Primary Fuse selection

[Fuselink]

I have posted three different scenarios which a primary fuse can be chosen for a transformer. I know that the secondary CBK will need to protect the overload portion of the XFR and primary S&C fuse (SM5-Slow Speed) will take care of the fault through currents. However choosing a right fuse will determine the short time delay and long time delay settings of the main breaker.
The curves were originated by S&C free software (Coordinaide)
I can see that 40A offers the best protection, however I was told to choose 1.4 to 1.5 times of the primary current for fuse ratings. I appreciate any comments on this.

Thanks

 

[dpc]

It would help if we knew the rating of the transformer. Is that shown somewhere?

 

[Fuselink]

Rating is shown on all graphs.
1500KVA- 27.6KV to 600V
Delta-Wye Grounded

 

[stevenal]

I tend to fuse conservatively. The 40 is above the inrush curve. If it allows the short term overloading you expect, I'd go with it.

Doesn't look like the 1.4 to 1.5 range will hit a standard size. Gotta round either up to 50 or down to 40. 65 is clearly too high.

 

[JamesEE]

Fuselink points out that secondary overcurrent protection will be provided; therefore the primary fuse is applied for (1) primary faults (as primary protection), (2) for secondary faults (as backup protection to secondary overcurrent protection), and (3) for secondary faults from the secondary windings to the first secondary overcurrent device (as primary protection). Even the 65 amp fuse provides primary line-to-line and three-phase protection through 70 seconds...anything that long is likely a secondary fault. Providing tighter protection (40 amp or 50 amp) is better for transformer protection (and for secondary protection from the secondary transformer windings to the first secondary overcurrent device), but the smaller fuse size provides less reliability for the connected loads (as the fuse may exhibit nuisance melting for a downstream fault or light overloads).

What type of transformer is being protected? If it is a dry type, it has little overload capability, so I'd tend to keep the fuse size lower (say a 40 or 50 amp). If the transformer is liquid insulated, it has substantial short-term overload capability, and if you desire to use that capability, then a 40 amp fuse is too low, so you may want to consider a 50 or 65 amp fuse.

Since secondary overcurrent protection is being provided, then I'd start with a 40 amp fuse and then perform a coordination study with the secondary protection. If it fits well, then your job is done. If you feel you need more time margin between the primary and secondary trip curves you could go up to a 50 or maybe even a 65 amp fuse. Generally, 0.3 sec is the minimum time margin you'd want between the primary minimum melt, and the maximum clearing of the secondary breaker (at the maximum secondary fault current).

What this boils down to is that I'd suggest that the settings of the secondary protective device will steer you to the proper fuse selection for the primary windings. The secondary protection setting is determined by the transformer type (dry or liquid), your desire for the use of any overload capability within the transformer and your protection & reliability philosophies. Don't forget to coordinate the primary transformer fuse with the upstream protection...this may be your limiting factor unless the upstream protection is set very high.