Fuse sizing 3

[buzz53]

Do fuses have 75% or 80% ratings? For E-rated fuses,a rule of thumb for fuse sizing can be 1.25% to 1.33% of rated primary full load amps of the transformer or can be as high as 600% based on secondary protection of the transformer. So if 100E fuses are used and it sees a spike of 75A off and on. Does this weaken the fuse element and fuse may open on the next spike?
I have read the Bussman fuse reference but it does not address my question of weaken fuse elements if current were to reach 75% of fuse rating. Bussman does go into melting minium points and maxium clearing time curves.

Is there any other fuse reference material?

 

[mykh]

Switch operations definitely deteorate fuse element by changing it's resistance caused by mechanical stress endured during on/off cycles. As an example, typical 100A rated fuse is capable to handle no more than 500 motor starts. Other factors such as fuse ambient temperature, start duration etc should also be considered if more accurate estimate is required. Ask fuse manufacturer for characteristic showing number of expected cycles to fuse failure as a function of % of nominal melt current.

 

[alehman]

mykh,
I've sometimes suspected this to be the case, but never seen any published data. Can you point me to a specific source where this type of information is availble?

It seems like there would be a minimum current (as percentage of fuse rating), below which the damage does not occur.

 

[dpc]

Not sure if your application is limited to low voltage, but you might check with Cutler-Hammer. They make both current-limiting and expulsion type medium-voltage fuses and I have found them to be more forthcoming than other fuse suppliers on these types of questions.

My (limited) understanding is that fuse damage is more an issue with current-limiting fuses than with expulsion fuses. But all low-voltage fuses will be current-limiting, probably.

 

[mykh]

alehman,

I used the plot below to estimate number of cycles before fuse operation. It is coming from paper named The Effects Of Cycling Loading On Motor Circuit Fuses issued long time ago by Gould Shawmut company.

The fuse sizes are typically selected so as not to exceed 60 to 65% of the fuse melt current based on the motor starting time. These sizes will give over 100,000 operations.

 

[alehman]

Thanks, mykh. Very intersesting. I will ask Gould for a complete copy of the report.