A critical aspect of our panel design service is making sure control panels have an accurate short circuit current rating (SCCR). Any UL listed control panel will indicate the SCCR on the nameplate. By knowing what the available fault currents are when making electrical additions to a facility, it is possible to proactively coordinate design requirements with project teams, allowing facility owners to mitigate the risk of purchasing underrated equipment.
Determining the SCCR on a custom control panel is typically coordinated between the panel designer and power engineers before panel design starts. Knowing the available incident energy enables the panel designer to proactively select and use components such that the SCCR of the panel will meet or exceed the required levels. Short circuit ratings are determined by performing an arc flash study on the facility’s power system to identify the incident energy available where the equipment would be installed. According to the NEC, control panels can be evaluated as an assembly by a third party or have their SCCR calculated per an approved standard such as UL508a Standard Supplement SB.
The UL508a standard now requires listed custom panel shops to have at least one Manufacturers Technical Representative (MTR) on staff at each fabrication facility. This certification process includes completing UL training and an exam on the understanding of the UL standard. With MTR certified individuals on Interstates’ control panel design and construction teams, we meet the UL requirements and ensure that we understand and apply the UL508a standard to our panel designs.
For mass-manufactured control panels designed to be repeatable, off-the-shelf items, the cost is a consideration for component selection. Unfortunately, components are chosen for their lower price point likely also have a lower short circuit rating than higher-priced counterparts. When the resulting panel design is evaluated and labeled with a SCCR, it may be technically correct but very likely below what is required by a facility.
Unless specific requirements were given during the quotation and purchasing process, a facility might very well end up with an underrated panel arriving onsite for installation.
It is common to see 10kA ratings and even lower because this information was not given as a part of the specifications. When an underrated panel or piece of equipment is found, there are two typical scenarios a project team may pursue:
- Seek engineering recommendations for changes to the power distribution to lower the incident energy available. This could mean adding a transformer ahead of the installation point.
- Purchase a new panel from the supplier that is higher rated.
Short circuit ratings are a detailed piece of information that are not always included when procuring systems with electrical components. Our experienced professionals at Interstates can consult with you to fully understand your installations needs and help coordinate solutions that are properly rated for your facility and help mitigate what could be costly remediations if an underrated panel was supplied.
Jamie Schmidt, P.E., Engineering Manager
This blog was originally published in the Current Connections Winter 2021 issue.