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Power Electronic Device Circuit Protection

Variable frequency drives, soft starters, and other power electronic devices are becoming increasingly more common in motor circuits. These power electronic devices are much more sensitive to the damaging effects of short-circuit currents and therefore require a level of protection that may not be provided by circuit breakers or conventional fuses. During the design and installation stages, it is important to check the data sheets, label, or manual of the power electronic device to understand the short-circuit protection options. With the proper fuse selection, a safer installation may result, with better power electronic device protection. This can result in more productive operation and higher short-circuit current ratings.

Short-Circuit Testing

UL 61800-5-1, the standard to which drives, and soft starters are listed, provides at least two levels of short-circuit protection. The Standard Fault Current test is mandatory to be listed, and there is an optional High Fault Current test which can be performed during the listing of the device.

The Standard Fault Current tests evaluate the drives at rather low levels of fault current, and significant damage to the drive is permitted – i.e. the drive does not have to be operational after the testing. Examples of the level of fault currents are 5000 amps for 1.5 to 50Hp drives and 10,000 amps for 51 to 200Hp drives. The drive must be marked with the maximum short-circuit current rating (at which it was tested). It does not have to be marked with the type overcurrent protective device if it has followed certain procedures. However, the manufacturer can list the drive with fuse protection only and then the label will be marked to identify that branch-circuit protection shall be provided by fuses only (either high-speed or branch circuit types).

The High Fault Current tests can be at any level of short-circuit current above the standard fault current tests. Significant damage to the drive is permitted – i.e. the drive does not have to be operational after the testing. The drive must be marked with the short-circuit current rating at which it was tested. In addition, it must be marked with the type overcurrent protective device(s) that were used for the test.

Protecting Drives and Soft Starters

A drive can be listed and marked for either fuses or circuit breakers or both. Many drives are marked for protection only by fuses since current-limitation is necessary to meet the requirements set forth in the product standard. If the unit is marked for fuse protection only, then only fuses can be used for protection of that drive unit and the proper type and size must be used (for high-speed fuses). Some drives will be marked for protection by a specific amp and class fuse (for branch circuit fuses).

For protection of small drives (below 200 hp), high-speed Class J fuses (often referred to as Class J drive fuses) are often used since these fuses provide performance similar to that of a high speed fuse but these fuses are a Listed Class J fuse. Unlike traditional high-speed fuses, Class J drive fuses are suitable for branch circuit protection (per the NEC®), and fits in standard Class J fuse clips, holders and disconnects. In addition, these fuses are not required to be provided as an integral part of a single listed assembly.

Traditional high-speed fuses come in many different shapes and sizes. They can be recognized to UL and CSA standard 248-13. This standard does not contain requirements for overload performance or dimensions; therefore, these fuses are not considered branch circuit protection per the NEC®. However, NEC® article 430, which covers motor circuits, does allow high-speed fuses to be used in lieu of branch circuit protection when certain conditions are met.

The use of high-speed fuses (also referred to as semiconductor fuses) for protection of power electronic devices, such as soft starters, in lieu of normal branch circuit overcurrent protective devices is allowed per NEC® 430.52(C)(5), which states that “Semiconductor fuses intended for the protection of electronic devices shall be permitted in lieu of devices listed in Table 430.52, provided that the marking for replacement fuses is provided adjacent to the fuses.”.

Per NEC® 430.130(A)(4), semiconductor fuses are permitted in accordance with the adjustable speed drive manufacturer’s instructions for use as the branch circuit short-circuit and ground-fault protective device for listed power conversion equipment, provided the fuses are an integral part of a single listed assembly incorporating both the protective device and power conversion equipment.