Current-Limiting vs Expulsion Capacitor Fuses

Two ways to interrupt current

An IEC-oriented fuse technical guide puts the difference in one line: expulsion fuse-links break the current by exhausting gases out of the fuse area, whereas current-limiting fuses use the effect of fuse-element evaporation in loose materials.

One manufacturer's capacitor fuse selection guide walks through the expulsion sequence. The link melts, the arc contacts the fiber tube lining and generates pressure, and the link is cooled and stretched as it is forced out of the tube. The fuse continues to conduct until a natural current zero. Parallel stored energy is absorbed in the fuse operation and the failed unit, and most of it is absorbed in the failed capacitor.

A current-limiting fuse acts within the fault itself. Manufacturer literature describes current-limiting fuses as able to limit system fault currents, frequently to a fraction of the system's fault capability; the reduced peak is the let-through current. The same literature describes such fuses as noiseless and expelling no hot gases or burning particles while interrupting. The same selection guide distinguishes two current-limiting designs: a back-voltage design that absorbs a large portion of the energy itself, and a current-chopping design that leaves trapped voltage on the other units.

Parallel stored energy decides a lot

When a unit fails, its parallel group discharges into it. The selection guide calculates that energy at 1.1 times the crest of rated voltage, 3.19 joules per kvar of parallel capacitance, and states its application rule: if parallel energy is above 20 kJ or 6000 kvar, it applies current-limiting fuses; if parallel energy is below 20 kJ and the available fault current is within its expulsion fuse's rating, it applies its expulsion fuse. That expulsion fuse has a stated capability of 30,000 joules but is not normally applied above 20,000 joules because of the possibility of capacitor case rupture.

Other sources give other numbers. A protection paper gives 15 kJ for all-film and 10 kJ for paper or paper-film dielectric at peak maximum voltage for externally fused banks. PJM's shunt capacitor guideline says less than 4,650 kvar of effective capacitance at nominal voltage, 15,000 watt-seconds, should generally be connected in parallel so the expulsion fuse's energy withstand is not exceeded, and notes that 6,300 kvar in parallel has been used successfully since 1993 with standard expulsion fuses. Another maker rates its capacitor expulsion fuse tubes at 20 kJ (50 A tubes) or 30 kJ (80 A tubes) when used with the unit spacing recommended per IEEE Std 18-2012; its 1990 predecessor listed 10,000 J and 15,000 J tubes. Quote whichever figure you use with its source, and do not merge them.

The long uniform-element, current-chopping design described in the selection guide is usable with essentially infinite parallel stored energy, but it cannot be used in inductively limited fault applications and needs several capacitors in parallel to avoid flashover from its high back voltage.

Worked example: when parallel energy pushes toward current-limiting

Take a bank built from 400 kvar units. With 16 units in a parallel group, a failed unit sees the other 15: 15 x 400 = 6,000 kvar, and 6,000 x 3.19 = 19,140 joules. That sits just under the 20 kJ figure in one maker's practice but above PJM's 4,650 kvar guideline and the 15 kJ all-film figure in the protection paper. Depending on which source governs the design, the bank is at or past the point where expulsion fuses are normally applied.

Cutting the group to 8 units gives 7 x 400 = 2,800 kvar, or 8,932 joules, within every limit in the table. But fewer units in parallel means a bigger voltage rise when one fuse opens. For a bank with many series groups the rise approaches P / (P - F), so 8 / 7 = 1.143, a 14 percent rise, against the protection paper's rule of more than 10 units in parallel to keep one open fuse within 10 percent. A relay maker's paper lists the ways out when parallel energy gets too large: more series groups, a double wye, or current-limiting fuses.

Fault current and bank connection

The available fault current a fuse must interrupt depends on how the bank is built. In ungrounded banks or banks with several series groups, faults are capacitively limited, typically to less than two or three times the bank load current. In single series group grounded wye or delta banks, faults are inductively limited and reach the full available system fault current.

The selection guide turns that into application rules for its own fuses. Its expulsion capacitor fuse should be used only on ungrounded wye banks, grounded wye banks with two or more series groups, or single series group grounded wye banks where the available fault current is within its rating. Its current-chopping fuse should not be used on single series group grounded wye or single series group delta banks.

