How Machine Design Influences SCCR

How Machine Design Influences SCCR

Learn from how a packaging OEM fortified electrical safety and achieved Short-Circuit Current Rating NEC compliance by using compact circuit protectors applied to servo drives.

By Joe Graveline, senior applications engineer, and Erik Barnes, marketing manager, Bussmann Division, Eaton

Editor's Note: This article is adapted from the white paper, “Solving SCCR,” from Rockwell Automation EncompassTM Product Partner Bussmann Division, Eaton. Download the free, comprehensive white paper that provides educational information about circuit protection and electrical safety, breaks down the SCCR analysis made for the project described in the following article, highlights how the solutions were determined, and demonstrates how proper planning can help achieve SCCR compliance with the NEC®.

Not only does the National Electrical Code (NEC®) require proper equipment and machinery short-circuit protection, it's also critical for safety of personnel and fire prevention. Proper planning in the design phase can avoid challenging jobsite corrections, and often can result in minimal-to-no impact on design, layout and material costs.

Viking Masek, Oostburg, Wisconsin, manufactures custom automated packaging equipment. The company recently had a project that needed a 65,000A Short-Circuit Current Rating (SCCR). Working with Eaton's Bussmann division, a Rockwell Automation EncompassTM Product Partner, an SCCR analysis of their design was performed. They identified several “weak link” components that limited the panel's SCCR to just 5,000A.

These were resolved when Viking Masek made several component substitutions that included using a Compact Circuit Protector (CCP) solution that permitted applying a 200,000A SCCR to several Allen-Bradley® Kinetix® 5500 servo drives from Rockwell Automation. This and other component changes improved the overall panel SCCR to 65,000A without affecting panel layout or material costs.

Many engineers at original equipment manufacturers (OEMs) can find it difficult to achieve SCCR compliance. struggling to determine when and where to apply the current-limiting rules that raise SCCR values. A single mistake can result in an incorrect panel rating.

Initial SCCR analysis revealed “weak links” that must be addressed to achieve the desired 65kA for packaging OEM Viking Masek. [Click to Enlarge]

Engineers also struggle with locating component SCCR information, which makes it difficult to find suitable substitutions to address low SCCR without requiring significant changes to the panel layout or overall material costs.

Another challenge is the limited information about the available short-circuit current, also known as the available fault current, at the location in the electrical system where the equipment is being installed. The NEC® prohibits any equipment installation where the available short-circuit current exceeds its short-circuit current rating (NEC 409.22).

SCCR Analysis

Eaton's Bussmann Application Engineering team provides free SCCR analysis, and Viking Masek' s leaders asked them to help. The Eaton Bussmann team performed an analysis and determined the panel's SCCR was just 5A.

The analysis revealed the following, identifying which components must be addressed to achieve the desired 65kA equipment SCCR value (see illustration):

  1. The miniature circuit breakers/supplementary protectors, rated at 10kA, must be replaced with overcurrent protective devices having an interrupting rating not less than 65kA and with comparable dimensions.

  2. The Kinetix 5500 servos had a 5kA rating as applied. The presence of the current-limiting 30A Class J fuse in the feeder circuit wasn't able to improve the 5kA rating. This required further investigation to resolve.

  3. Although the power circuit components downstream of the power transformer had SCCRs less than 65kA, the 10kVA power transformer raised their ratings to a level sufficient to achieve the desired 65kA equipment SCCR.

Overcurrent Protection Solution

The Eaton Bussmann team researched design solutions to address weak links limiting the panel's SCCR below 65kA, starting with the miniature circuit breakers/supplementary protectors. Considering the size, functionality and costs of these existing components, the Bussmann series Compact Circuit Protector (CCP) solution, with KTK-R Class CC fuses, provided the best solution when compared with a molded case circuit breaker or a fuse and fuse block solution. This component substitution raised the interrupting rating to 200kA without any significant effect on costs or the panel layout.

Next, they looked for a solution for the 5kA SCCR rating determined for the Kinetix servo drives, which have a 200kA SCCR value when applied with a specified overcurrent protective device.

As applied in the existing design, the servos being fed by a single circuit breaker resulted in a 5kA SCCR value. After reviewing the instruction manual, it was determined that a 200kA SCCR value could be applied using specified fuses as the overcurrent protection to the servos grouped by model number. All except the 40A servo drives were fed by UL 98 Listed Compact Circuit Protector disconnect switches using Bussmann series KTK-R Class CC fuses. The 40A servo drives were fed with Bussmann series Low-Peak LPJ-35SP Class J fuses.

The Right Tools

Safety and code compliance is key for all involved with equipment specification, manufacturing, installation and use. Although SCCR compliance might be challenging at times, proper short-circuit protection is a critical safety element of any machine or equipment.

For Viking Masek, its panel's SCCR was improved from 5kA to 65kA with just a few component changes and minimal impact to the design. The company now can send its MS400 equipment to the customer's jobsite and be confident it's suitably designed for fault current levels up to 65kA.

The SCCR analysis validated the panel's enhanced SCCR, component by component, to aid in the equipment's installation and verification during the inspection and approval process.

With the right resources and tools, SCCR compliance can be achieved more easily with minimal-to-no impact on design layout or material costs.

Bussmann Division, Eaton is a participating Encompass Product Partner in the Rockwell Automation PartnerNetwork™. The company supplies fusible circuit protection and electrical safety solutions to protect equipment and provide reliable, efficient power distribution.

The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.

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