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Short Circuit Current Rating (SCCR) and how does it apply to AC-DC Power Supplies and EMC/EMI filters?

Monday, May 20, 2024

The SCCR (Short Circuit Current Rating) of industrial control panels, machinery electrical panels and some motor control panels is required to comply with UL508A. Equipment must be evaluated and marked with a SCCR rating that exceeds the available fault current at the installation point. Failure to do this can endanger the safety of workers or cause damage to equipment in the vicinity during a large current overload. Note the SCCR is different from the interrupting ratings of fuses, breakers and other over-current protection devices.

TDK-Lambda does occasionally receive requests for an SCCR value for our AC-DC power supplies (Figure 1). As these power supplies are certified by the relevant safety bodies (UL, TUV etc), the safety report will note the testing conditions. This would include a statement detailing the current rating of a protective device as part of the building installation (usually 20A). That external protective device will be part of the SCCR for the end equipment assessment and hence our AC-DC power supplies will not have a short circuit current rating. Normally in the event of an internal power supply fault, the power supply’s internal AC fuse would open first, isolating it from the equipment’s input voltage.

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Figure 1: TDK-Lambda DRB DIN rail mounted filters

To reduce conducted EMI (Electro Magnetic Interference), or to minimize high voltage noise pulses, one or more EMI/EMC filters may be fitted in the equipment. As these single or three phase filters are placed directly in the AC supply line, they are required to be tested to determine their SCCR.

Many of TDK-Lambda’s R series of single and three-phase filters (Figure 2) recently were tested to UL 508 18th Edition Clause 53 (Standard Fault Short Circuit Testing) and 54 (High Fault Short Circuit Testing). As most basic EMI filters do not have internal protection, UL again tests with external branch circuit protection. In the case of many of these filters, an inverse time circuit breaker was used. This type of breaker has a time delay to avoid nuisance tripping and that delay decreases as the current level increases. These R series filters were certified by UL with SCCRs of up to 35,000A (35kA).

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Figure 2: TDK-Lambda’s R series single and 3-phase EMI/EMC filters

The R series encompasses a large range of EMI/EMC filters for multiple applications and uses with current ratings from 0.5A to 300A.

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