Fuse sizing

[pfalcon]

I am looking for some confirmation or refutation on general fuse sizing.

Looking at the melting curves on several types of fuses, the amp rating is a current that will not melt the fuse in under 1000 seconds (top of chart) and presumably or theoretically never will.

Off the cuff and short details that would imply a 100 amp fuse on a 100 amp transformer ought to be sufficient. Table 450.3(B) permits a 250 amp fuse with secondary protection.

Through the hard knocks school of training and subsequent research the 250 amp permissable is not for the purpose of handling the inrush current. Inrush takes the appropriate fuse type not just the appropriate fuse current rating. That it is not inrush is also supported by the 125 amp restriction if we qualify to dump the secondary protection.

So, after long winded wind-up, what is the basis for the 250% (or 125%) permissable fusing? Our desire to be able to overdrive equipment? Our lack of confidence in line/load quality? Quit asking cause it works?

 

[davidr43229]

So, after long winded wind-up, what is the basis for the 250% (or 125%) permissable fusing

I read your post 3 times and will try to answer this for you. NEC 450.3 does allow for 250% on the primary and 125% on the secondary as maximums for transformers 9 amps or greater. These values are stated to prevent fires in an overload situation.
The RK-5's have the longest time delay for your use.
http://www.bussmann.com/pdf/814df254-27f9-4f1d-a922-2478ce169e70.PDF

Just my $.02

 

[jim dungar]

Transformers effectively need to be protected at two points, full load and inrush.

The NEC requires that the transformer be protected by protective devices sized at no more than 125% of FLA. This must be done with the primary device.

However, the transformer inrush current needs to be considered and the primary fused selected by type or size to accomodate the inrush. If a 125% FLA fuse can not handle the inrush of the transformer it is permissable to increase this fuse up to 250% AS LONG AS a secondary protective device sized at 125% FLA is provided.

Fuse curves are published based on open air. Enclosed fuses must be derated to 80% just like circuit breakers.

 

[pfalcon]

I cannot find the reference for derating fuses that are enclosed. Please forward the reference.

Barring the derating for temperature the Bussman site recommends fusing for a 4:4 ratio as opposed to the 5:4 ratio from the NEC. ATM I can only speculate that the 80% rule is being used to set the table limits. Probably from field experience that 4:4 has too many nuisance trips. I can find no source for justifying the 10:4 ratio when secondary fusing is used.

I can apply the code but I'm kinda a "WHY" person. I hate doing something without knowing the underlying basis.

 

[jim dungar]

I cannot find the reference for derating fuses that are enclosed. Please forward the reference.

Barring the derating for temperature the Bussman site recommends fusing for a 4:4 ratio as opposed to the 5:4 ratio from the NEC. ATM I can only speculate that the 80% rule is being used to set the table limits. Probably from field experience that 4:4 has too many nuisance trips. I can find no source for justifying the 10:4 ratio when secondary fusing is used.

I can apply the code but I'm kinda a "WHY" person. I hate doing something without knowing the underlying basis.

The 80% comes from testing (i.e. UL) procedures for enclosed protective devices. There is actually no derating of devices rather they chosen based on 125% of the continuous load. Look at Bussmann's SPD where they discuss fuse ratings, they say to use the 125% number except when the fuse/switch combination has been approved for 100%.

When transformer primary and secondary protection is provided: the transformer is protected against overcurrent by virtue of the secondary device not exceeding 125% FLA; the transformer is protected against short circuits (internal faults) due the the primary device being limited to 250%.

While I regularly use Bussmann's SPD as a reference, it is not the only source for fuse information. I also like the Advisor Guide from Ferraz-Shawmut.

 

[brian john]

Through the hard knocks school of training and subsequent research the 250 amp permissible is not for the purpose of handling the inrush current. Inrush takes the appropriate fuse type not just the appropriate fuse current rating. That it is not inrush is also supported by the 125 amp restriction if we qualify to dump the secondary protection.

Speaking from field experience, and at this time I could not quote the values of current..BUT it is possible to have transformer inrush currents that exceed the 125% rating.

This generally occurs where the transformer has a high K rating, ie. not your standard off the shelf transformer, the ATS is a fast transfer switch like an ASCO ATS (no center off position delayed transfer), both sources are available and the customer did not specify an in-phase-monitor as an option.

 

[pfalcon]

Thanks Jim. That clears a lot up and gives me a good resource to study.

A couple years ago I was moved laterally inside Megacorp to handle transformer problems on machine tools. The event that triggered it was a machine tool that blew primary transformer fuses. It only worked if the fuses were brand new and the machine had been sitting off for awhile. The vendor was here and assured us we were using identical fuses and that it worked just fine at their site. The ELs and Engrs on site were stumped.

Bottom line: CC fuses handle 20:1 inrush while CC-J fuses handle 85:1 inrush. Transformers depending on who you ask pull from 50:1 to 85:1 inrush. The vendor had lied about how it worked at his site. Once they had it running they wired the rest hot.

 

[jim dungar]

A couple years ago I was moved laterally inside Megacorp to handle transformer problems on machine tools. The event that triggered it was a machine tool that blew primary transformer fuses.

Control power transformers (CPT) often have inrush currents of 14x up to 20x FLA versus normal power transfromers which are typically less than 10x. Also the difference between "cold" and "warm" starting is more pronounced in CPTS. These are part of the reason that the NEC has different selection allowances for transformers rated <9A. In general you will have problems if you try to protect a CPT using the same procedures as with power transformers.