RES Technical Corner by Brett Eliasz, PE

Technical Corner

For the Electrical article this month we will discuss preloading of medium voltage (MV) fuses and the effects the preloading has on the TCC curves.

Unfortunately, the reference material is too large to attach to this article, but I can provide the links:

S and C TCC curve for standard speed SMU fuse units-TCC 153-2 https://www.sandc.com/globalassets/sac-electric/documents/sharepoint/documents---all-documents/tccnumber-153-2.pdf?dt=637838914218842534

These fuses appear to be able to handle about 200% of their rated current continuously, however if you look below under the BASIS verbiage these fuse curves were based on 77 degrees F and NO INITIAL LOAD.

If you keep reading down to APPLICATION, you will notice another S and C information Bulletin 242-190 located at the link below: https://www.sandc.com/globalassets/sac-electric/documents/sharepoint/documents---all-documents/informationbulletin-242-190.pdf?dt=637201464078109714

- This link provides tables that are to be used to size your fuses and is based on the continuous current.

- We will be using table 7 as these correlate to the specific TCC curve and type of fuse

- These ratings appear to drop (or shift the curves) from the 200% ratings listed in the TCC curves which again had NO INITIAL LOAD… - These tables properly illustrate the characteristics of a PRE-LOADED fuse…meaning this fuse is installed and carrying some degree of current which reduces the heat-dissipating ability which reduces the melting time (shifts the curves)

- In addition to the base continuous current we can see these fuses can handle Daily peaks a little higher than the continuous ratings.

- The emergency values listed should only occur once or twice per year. -

A transformer primary fuse is selected to accommodate-NOT TO INTERRUPT-peak loads.

In conclusion, a 100E fuse should be able to handle 130 amps continuously (table 7) as well as daily and limited emergency peak loads often experienced in a medium voltage system. Solely using the TCC curve tells me the same 100E fuse can handle roughly 200 amps continuously which is NOT how we should be selecting these types of fuses.

The more heat from a pre-loaded fuse and the peak load experiences the fuse has encountered anneals the fuse metals and shifts the curve. If we size per the continuous ratings in the tables, we can ensure the longevity of fuses.

Hopefully this article finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the Rochester Engineer and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

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Brett Eliasz, P.E., LEED AP BD+C , RES Director

APRIL 2022 The ROCHESTER ENGINEER | 11