Capacitor Fuse Glossary: Terms for Capacitor Banks and Fusing

A to C

• Apparent power (kVA): the vector sum, not the arithmetic sum, of real power (kW) and reactive power (kvar), so kVA = sqrt(kW^2 + kvar^2).
• Asymmetrical current: fault current that includes a DC offset. The IEEE fuse working group tutorial notes the crest of a symmetrical fault is 1.414 times its rms value, and the crest of a fully asymmetrical fault may be about double that, depending on circuit X/R.
• Back-to-back switching: energizing a capacitor bank next to a bank that is already energized. The inrush is limited only by the small inductance between the banks, and published application guides put it on the order of tens of kiloamperes at kilohertz frequencies.
• Back-up current-limiting fuse: a fuse that can interrupt any current between its rated minimum interrupting current and its rated maximum interrupting current. Lower currents may not be interrupted, so it is often paired with another device.
• BIL (basic impulse insulation level): the impulse voltage a unit's insulation withstands. One capacitor maker catalogs 2,400 to 14,400 V units at 95 kV BIL and higher-voltage units at 125 or 150 kV BIL.
• C rating: a capacitor fuse designation used in manufacturers' catalogs. No standard defines it. DENCO's catalog defines its DNX C number by melting time: the fuse melts in 1000 seconds at 1.7 to 2.4 times its C number.
• Capacitive reactance (Xc): 1 / (2 pi f C), in ohms. It falls as frequency rises, which is why harmonic currents flow into capacitors. At 60 Hz, Xc = 2653 / C with C in microfarads.
• Capacitor unit: one sealed can of capacitor elements, with its own kvar and voltage rating. Units connected in parallel form a parallel group.
• Case (tank) rupture: bursting of a capacitor case due to gas pressure caused by internal arcing.
• Case rupture curve: a current-time curve published by the capacitor maker for the internal gas pressure limit of a unit. Faults cleared to the left of it carry minimal likelihood of rupture. It cannot be calculated by the engineer setting the protection.
• Clearing (total operating) I2t: the melting I2t plus the arcing I2t of one fuse operation. It is also the let-through I2t that the failed capacitor sees.
• Critical step: in one relay maker's method, the number of failed units at which the surviving units in a group reach the maker's maximum working voltage, generally 1.1 per unit.
• Current-limiting fuse: a fuse that interrupts by the evaporation of its element in a loose filler material. Published manufacturer data describes it limiting fault current, frequently to a fraction of what the system could deliver.

D to F

• Definite-purpose breaker: a circuit breaker rated for back-to-back capacitor switching, with a higher inrush current and frequency limit than a general-purpose breaker.
• Delta bank: a bank with a single series group of units rated at line-to-line voltage, generally used at distribution voltages. With one series group, losing a unit does not overvoltage the others, so unbalance detection is not required for unit loss.
• Dielectric absorption: the property of certain capacitors to recharge after being discharged. It is one reason removed units are kept shorted.
• Discharge device: a resistor, usually internal, that drains residual voltage after disconnection. In the 2023-cycle NEC Article 460 text the target is 50 V within 1 minute at 1000 V and under, and within 5 minutes above 1000 V. Cal/OSHA states it is not a substitute for shorting and grounding.
• Distribution class and power class: two IEEE fuse classes. Power class fuses are designed and tested for feeders and locations in or close to the substation, where X/R is high; distribution class fuses for locations some distance from it.
• Double (split) wye: a bank split into two equal wyes, with neutral current or voltage measured between them. The arrangement is insensitive to system voltage unbalance.
• Duty class: one capacitor maker's grades of unit (standard, heavy and extreme duty) that differ in continuous overvoltage capability and in how far their tank rupture curves extend in current.
• Element: a basic wound section inside a capacitor unit. A film element usually fails short when its foils weld together; in an internally fused unit its own fuse then removes it.
• Expulsion fuse: a fuse that breaks current by exhausting gases out of the fuse area. It conducts until a natural current zero, and most of the parallel stored energy is absorbed in the failed capacitor.
• Externally fused bank: a bank in which each capacitor unit has its own fuse, mounted between the unit and the bank's fuse bus. The blown fuse identifies the failed unit.
• Fixed (unswitched) bank: a bank left permanently connected, sized for the kvar needed at minimum load.
• Full-range current-limiting fuse: a fuse that can interrupt any continuous current between the minimum current that melts its element, at maximum application temperature, and its rated maximum interrupting current. It does not need another device for overloads or high-impedance faults.
• Fuseless bank: a bank with no fuses, built from long series strings so that a shorted element leaves only a small voltage rise on the rest. One relay maker's paper says it is not usually applied below about 34.5 kV.
• Fusing factor (protective margin, fusing ratio): the fuse's continuous current rating divided by the capacitor current. Published values include 1.3 to 1.65 per unit in one manufacturer's guide, 1.35 in several medium-voltage application methods, and 1.25 or less for coordination with one unbalance relay.

