Although short circuit current ratings (SCCRs) for variable frequency drives (VFDs) are widely discussed, many people are still unclear about what these ratings mean and why they are important. SCCRs concern safety issues dealing with prospective short circuit current (PSCC) at the point where a VFD is connected to an electrical power feed.
SCCR refers to the amount of PSCC that a device such as a VFD is rated to withstand. The National Electrical Code® defines the SCCR as, “The prospective fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.” This means that a chassis-mounted VFD or enclosed VFD must not create a shock hazard and must contain and flame, fire or explosion hazard during a short circuit event.
The Standard Technical Panel for UL508C was established in late 2012 to ensure that all VFD manufacturers conduct tests and provide consistent SCCR data to customers across the industry. These clarifications have been written into UL 61800-5-1, which is a new UL standard for variable frequency drives. These standards specify marking information (either on the VFD, itself, or on accompanying documents) and detail how the drive should be installed based on maximum PSCC.
Circuit breakers are often installed with VFDs to meet code requirements for overcurrent protection. These devices open the circuit to stop the flow of the current. However, when the PSCC value at the connection to the VFD is higher than the input rating, a line reactor or DC choke must also be used to limit current and reduce PSCC to meet the VFD rating.
In conclusion, matching the VFD input rating to the PSCC is not enough for proper drive installation. Careful consideration of both the input rating and the containment rating clarified in UL 61800-5-1 is essential. These ratings will impact your choice of overcurrent protection device (such as fuse or circuit breaker), line reactor or DC choke, as well as possible enclosures.
If you are considering a VFD application and want any assistance in working through these selection and installation matters, please contact your local Kaman sales representative.
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