include single-phase circuits and power generation.

05/25/2016

Current-limiting devices can have a significant impact on the Short-Circuit Current Rating (SCCR) of an industrial control panel. However, many struggle to understand how to properly apply current-limiting devices to raise SCCR. This webcast will simplify the process of applying a current-limiting device to raise SCCR, explain the limits of use, and demonstrate how current-limiting devices can help solve SCCR challenges, as well as increase design and manufacturing flexibility with minimal to no impact on cost.

Erik Barnes, Marketing Manager for Bussmann series products

Question: Is this about improving a SCCR of an industrial control panel or electrical equipment in general such a heavy duty safety switches?

Erik Barnes: The context of this webcast is regarding the industrial control panel for equipment or machinery as described in UL 508A.

Question: Can we use current limiting reactors to reduce the available fault current on the panel?

Barnes: Reactors are not considered in the SCCR determination of the industrial control panel, but they are not prohibited from consideration in determining the available fault current. However, reactors may affect the circuit, components, or loads in other ways. A qualified resource should be consulted when applying reactors.

Question: In a light commercial application would the local utility need to be contacted first to get the available fault current?

Barnes: The fault current on the primary of the transformer in question will affect the fault current determination at the secondary of the transformer. However, if the fault current is unknown, the fault current can still be calculated for the secondary using a formula which assumes infinite fault current on the primary side. Also, keep in mind that the upstream fault current is subject to change.

Question: Do terminal blocks have SCCR?

Barnes: If terminal blocks are applied in the power circuit, then they must be considered in the evaluation. Their assumed value from Table SB4.1 is 10kA, but Eaton offers terminal blocks with SCCRs up to 200kA. See the contact number at the bottom of this Q&A for more information.

Question: I recently came across a packaged air compressor, with a star-delta controller, but no built-in OCPD. The vendor's instructions state that two sets of fuses are to be placed in series in the supply conductors (downstream of the owner's MCCB) in order to meet the SCCR rating of 65kA (before model year 2014) for CSA/UL certified compressors. The source is a 2.5MVA 12.47kV-480V transformer where the maximum let-through current plus 100% motor contribution is below 65kA. Do I really need the fuses? One or both sets?

Barnes: Equipment should be installed in accordance with the manufacturer's instructions. The manufacturer should be consulted regarding questions about installation or ratings for their equipment.

Question: What is the Oscar tool?

Barnes: Bussmann series Oscar 2.1 Online Compliance Software is a web based calculation program that determines the SCCR for industrial control panels. See the Oscar link at the bottom for more information.

Question: Are there coordination issues with SCCR applications?

Barnes: Applying a current-limiting device in the feeder may result in loss of power to all downstream circuits in the event of a short-circuit. Although not required by code to selectively coordinate within an industrial control panel, control panels can be designed to achieve selective coordination between the overcurrent protective devices inside the control panel. One simple method is to select overcurrent protective devices using the selectivity ratio tables found in the SPD Electrical protection handbook (publication number 3002).

Question: Why does slide 32 show a 200kA SCCR rating for panel B? Shouldn't it be 65kA?

Barnes: Correct. That should state 65kA.

Question: Should you use peak current or RMS?

Barnes: The UL 508A standard publishes peak let-through current values for fuses in table SB4.2. This standard also specifies the peak let-through value be applied from the manufacturer's published data when evaluating current-limiting circuit breakers for this analysis.

Question: Why do I have to consider the SCCR rating of, for example, bus bars and not the wiring/conductors that will be used in control panels?

Barnes: UL 508A requires busbar be included in the determination, and assigns a default value of 10kA if the SCCR of the busbar is unknown. Wiring is not considered in the evaluation. Wiring is evaluated according to a separate UL standard.

Question: Are single-phase circuits (control) under the same rules as the three-phase motor circuits?

Barnes: UL 508A requires the power circuit be evaluated when determining SCCR (SB4.2.1). The control circuit is not considered. However, the overcurrent protection on the primary side of a control transformer or power supply must also be evaluated when determining SCCR (SB3.2.1).

Question: Is there any documentation available for SCCR with examples?

Barnes: Yes, there are several application notes and additional information available at www.eaton.com/sccr.

Question: Why are over-current protective devices in branch circuits not adjusted by the let thru currents of feeder fuses?

Barnes: It is unknown how the downstream overcurrent protective device will perform in conditions where the fault current exceeds its interrupting rating, even in conditions where a current-limiting feeder is applied. This can be determined through testing (known as series rating), but is not permitted for consideration within UL 508A. A current limiting feeder will limit the amount of let-thru current but not the rate of current rise that the downstream device experiences. This could result in the inability to safely open the circuit. Also, during a short-circuit event it is possible that motors in adjacent branch circuits will contribute to the short circuit, bypassing the upstream current-limiting feeder. There may be other factors that may also impact the outcome of such an arrangement.

Question: How does current limiting devices work with arc flash mitigation (ARM) devices?

Barnes: Current-limiting devices should be considered in the evaluation for arc flash. However, the presence of an arc flash mitigation device does not replace the need for sufficient SCCR or sufficient interrupting ratings on overcurrent protective devices. They both require independent evaluation and consideration A control panel with insufficient SCCR may still expose hazards to personnel or equipment even if an arc flash solution is in place.

Question: For single-phase control panels consisting only of electronics such as PLCs and relays, is the incoming power from a CB panel still considered a feeder-circuit? Does it end at the first overcurrent protective device?

Barnes: UL 508A requires the power circuit be evaluated when determining SCCR (SB4.2.1). A power circuit can be identified by determining if the circuit in question leads to an external load. However, the overcurrent protection on the primary side of a control transformer or power supply must also be evaluated when determining SCCR (SB3.2.1).

Question: We have different class circuit breakers with different interruption ratings. Is that the same as current limiting?

Barnes: No, interrupting ratings do not indicate current-limitation. Circuit breakers that are certified as a current-limiting circuit breaker are marked as such, and their respective peak let-through current can be found in data provided by the manufacturer. Per UL 508A, only circuit breakers marked as current limiting can be applied to raise a branch component SCCR.

Question: Does this concept applies to variable frequency drive (VFD) applications with SCCR specs for mounting VFD's on the wall without mounting in an additional metallic enclosure?

Barnes: Equipment should be installed in accordance with the manufacturer's instructions. The manufacturer should be consulted regarding questions about installation or ratings for their equipment.

Question: Is an infinite bus assumed, or can one use the utility report fault current into the utility transformer?

Barnes: Eaton's Bussmann series Available Fault Current Calulator (FC2) tool allows for either infinite current or a known current be applied to the primary. FC2 is free to download for mobile devices.

Question: Why can't current limiting devices raise the SCCR of other overcurrent devices downstream of the current limiting device?

Barnes: It is unknown how the downstream overcurrent protective device will perform in conditions where the fault current exceeds its interrupting rating, even in conditions where a current-limiting feeder is applied. This can be determined through testing (known as series rating), but is not permitted for consideration within UL 508A. A current limiting feeder will limit the amount of let-thru current but not the rate of current rise that the downstream device experiences. This could result in the inability to safely open the circuit.

Also, during a short-circuit event it is possible that motors in adjacent branch circuits will contribute to the short circuit, bypassing the upstream current-limiting feeder. There may be other factors that may also impact the outcome of such an arrangement.

Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

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