G to L

• General-purpose current-limiting fuse: a fuse that can interrupt any fault current between a current that melts it in not less than 1 hour and its rated maximum interrupting current.
• Grounded wye bank: a wye bank with the neutral grounded. It gives a low-impedance path for surge currents and for triplen harmonic currents, and reduces recovery voltage for switching equipment.
• Group fusing: fusing a bank by phase rather than unit by unit. The USDA RUS bulletin notes pole-mounted units are normally fused this way.
• H-bridge: an unbalance scheme with a current transformer connected between two equal strings of the same phase near their midpoint. It is used on large banks with many units in parallel.
• Harmonic filter (tuned bank): a power factor correction capacitor combined with a series iron core reactor; one maker's filters are almost always tuned below the 5th harmonic. IEEE 1531-2020 is the application guide.
• Hot spot: heating at a loose fuse connection that makes a link behave as though heavily preloaded, so a transient that would not normally affect it blows it.
• I2t (ampere-squared seconds): a measure proportional to the energy delivered to a fuse element. One manufacturer's guide says it applies only to operations faster than about 0.1 or 0.01 seconds.
• I2t withstand (fatigue): each time an element is heated near its melting point it deteriorates slightly. Makers can plot how many such pulses a fuse survives at a given percentage of its melting I2t.
• Inherent (natural) unbalance: the standing unbalance signal from system voltage unbalance and capacitor manufacturing tolerance, plus sensor tolerance and unit temperature differences.
• Inrush current: the high-frequency charging current when a bank is energized. One manufacturer's application guide says a single isolated bank's inrush is generally less than 5 kA and 1 kHz.
• Inrush (current-limiting) reactor: a series inductor that reduces the magnitude and frequency of inrush and outrush current.
• Internally fused unit: a unit in which each element has its own fuse inside the can. A failed element is disconnected and the unit stays in service with a small loss of capacitance.
• K link: a fast expulsion fuse link in the EEI-NEMA series. Tin K and T links carry 150% of their rating continuously; silver links carry 100%.
• kvar: kilovolt-amperes reactive, the reactive power rating of a capacitor. For a single-phase unit, kvar = 2 pi f C V^2 / 1000 and current = kvar / kV.
• Let-through (cut-off) current: the highest instantaneous current reached while a fuse-link breaks the circuit.
• Line fuse: in IEC 60549, a fuse that clears a fault on the whole capacitor bank and isolates it from the system.
• Lockout delay: a time delay in an unbalance relay that lets the unit fuse of a failing unit operate before the relay trips the bank. One relay's delay is factory set at 10 seconds.

M to P

• Melting (pre-arcing) I2t: the Joule integral from the start of the overcurrent until the arc forms. A fuse must not reach it during inrush or outrush.
• Minimum melt and total clearing curves: the two time-current curves for a fuse. Per the IEEE fuse working group tutorial, these curves typically start at 0.01 s and end at 1000 s.
• Neutral current unbalance (60N): a relay signal taken from current in a grounded wye neutral or between the neutrals of a double wye.
• Neutral voltage unbalance (59N): a relay signal taken from the neutral-to-ground voltage of an ungrounded wye bank.
• Outrush current: the discharge of a capacitor bank into a nearby fault or into a failed unit, at a frequency of a few kilohertz.
• Overvoltage factor (kOV): the voltage on the most stressed healthy unit during a failure divided by its voltage in a healthy bank.
• Parallel energy (parallel stored energy): the energy in the healthy units of a group that discharges into a failed unit and its fuse. One manufacturer's guide computes it at 1.1 times crest of rated voltage, 3.19 joules per kvar.
• Parallel group: units connected in parallel to make up the kvar of one series section.
• Parallel resonance order (h): h = sqrt(kVAsys / kvar), the harmonic at which a capacitor resonates with the system inductance.
• Power factor: the ratio of real power (kW) to apparent power (kVA), equal to the cosine of the angle between voltage and current.
• Pre-insertion inductor or resistor: an element switched in briefly while a bank closes, to damp the energization transient.

