An expulsion fuse cutout combines a fuse link inside an insulating tube with a hinged holder and fixed contacts. In the illustrated fault-clearing sequence, the link melts and an internal arc is interrupted before the tube drops open. The calculator compares nominal single-phase transformer current with an entered fuse rating; it does not select a fuse or predict a trip.
Electric Fusible Cut-out Interactive Calculator
Compare nominal single-phase transformer current with an entered fuse-link rating. Watch a separate, illustrative internal interruption and tube-dropout sequence.
Equation Used
- Single-phase apparent power and winding voltage.
- Fuse-link rating is an entered comparison value.
- No fault-current, time-current or coordination model.
- Illustrative tube proportions and replay timing.
Nominal load comparison only. The independent animation is not a predicted trip or automatic reclosure.
A fixed insulator and a hinged fuse tube
The ribbed insulator supports separate upper and lower contact assemblies. The tube is latched at the top and retained by the lower hinge. Metal ferrules and a lifting ring are shown on the moving tube; the mounting bracket and contact supports stay fixed.
When a fault causes the link to melt, current can continue briefly through an arc inside the tube. In an expulsion fuse, interaction with the liner helps extinguish that arc. The illustrated operating sequence separates internal interruption from the subsequent mechanical dropout.
The enlarged section shows the link parting and an internal arc extending before it disappears. Only then does the main tube rotate down about its lower hinge. The replay resets the illustration; a real fuse cutout is not an automatic recloser.
This is a simplified explanation based on manufacturer cutout construction, not a dimensional reproduction of one model. The animation does not model flipper dynamics, arc plasma, fault current or transient recovery voltage.
Compare nominal transformer current
Enter the single-phase transformer apparent-power rating in kVA, the voltage across that primary winding and a fuse-link current rating. The four results show nominal primary current, its percentage of the entered link rating, the arithmetic current difference and any percentage above the rating.
The voltage must be the voltage across the winding under consideration. A three-phase total kVA rating requires a different calculation; this page does not apply the single-phase formula to a three-phase bank.
These arithmetic comparisons are useful for understanding the quantities. They are not a protection-coordination study, an interrupting-capacity check or a recommendation to fit a particular link.
Single-phase current and rating comparison
Nominal primary current is I=1000S/V, with S in kVA and V in volts. The load-to-link ratio is 100I/F percent, where F is the entered link rating in amperes. The arithmetic difference is F−I; a negative result means nominal current exceeds the entered rating.
The amount above the rating is max(0,100I/F−100) percent. It is not a probability of failure or an overload-risk score. Use the percentage only to compare nominal load with the entered current rating.
Fuse operation depends on the actual time-current characteristic and operating conditions. An input combination above 100% does not establish an immediate opening time. Transformer inrush, coordination, available fault current and equipment ratings are outside this calculator.
Example: compare a 25 kVA single-phase winding
With 25 kVA and 7200 V across the primary, the nominal current is 25000/7200=3.4722 A. Against an entered 6 A fuse link, this is 57.87% of the rating, with an arithmetic difference of 2.5278 A and zero percentage above the rating.
If the entered transformer size doubles to 50 kVA at the same voltage, nominal current doubles to 6.9444 A. That is 115.74% of 6 A, a difference of −0.9444 A and 15.74% above the rating. Neither example selects a suitable fuse or predicts its operating time.
What this comparison does not establish
A current rating alone does not describe the fuse’s complete time-current behavior, interruption capability or coordination with other protection. Product selection requires the relevant manufacturer characteristics and system study.
The animation is an illustrative fault sequence regardless of the selected nominal-load values. Its stage durations and tube rotation speed are chosen for visibility, not taken from a clearing-time calculation.
A hanging open tube is a visible mechanical condition; it does not by itself establish that all parts are de-energized. This illustration provides no switching, testing or installation procedure.
Fuse cutout questions
Why does the animation open even when the displayed load is below the fuse rating?
The animation demonstrates an independent fault-clearing sequence. The inputs calculate nominal load, not fault current. The animation does not claim that nominal load caused the operation.
Does the tube opening extinguish the initial fault arc?
The illustrated sequence shows interruption inside the fuse tube first, followed by dropout. The internal interruption and subsequent mechanical dropout are separate stages.
Does a result above 100% mean immediate operation?
No. The actual fuse characteristic and current history determine its response. This calculator supplies neither a trip-time nor a protection rating.
Can this calculate a three-phase transformer bank?
No. It uses single-phase apparent power and voltage across one winding.
Why does the demonstration start again?
It is an autoplay replay of the explanation. It does not imply automatic replacement of a melted link or automatic reclosing.
References
- S&C Type XS Fuse Cutouts descriptive bulletin — fixed supports, tube ferrules, flipper, hinge and dropout after interruption; construction photograph on page 7.
- Eaton: distribution cutouts — insulator, fuse-holder and contact construction.
- Eaton: fundamentals of fuses — expulsion-fuse interruption principles.
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