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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. In the past, manufacturers of these devices provided internal protection in the form of high-speed fuses, which are much more current limiting than conventional branch circuit fuses. However, as drives and soft starters have grown smaller and smaller, the internal fuses have been omitted by starter manufacturers in favor of short-circuit testing to UL standards with external protection. Now, in many cases, drives are shipped without fuses, and it is the responsibility of the installer or owner to provide this protection. 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. UL 508E can be used to verify Type 2 (no damage) protection when protected by a specific current-liming overcurrent protective device.

  1. 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).
  2. 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. If current-limiting branch circuit fuses (such as Class J, T, CC, etc.) are used, then the tests are conducted with special umbrella fuses. Umbrella fuses have energy let-through levels greater than the UL limits for various classes and amp rated fuses. These umbrella fuses have energy let-through levels that are greater than commercially available fuses.
    A drive can be listed and marked for either fuses or circuit breakers or both. Typically, the 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. Some drives will be marked for protection by a specific amp and class fuse (for branch circuit fuses).
  3. Type 2 Protection (no damage) is the best level of protection. With this protection, the drive cannot be damaged, and the unit is tested and marked with a high short-circuit current rating. It must be able to be put into service after the fault has been repaired and the fuses replaced.

A clear understanding of semiconductor device types is needed when considering Type 2 protection (coordination) with variable speed drives. Only silicon-controlled rectifier (SCR), gate turn-off thyristor (GTO) and diode-based devices can achieve Type 2 protection, and it is only possible with properly selected high-speed fuses. Thyristor type devices can effectively share energy equally across the PN junction. They have short-circuited energy withstand levels that are lower than conventional branch circuit fuse let throughs, however, Type 2 protection can be achieved with properly selected high-speed fuses.

Equipment that uses insulated gate bipolar transistors (IGBTs), high frequency devices, cannot presently achieve Type 2 protection levels. IGBTs do not have enough surface area contact with the actual junction to help share energy evenly. IGBTs share energy very well during long duration pulses, but during short duration, high amplitude faults most of the energy is being carried by an individual bonding wire or contact. Current fuse technology cannot effectively protect the bonding wires of IGBT based equipment from overcurrent conditions, and therefore Type 2 no damage protection is not possible. However, current high-speed fuse technology can protect IGBTs from case rupture under short-circuit conditions.

Protecting Drives and Soft Starters

There are two important considerations when selecting protective devices for drives and soft starters:

  1. The device must be able to withstand the starting current and duty cycle of the motor circuit without opening.
  2. The device must be able to clear a fault quickly enough to minimize damage to the drive or soft starter. The melting time current characteristic curve can be used to verify a fuse’s ability to withstand starting currents and duty cycle, while clearing I2t at the available fault current can be used to verify the various levels of protection described earlier.

There are two types of faults that can occur with drives and soft starters – internal faults and external faults. Internal faults are caused by failures of components within the drive or soft starter, such as failure of the switching components (capacitors, SCRs, thyristors, IGBTs, etc.) External faults occur elsewhere in the circuit, such as a motor winding faulting to the grounded case.

Most soft starters utilize either silicon-controlled rectifiers (SCRs) or gate turn-off thyristors (GTOs) for power conversion. These devices depend on high-speed fuses for protection from both internal and external faults. If high-speed fuses are properly selected, Type 2 protection may be achieved.

Modern adjustable speed drives often utilize insulated gate bipolar transistors (IGBTs) as the main switching components. IGBTs have drastically lower energy withstands than SCRs and GTOs, which makes protection of these components very difficult. For external faults, drives using IGBTs incorporate electronic protection that shut off the switching components when fault currents are detected. However, over time, transient voltage surges can lead to the electronics’ inability to shut off the IGBT switching. This can lead to internal faults as the IGBTs fail and rupture. The violent rupture of IGBTs can cause additional faults to adjacent components as a result of the expelling of gases and shrapnel. High-speed fuses may not be able to prevent the IGBT from failing, but properly selected high-speed fuses can prevent the violent rupture of IGBT devices and the resultant additional faults and safety hazard.

Large adjustable speed drives often include internal high-speed fusing in order to protect against rupturing of components. However, small drives (below 200 hp) often do not include internal fusing, so the user must supply protection. With properly sized and applied high-speed fuses, repair, replacement and lost productivity costs will be minimized. Where high-speed fuses are used for protection of adjustable speed drives, they must be provided as an integral part of a single listed assembly incorporating both the protective device and power conversion equipment.

Note: 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.

Complying with the NEC®

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.

Per NEC® 430.124(A), if the adjustable speed drive unit is marked that it includes overload protection, additional overload protection is not required.

NEC® 430.128 states that the disconnecting means for an adjustable speed drive system shall have a rating not less than 115% of the rated input current on the drive unit. This means that the disconnect required in front of each drive unit must be sized in accordance with the drive unit rated input current, not the motor current.

When connecting conductors between the disconnecting means and the drive, NEC® 430.122(A) states that “Circuit conductors supplying power conversion equipment included as part of an adjustable speed drive system shall have an ampacity not less than 125% of the rated input to the power conversion equipment.” This means that the conductors shall be sized to the rated current on the conversion unit nameplate and not the motor rating.