Here are some common questions regarding current-limiting fuses, hazard analysis and safe work practices, along with answers provided by Ferraz Shawmut engineers. Have a question you don't see answered? E-mail us or call Technical Services.

What is the reliability of overcurrent protection over time?

I've heard that under certain conditions, current-limiting fuses can allow for high incident energies when the fault current is low. What can we do in these cases?

For arc flash calculations, are fuses from various manufacturers interchangeable as long as the UL class stays the same?

It seems like selectivity is always lost when current-limiting fuses are used. True?

During a flash hazard analysis, is it necessary to calculate arc fault current when using current-limiting fuses?

When using a current-limiting fuse, how can you calculate the arc flash current when you know the short circuit current?

We have a small organization with various 480V at 600 MCCs. Is it possible to perform our own study or is do we need to enlist an engineering company?

Are arc flash calculations required for control panels containing only finger-safe terminals and insulated cable?

While checking for "no voltage" after deenergizing a circuit, do the requirements for PPE still apply?

What is required of infrared thermographers who do not open or touch equipment, but only view the equipment through an IR imager (camera) after the panels have been opened?

What is the reliability of overcurrent protection over time?
Amp-Trap 2000® fuses can be expected to provide reliable current-limiting performance for over 20 years if they are not physically damaged or continually utilized above their ampere rating. Since the current-limiting action of a fuse requires a fixed mass of copper or silver to melt, the fuse's speed of response is not affected with the passage of time. Consequently, there's no need for the periodic recalibration that might be required for an electromechanical device.

I've heard that under certain conditions, current-limiting fuses can allow for high incident energies when the fault current is low. What can we do in these cases?
Tests have shown that under most conditions, Class J, Class RK1 and Class T fuses can limit incident energies to low levels at typical working distances. The key to acceptable arc flash mitigation is to apply the fuse where the expected level of arc fault current will be large enough to drive the fuse into current-limiting mode.

Many times, the switch from Class RK5 fuses to Class RK1 or Class J fuses addresses the issue of lower fault currents. Going to lower ampere ratings where load currents allow can also address these issues. For applications where the expected arc fault current would be lower than current-limiting threshold of these fuses, consider the Ferraz Shawmut's recently introduced Amp-Trap® High-Speed J fuse.

For arc flash calculations, are fuses from various manufacturers interchangeable as long as the UL class stays the same?
For most applications this is true. However, there may be cases where certain brands may have superior performance compared to other fuses of the same class. An example is the Amp-Trap® High-Speed J.

Originally designed to provide protection for semiconductors in soft-starters, the High-Speed J has a low current-limiting threshold that provides a wider range of low arc flash energy protection. Plus, it's usually easier to ensure fuse coordination when you standardize on one brand of fuses.

It seems like selectivity is always lost when current-limiting fuses are used. True?
False. In fact, quite the opposite is true. Amp-Trap 2000® fuses will be selective for all levels of overcurrents, as long as the ampere rating of the upstream Amp-Trap 2000 fuse is twice that of the downstream fuse.

During a flash hazard analysis, is it necessary to calculate arc fault current when using current-limiting fuses?
When using Amp-Trap 2000® fuses, it's possible to perform the analysis knowing only the bolted-fault current, if that current is above the minimum shown in the incident energy charts. These charts are based on a model that was tested extensively in our lab under conditions believed to yield worst-case results. Be sure to read the criteria carefully to ensure that these charts apply to your applications. To get projected energies, use our online Arc Flash Calculator.

More about arc flash energies
Go to Arc Flash Calculator

When using a current-limiting fuse, how can you calculate the arc flash current when you know the short circuit current?
Arc fault currents can be estimated through various models, such as the method identified in IEEE 1584, Guide for Performing Arc Flash Calculations. This would be necessary if the calculated bolted-fault current is lower than the minimum bolted-fault current in the incident energy charts provided for the current-limiting fuse.

Download our white paper, "Improved Method for Arc Flash Hazard Analysis"
Go to Arc Flash Calculator

We have a small organization with various 480V at 600 MCCs. Is it possible to perform our own study or do we need to enlist an engineering company?
Although it is certainly possible to perform your study using tools such as those provided with IEEE 1584, Guide for Performing Arc Flash Calculations, there may be benefits in working with an engineering firm. They include the extra manpower, the availability of expensive software tools, better familiarity with standards and regulations, and improved efficiency.

Are arc flash calculations required for control panels containing only finger-safe terminals and insulated cable?
At this time, most experts recommend calculations for such panels because of field experiences. Although IP20 grade finger-safe devices do minimize the probability of incidental contact, they are likely to be compromised in the presence of an arc flash explosion. If there is a possibility of an arc being established, you will need to take the steps necessary to protect workers from the potential hazard.

While checking for "no voltage" after deenergizing a circuit, do the requirements for PPE still apply?
Yes. Until it is proven that the equipment is electrically safe, workers must use the PPE that would be appropriate for the task if the equipment were in an energized state. Errors in one-line diagrams, labeling errors, opening the wrong disconnect device, and back-fed equipment are some of the situations where equipment could still be energized while workers are doing the verification. Without the proper PPE, workers could be seriously injured.

More about OSHA regulations
More about NFPA 70E

What is required of infrared thermographers who do not open or touch equipment, but only view the equipment through an IR imager (camera) after the panels have been opened?
If thermographers go within the safety boundaries delineated in NFPA 70E, they need to comply with the appropriate guidelines. If, for example, they need to operate within the flash-protection boundary, they need to wear the PPE and fire-resistive clothing dictated by the calculations at their closest working distance.