E Rating

[SagDEG]

Does anyone know the origin of the "E" in E Rating? I know Edison invented the first safety fuse, but I don't think the E was established from his name. I don't have the NEMA spec to see if they specify.

Thanks,

SK

 

[alehman]

Are you referring to class E high-voltage fuses?

This is one of several classes. Others are R, K, T and C. These are defined in ANSI C37.42. I'm not sure the "E" is significant of anything other than the time-current characteristics of the fuse.

 

[SagDEG]

So the "E" is just assigned to that rating with no origin of significance. Thanks for the info on ANSI alehman. I'll take a look. Hopefully, I'll find a good definition.

SK

 

[alehman]

I may be wrong, but so far as I know "E" only designates the class. There may be some historical significance of which I'm not aware.

Class E is designed primarily for transformer protection, R is for motors, K and T are mostly used for distribution cutouts, K being relatively faster than T. There is also a class H that is used where high surge currents are encountered.

 

[wbd]

For distribution cutouts there is also a N fuse. I have run into a D fuse that is by Cooper and is for high lightning areas.

 

[stevenal]

I believe the T designation may have come from the tin element originally used in this class. Other materials with different TC characteristics took on other letters which don't seem to relate to their makeup.

But the E class alone does not designate the TC characteristic. S&C Power fuses, all with the E rating, can be obtained in standard, slow, and very slow flavors.

 

[davidbeach]

The E characteristic is defined at long time only. As stevenal mentioned S&C has various speeds; I also understand that others make current limiting E fuses. All are possible as long as the long time requirement is met.

 

[alehman]

The only TC requirements for an E designation are :
For ratings <=100A, melt in 300s for current between 200% and 240% of rating
For ratings >100A, melt in 600s for current between 220% and 260% of rating

 

[davidbeach]

Thanks alehman.

 

[stevenal]

Found it. C37.46 supports alehman's statement with a little rounding. The last number stated in the standard is 264%.