This is a common problem when dealing with older electrical systems. Here's a breakdown of how to approach this situation and what you can do

1. Prioritize Finding the CRN Fuse File:
Continue Searching Your Own Archives

Double-check your own files, older projects, and backups thoroughly. Sometimes these things are hiding in unexpected places.

Colleagues and Industry Contacts

Reach out to colleagues, former colleagues, and others in the electrical engineering community. Someone might have encountered this before or have legacy files. Consider posting on relevant online forums and LinkedIn groups frequented by power system engineers. Specifically mention that you're looking for an SKM database file (.DBF or .DAT files associated with the fuse library).

Electrical Equipment Suppliers

Contact local electrical equipment suppliers who might have carried CEFCO products in the past. They might have old catalogs or even old SKM files used for coordination studies.

SKM Support

Contact SKM technical support directly. Even though the fuse is obsolete, they might have it in their internal database or know of resources to find it. They might suggest alternatives within SKM.

2. Workarounds When the Exact CRN Fuse File is Unavailable



Datasheets and Fuse Characteristics



Focus on Specifications

Even without the SKM file, try to find datasheets for the CEFCO CRN fuse. Look for:

Time-Current Curves (TCC)

This is the most important information. Find a scanned copy if necessary.

Ampere Rating Voltage Rating Interrupting Rating I²t Let-Through Current

This is the energy let through during a fault.

Create a Custom Fuse Curve (If Possible)

SKM might allow you to input a time-current curve manually based on datasheet information. This is time-consuming but can be the most accurate approach. Check the SKM documentation on how to define custom fuse characteristics. You'll need enough points on the TCC to create a smooth curve.

Approximation Using Similar Fuses



Identify Similar Fuse Types

Based on the ampere rating, voltage rating, and general application (e.g., motor protection, general purpose), try to find similar fuses from other manufacturers that are in the SKM library. Consider fuses from Ferraz/Mersen, Bussmann, or Littelfuse.

Choose a Conservative Fuse

Err on the side of caution. If you're unsure, select a fuse with a slower clearing time (further to the right on the TCC) for coordination purposes. This will ensure that the upstream protective devices coordinate properly.

Document Your Reasoning

Clearly document your assumptions and why you chose the substitute fuse in your SKM model documentation. This is crucial for transparency and future reference.

Use a Generic Fuse



Generic Inverse Time Curves

SKM (and other power system software) often have generic fuse models based on inverse time characteristics (e.g., "Generic Slow Blow Fuse"). You can adjust the time constant to approximate the behavior of the CRN fuse. This is less accurate but can be a reasonable starting point, especially if you are mainly interested in coordination with upstream devices.

Coordination Override (Least Preferred)



Manually Check Coordination

If you absolutely cannot find a suitable fuse model, you can manually check coordination. Calculate the fault currents at the point of the fuse and manually compare those currents to the published TCC of the upstream device. This is highly error-prone and should only be used as a last resort.

3. Important Considerations



Obsolescence and Replacement

Consider whether the CRN fuses should be replaced with modern alternatives. This is especially important if you are doing a major upgrade or renovation of the building's electrical system.

Accuracy vs. Effort

There's a trade-off between the accuracy of your model and the time you spend trying to find the exact fuse file. If the CRN fuse is a relatively small part of the overall system, a reasonable approximation might be sufficient. However, if it's critical to the protection of sensitive equipment, you'll need to put in more effort to find accurate data.

Documentation is Key

Whatever approach you take, document your methodology, assumptions, and any limitations in your SKM model. This is essential for understanding the results and making informed decisions about the electrical system.

Code Compliance

Ensure that any changes you make to the electrical system comply with applicable electrical codes and standards (e.g., NEC).

Example of How to Approximate using Similar Fuses


Research the CRN Fuse

Let's say you find a datasheet (or a partial datasheet) stating the CRN fuse is a 100A, 600V, Class H fuse, typically used for general-purpose circuit protection.2.

Look for Similar Fuses

In SKM, search for Class H fuses from manufacturers like Bussmann or Littelfuse. Pay attention to the available amp ratings and voltage ratings.3.

Compare TCCs

If possible, get a TCC for the CRN fuse (even a hand-drawn one). Compare it to the TCCs of the candidate fuses in SKM. Try to find a fuse with a similar time-current characteristic.4.

Document: In your SKM model, clearly state that you are using a Bussmann (or Littelfuse) Class H fuse as a substitute for the CEFCO CRN fuse and provide the reason for your choice.By following these steps, you can increase your chances of finding the CRN fuse file or developing a reasonable workaround for your SKM model. Good luck! Flag for review