E-Rated Transformer Protection Fuses

It is a melting characteristic defined by standard, not a manufacturer designation. Under ANSI/IEEE C37.46 an E-rated fuse of 100 A or below must melt within 300 seconds at an RMS current between 200% and 240% of its continuous current rating; above 100 A it must melt within 600 seconds at between 240% and 264%. Because the criterion is defined rather than proprietary, an E rating means the same thing whoever made the fuse.

That is a separate question from the E rating, and ANSI C37.40 defines the three classes by how far down the current range the fuse can be relied on to interrupt. A full-range fuse interrupts any current that will melt its element. A general-purpose fuse interrupts down to the current that melts the element in one hour, so a lower overcurrent needs another device. A back-up fuse interrupts only down to a stated minimum interrupting current and must have a low-current device in series with it. Which class applies to a specific DENCO part is confirmed by DENCO against that part.

An expulsion fuse interrupts using the gases its own arc produces. It limits how long the overcurrent lasts but not how large the fault current becomes. A current-limiting fuse melts and abruptly introduces a high resistance, cutting both the magnitude and the duration of the current before interrupting it — but only while the current is inside its specified current-limiting range. Replacing an expulsion cutout link with a current-limiting design changes the let-through energy the rest of the installation sees, so it should be assessed rather than assumed.

The E rating fixes the melting characteristic and nothing else. Voltage rating, interrupting rating, barrel count, overall length, clip or bolt centres, and any indicator or microswitch requirement all still have to match, and a fuse that physically fits is not necessarily electrically equivalent. Send us the part number or a photograph of the existing fuse and its mounting and we will confirm against it rather than answer from a table.