Streamlining panel design equals higher SCCR

—
Sean Ghoddoucy, Segment Manager, National Alliance Accounts, ABB
Daniel Lightsey, ABB Ability(TM), Smart Power

Panels with higher AIC / SCCR ratings provide superior safety, helping protect employees and equipment while avoiding production losses and scrap. OEM machine builders and panel builders can pursue several paths to achieve those higher ratings.

Streamlining panel design equals higher SCCR
“Safety first” is a slogan prominently posted in many industrial facilities. But increasing safety usually comes at a cost. The cost/benefit equation must therefore always be considered to determine what safety investments will provide the greatest payoff.

One of the ways electrical power distribution and control panel builders seek to increase safety – both for connected equipment and operators – is to increase the Ampere Interrupting Capacity (AIC) or Short Circuit Current Rating (SCCR) of the panels. This can be an excellent approach to helping enhance safety with a favorable cost/benefit ratio.

Creating enhanced panel safety through higher fault withstand ratings is particularly important in a variety of applications, including facilities where even a brief power loss due to an irreversible damage caused by a high current fault, would result in high scrap production. The loss of a power distribution panel in a semiconductor production facility, for example, would be extremely expensive.

AIC versus SCCR
Before exploring options for increasing these ratings, it’s worthwhile to clarify the difference between them. AIC (expressed in kA) relates to individual current-interrupting devices, including protective devices like circuit breakers and fuses. Components are rated for a maximum fault current and what the protective device can handle on the load side without being damaged or destroyed, or creating a safety risk for personnel or equipment. Devices like relays aren’t AIC rated because they don’t provide overcurrent protection.

SCCR, on the other hand, is a rating of components or combination of components in a protective sub-unit or system. This rating, also expressed in kA, indicates the ability to safely handle a specific current for a specific time, or until a protective device opens the circuit and clears the fault. It’s more than withstand, which means the breaker can see the fault, open the circuit, and not cause a catastrophic event. It’s the ability to safely interrupt the fault to reduce the energy flowing downstream. This rating is a case of the chain only being as strong as the weakest link; the panel rating is equal to the lowest-rated overcurrent protective device in the power distribution circuit of the panel.

Design for higher ratings
Too many panel builders rely on past designs as the starting point for current panels, an approach that works against increased ratings. Instead, a better approach is to begin with a blank slate with an eye to achieving the highest rating possible. Review the power distribution scheme to identify and eliminate components that limit your ratings.

Remove weak links
As part of your refocused design efforts, root out the weakest links. It’s often convenient to add terminal blocks or multi-connections to a panel to simplify wiring. But that untested, unrated connection point becomes the weak point in the panel design, and could result in a panel rated at a mere 1 kA. Eliminating these terminal blocks, and directly wiring from component to component, can provide a significant, and easily attained, ratings boost.

Select tested combinations
The best approach when specifying components is to rely on series tested breakers and contactors, and main breakers and feeder breakers. These combinations have been evaluated and certified by standards organizations to provide a proven level of protection. It’s possible for panel builders to have their custom designed panel combinations tested and certified, but this is a very costly and time consuming approach. An additional disadvantage is that if any of the components in the system is changed, the test data is invalidated.

Select higher-rated components
Manufacturers are striving mightily to provide higher-rated components to meet customer expectations. Twenty years ago, 22 kA was an acceptable rating. Today, panel builders who design for machine builders are typically working in the 35 kA and 65 kA range, and sometimes as high as 100 kA. Some devices, like current-limiting circuit breakers, can provide higher ratings as well as added protective features. After sensing the fault, they open in as little as one quarter of a cycle, drastically reducing the overcurrent downstream.

Control circuits
Streamlining the panel design where the power distribution circuits are separated from the control circuits in the panel is another way to reach higher SCCR ratings. The lowest-rated and/or untested devices are typically on the control side. If those components rely on power from the power distribution side, then the rating of the panel can be no higher than the lowest-rated protective device on the control side. Control panel transformers (CPTs), can help resolve this issue by separating the control portion from the power distribution side of the panel.

Add Fuses
Additional fuses are a simple way to increase ratings, and may be the best solution in some applications. The drawback is that they require additional space, cost, and an inventory for spare fuses. In heavy industrial applications, that’s not an issue, but when panel space is tight, finding room for fuse blocks can be a problem.

Build competitive advantage
Higher panel ratings will help enhance safety for both employees and production assets, and help avoid production delays and waste. Increased SCCR provides benefits in every application, although the cost/benefit ratio is higher in some than others.

To increase ratings, panel builders should reevaluate their panel design approach. Doing so can increase SCCR, providing a benefit to customers and end users, consulting engineers and inspectors at a minimal cost. Streamlining panel designs whenever possible by removing low or unrated components and selecting higher-rated devices, including current-limiting circuit-protection devices. Rely on your electrical component providers to provide you with product information and design guidance, including recommendations for series-tested component combinations.

Streamlining panel design and component selection enables creative panel builders to achieve the competitive advantage of higher SCCR in every panel they design and manufacture.