R to S

• Reactive power: the leg of the power triangle at right angles to real power, measured in kvar. Capacitors supply it; inductive loads draw it.
• Safe zone: in the 2023-cycle NEC Article 460 text, low probability of damage other than a slight swelling of the capacitor case, as identified by the case rupture curve of the capacitor.
• Self-excitation: an induction motor with too much connected capacitance acting as a generator after it is disconnected. The motor maker's maximum kvar avoids it.
• Series group: parallel groups connected in series to reach the bank's nameplate voltage.
• Sneakout: a term used in one fuse maker's literature for a nuisance fuse operation caused by capacitor bank inrush or outrush current.
• String: the chain of series groups forming one path in a phase. R is the number of strings in parallel per phase.
• Surge impedance: sqrt(L / C) of the switching circuit. The peak energizing current is the initial voltage divided by it.
• Switched bank: a bank with a switching device, control equipment and a control power source, switched in for heavier loads.
• Symmetrical current: fault current with no DC offset, rated in rms amperes. DENCO lists the DNX interrupting rating as 50 kA asymmetrical and the Types 1 to 9 ratings as symmetrical; the two are not converted into each other.

T to Z

• T link: a slow expulsion fuse link in the EEI-NEMA series. One capacitor maker's group fusing note says the transient-current criterion points toward a slow link such as a T link, while healthy-unit overvoltage on ungrounded banks pushes toward a fast K link.
• Unbalance compensation: subtracting the standing (inherent) unbalance from the relay signal. A relay maker's paper says to provide it when the inherent error approaches 50% of the alarm setting.
• Unbalance protection: relaying that detects the change in a bank's internal balance after units or elements fail, alarms on the first loss and trips before healthy units are overstressed.
• Unfused bank: series and parallel units with no fuses. One relay maker's paper says it is normally used below 34.5 kV, where series strings are not practical, or on higher-voltage banks with modest parallel energy.
• Ungrounded wye bank: a wye bank with the neutral not grounded. It gives no path for zero-sequence currents, the neutral must be insulated for full line voltage, and a fault through a failed unit is capacitively limited, typically to less than two or three times bank load current.
• Unit fuse: in IEC 60549, a fuse that clears a fault inside one capacitor unit so the rest of the bank stays in service.
• Visible break (isolating means): the 2023-cycle NEC Article 460 text calls for a visible gap in the electrical circuit adequate for the operating voltage.
• Voltage differential (87V): an unbalance method comparing a tap-point voltage within a phase with a fraction of the bus voltage.
• Voltage magnification: amplification of a switching overvoltage between a switched transmission bank and distribution banks downstream. A published case study shows 1.76 per unit.

Formulas used across the guide

The worked examples in the other articles use these relationships. Units matter: state them every time.

Terms that are often mixed up

• Fuseless versus unfused: both lack fuses, but a fuseless bank relies on many elements in series so one failure barely changes the voltage on the rest, while an unfused bank uses fewer, larger series and parallel units.
• Fusing factor versus interrupting rating: the fusing factor concerns continuous current; the interrupting rating is the largest fault current the fuse can clear.
• Melting I2t versus clearing I2t: melting I2t sets whether inrush or outrush damages the fuse; clearing I2t sets what the failed capacitor must absorb.
• Unit fuse versus line fuse: one removes a single failed unit, the other disconnects the whole bank.
• Symmetrical versus asymmetrical ratings: they describe different currents and are not interchangeable on a quotation.
• C rating versus E or R rating: E and R ratings are defined in IEEE fuse standards; the C rating is a catalog designation, and DENCO's definition is its own.

Frequently asked questions