Fuse time current curves 2

[Mbrooke]

Why does IEEE Standard C37.41 and IEEE Standard C37.46 require that a fuse's time current curve start at 200% for 100amp and below and 220% over 100amps? Also why are the permitted daily and emergency overload values well below 200%?

https://www.sandc.com/globalassets/...on-bulletin-242-190.pdf?dt=637141600507218968

 

[cuky2000]

Hi Mbrooke,

It recognizes implicitly that any fuse does not provide protection in the overload range. However, the IEEE standard sets a benchmark for the E-rated fuse melting time-current characteristics in the range of small overloads of the transformer. E-rated fuses are not intended to provide overload protection. They should only be applied in situations where it will not be required to open small overcurrents.
Below is a summary table with the characteristics of the E-rated fuses that I hope helps to provide some light to your post.

......

 

[Mbrooke]

It does. But why mandate 200 and 220%? Why not leave the discretion to the manufacturers?

 

[davidbeach]

So that E-rated fuses are somewhat similar. The higher current/faster tripping portion of the curves vary, but for continuous ratings it’s not necessary to know which 200E, just that it’s a 200E.

 

[cuky2000]

Probably the reason for that is for making the selection of MV application easy and allow code and standard to develop practical application rules.

It goes back to the applications for which the fuses are intended to allow for:
1) The fuse’s continuous current rating must be able to withstand transformer overloading and emergency operation and meet the NEC requirements.
2) To provide moderate overload protection and allow start under cold load pickup conditions without tripping the fuse.

 

[Mbrooke]

Ok- but I thought the NFPA/UL had no influence over the IEEE?

 

[cuky2000]

Although those organizations are independents, they providing application guides in the same marketplace for the same subject matter. The physics of the phenomenon drive the results that should be with reasonable similarity.

 

[Mbrooke]

@Cuky: Question- is this your site?

https://www.etechnog.com/2018/10/how-hair-dryer-works-hair-dryer-circuit-diagram.html

I like it, much respect!

 

[cuky2000]

Hi Mbrooke,

Sorry, this is not my site.
BTW, I like it too.

Regards

 

[Mbrooke]

You sure about that? Either way, I always enjoy seeing your personal notes.

 

[Mbrooke]

I just realized HPA type PF, PX and PM fuses start at around 110%, while PE fuses aligned with the IEEE start at 220%. My question being, why does Southern States start their fuses at 110% and not follow the IEEE?

 

[cuky2000]

The SS fuses PF, PX and PM are type N fuses (no E-fuses). Therefore, they meet the continuous current range per IEEE Std. See the excerpt below from the fuse manufacturer.

 

[Mbrooke]

I don't think I've ever been this blown away by a reply, but... I just was! Star Given. Go you!

So- I want to ask- why are there N fuses and why are there E fuses? Where would an N fused be applied? Why does S&C shy away from them when it comes to trafo protection?

 

[cuky2000]

Type N fuses conform to previous NEMA standards and have been superseded by Type K and T links in ratings of 5 through 200A for use in cutouts rated through 27kV.

SUMMARY:
[li]Type K fuses: Fast links within a nominal speed ratio of 7 are suitable for transformer & capacitor protection[/li].
[li]Type T fuses: Slow links within a nominal speed ratio of 12 are suitable for line protection.[/li]

 

[Mbrooke]

What about power fuses? Is type N obsolete for those applications?

 

[Mbrooke]

Ok, now I'm even more confused... why do these N curves start at 200%?

https://www.sandc.com/globalassets/...ts/tcc-number-167-6.pdf?dt=637191164616906426

I've always thought it was good practice to start links at 200%, however, I can't figure out why SS would offer 161kv fuses starting at 115% ish even for type N.