Compare interrupting ratings on the same basis. An IEEE working group tutorial notes that the crest of a symmetrical fault is 1.414 times the rms symmetrical value, while a fully asymmetrical fault may reach about 2.8 times, depending on X/R. DENCO DNX fuses carry a 50kA asymmetrical interrupting rating and DNXC fuses 50kA; a symmetrical rating and an asymmetrical rating are not the same number. One manufacturer's let-through example shows what limitation means in practice: on an 8.3 kV circuit with 20,000 A rms symmetrical available and an unlimited peak of 48,000 A, a 40 A current-limiting fuse lets through 7,800 A peak.

Interrupting and energy ratings from one maker's data

One manufacturer publishes both kinds of capacitor fuse in the same selection guide, which makes the scale difference easy to see. Its expulsion fuses interrupt a few thousand amperes, while its current-limiting classes are rated in tens of kiloamperes and carry stated parallel discharge energy limits.

Expulsion links used for group fusing hit a similar ceiling. One capacitor maker states that the largest links that coordinate with its units' tank rupture curves are 100K and 65T, and only up to 10,000 A symmetrical for its standard and heavy duty units at X/R of 0, falling to 6,500 A at X/R of 15. Above that it points to current-limiting fuses, ungrounded wye operation or moving the rack.

Side-by-side comparison

The table collects the operating differences described above. Use it to narrow the choice, then confirm with the capacitor manufacturer and a protection study.

Combination fuses and typical uses

The choice is not always one or the other. IEEE C37.42-2016 covered expulsion, current-limiting and combination-type external capacitor fuses in its scope. A utility's construction standard for a 600 kvar pole bank built from three 200 kvar, 7.2 kV units specifies an 8.3 kV, 30 A combination expulsion and current-limiting fuse that fits a universal cutout, and notes the utility changed to it from separate expulsion and current-limiting fuses in 2015.

One manufacturer's catalog for a full-range current-limiting capacitor fuse lists where such fuses are used: high fault areas, fusing of individual capacitors, capacitive circuits requiring positive isolation, circuits where let-through current must be restricted, areas where burning debris cannot be tolerated, and places where high noise levels are undesirable. A metal-enclosed bank maker states that each capacitor in its banks is individually fused with current-limiting fuses.

DENCO's catalog describes its C-rated capacitor fuses as current-limiting with silent operation and no discharge, suited to use indoors or outdoors within an enclosure, and lists capacitor and capacitor bank protection, power factor correction equipment, harmonic filtering equipment, induction heating systems, and high power drives and welders as applications.

Full-range or back-up

Current-limiting fuses come in classes that matter for capacitor duty. The IEEE working group tutorial on IEEE C37.48.1 (now superseded) describes a back-up fuse as able to interrupt any current between its rated minimum interrupting current and its maximum, warning that lower currents can lead to the fuse not interrupting. A full-range fuse can interrupt any continuous current that causes the element to melt, at the maximum application temperature, up to its maximum interrupting rating, and does not need another device for overloads or high-impedance faults. DENCO's DNXC is listed as a full-range current-limiting capacitor fuse.

This matters for capacitors because fault current is not always high. In an ungrounded bank or one with several series groups, faults are capacitively limited to about two or three times bank load current, as noted above. A back-up fuse needs a current at or above the minimum interrupting current that its maker specifies and marks on the fuse, so check that value against the lowest fault current the fuse will see, or use a full-range design.

Common mistakes

These come up when expulsion and current-limiting fuses are compared on one number.

• Applying one source's parallel energy limit to another maker's fuse. Each figure belongs to its own source and fuse type.
• Counting only the bank's own units. Adjacent banks on a common bus add parallel energy, as PJM's guideline notes.
• Comparing a symmetrical interrupting rating with an asymmetrical one as if they were the same number.
• Using a current-chopping design on a single series group grounded wye or delta bank, where one maker says it should not be used.
• Fixing a parallel energy problem by removing units from the group without rechecking the overvoltage on survivors after one fuse opens.
• Choosing a back-up current-limiting fuse for a bank whose faults are capacitively limited, without checking its marked minimum interrupting current against the lowest fault current.

Frequently asked questions