Capacitor Fuses from 1 kV to 5.5 kV: DENCO Types 1 to 9

What the Types 1 to 9 series covers

DENCO's low and medium voltage capacitor protection fuses sit on catalog pages 59 to 71, with Type EJ-5 / EJO-5 capacitor protection fuses listed on p.75. The catalog describes all of them as high interrupting capacity, current-limiting fuses designed to ANSI C37-41 specs; that phrase is the catalog's own wording, not a listing or certification claim.

The series is catalogued by maximum AC voltage and Type number rather than by a single catalog-number pattern. That makes it different from the DNX and DNXC, where the catalog number carries the voltage class and rating. For Types 1 to 9, the catalog numbers, dimensions and curves are on the catalog drawings and are not reproduced in this guide; ask DENCO for them against the rating you need.

DENCO's C-rated family lists the applications these fuses serve: capacitor and capacitor bank protection, power factor correction equipment, harmonic filtering equipment, induction heating systems, and high power drives and welders, indoors or outdoors within an enclosure. They are offered with or without a blown fuse indicator.

Ratings by voltage and Type

The table lists every Types 1 to 9 row as printed. Ampere ranges differ by Type even within one voltage: at 1.5 kV, Type 5 starts at 50 A while Types 1 and 2 start at 25 A. The Type number identifies the design within each voltage class, so do not assume that Type 1 at 1.5 kV and Type 1 at 3 kV are the same construction.

Choosing the voltage class

One manufacturer's guidance for low-voltage capacitor fuses sets three rules: the capacitor voltage should be less than or equal to the fuse voltage; the nearest available fuse voltage should be used, so that the voltage the fuse develops while interrupting does not damage the system; and on three-phase capacitors the fuse rating should equal or exceed the line-to-line voltage.

There is a second check. IEEE's capacitor subcommittee, interpreting IEEE 18-2002, said that because capacitor units are expected to stay in service for bank contingencies up to 110% of rated voltage, the fuses should be rated to withstand that overvoltage as well. For a 2400 V capacitor, 110% is 2640 V, which is above the 2500 V class; for a 4160 V capacitor it is 4576 V, above 4300 V. A 4800 V capacitor at 110% is 5280 V, inside 5500 V.

So for 2400 V and 4160 V capacitors the nearest class and the contingency check point in different directions. The worked table below shows both. Send the capacitor voltage, the connection and the system's operating voltage to DENCO and let DENCO engineering confirm which class applies; do not settle it from this article.

Current rating: margins for low-voltage capacitors

As at medium voltage, the fuse is sized from the capacitor current with a margin. For a three-phase capacitor, line current is kvar x 1000 divided by the square root of 3 times the line-to-line voltage, as one manufacturer's formula sheet gives it. The margin is where low-voltage practice differs.

One manufacturer's low-voltage capacitor fuse guidance says the continuous current rating should be 1.65 times the current flowing in each phase, to protect against harmonics and switching currents. Medium voltage unit fuse guides use 1.35. A fuse maker's capacitor application note sizes medium voltage capacitor fuses at 165% to 200% of capacitor current, and a plant engineer's guide gives 1.65 to 2.5 times rated current. None of these is a standard; they are published practices, and DENCO engineering will apply the one that suits the application.

The reason for the higher margins is switching. A contactor maker's guide reports that inrush in fixed low-voltage banks can reach 30 times nominal current, and in multi-step banks up to 100 times the nominal current of the step being energized, at 3 to 15 kHz for 1 to 2 ms. A fuse maker puts typical inrush at less than 25 times capacitor current for less than a quarter cycle. The fuse has to ride through that many times over without fatigue.

Worked examples

The table applies the arithmetic to five three-phase capacitors. It shows both margins and both voltage-class readings so the effect of each choice is visible. The listed ranges come from the ratings table above; whether a specific Type and rating fits is DENCO's call.

Interrupting rating and fault level

Types 1 to 9 interrupting ratings are stated as symmetrical: 40 kA at 1.5 kV, 40 to 50 kA for the 1 kV and 1.5 kV Types 3 and 4, 50 kA at 2.5 and 3 kV, and 65 kA at 4.3 and 5.5 kV. Compare them with a symmetrical fault current from your study. The IEEE C37.48.1 tutorial notes that the instantaneous crest of a symmetrical fault is 1.414 times the rms value and that a fully asymmetrical fault can reach about 2.8 times, depending on X/R, so the basis matters.

Low-voltage capacitor banks in plants often sit close to large transformers, where fault levels are high. A fuse maker's capacitor application note adds a further reason for individual fusing in that setting: in banks of parallel capacitors, if one capacitor fails the others discharge into it, and violent case rupture of the faulted capacitor can result. Individual capacitor fusing, it says, eliminates that problem.

Code and safety points at these voltages

NEC Article 460 splits capacitor rules at 1000 V in its 2023-cycle text (600 V in older editions), so a 1000 V application and a 4160 V application fall under different parts. Confirm the wording against the edition your jurisdiction has adopted; the points below are from the 2023-cycle text.

For 1000 V and under, 460.8(B) calls for an overcurrent device in each ungrounded conductor for each capacitor bank, rated or set as low as practicable, and 460.8(A) and (C) put conductors and the disconnecting means at not less than 135% of the capacitor's rated current. 460.6(A) requires residual voltage to fall to 50 V nominal or less within 1 minute after disconnection.

Over 1000 V, 460.25(A) requires a means to detect and interrupt fault current likely to cause dangerous pressure within an individual capacitor, and 460.25(D) asks that protective devices operate within the limits of the safe zone for individual capacitors. 460.28(A) requires residual voltage to fall to 50 V or less within 5 minutes.

None of that changes the work procedure. Disconnect, wait, short circuit and ground the capacitors before handling, as OSHA 29 CFR 1910.269(w)(1) describes, and follow NFPA 70E.

Where the DNX takes over

At 2.8 kV, and again at 4.3 kV and 5.5 kV, DENCO also lists the DNX clip-style current-limiting capacitor fuse: DNX2 at 1C to 45C and 50C to 100C, DNX4 at 18C to 35C and 40C to 100C, and DNX5 at 6C to 18C and 20C to 75C, all at 50 kA asymmetrical. Above 5.5 kV, the DNX and DNXC are the DENCO options.

Where both series are listed, the differences are in how the rating is expressed (amperes for Types 1 to 9, C numbers for the DNX), the interrupting basis (symmetrical vs asymmetrical), and the mounting. Send the existing hardware details and DENCO engineering will say which series fits.

What to send DENCO

For a 1 kV to 5.5 kV capacitor fuse, the request looks much like a medium voltage one, with more weight on switching. The list below is what lets DENCO engineering choose between Types, or between Types 1 to 9 and the DNX, without a second round of questions.

• Capacitor nameplate: voltage, kvar, single-phase or three-phase, frequency, and the manufacturer.
• Connection and operating voltage, including any sustained operation above nameplate.
• Bank arrangement: fixed or multi-step, number of steps, contactor or switch type, and any detuning or inrush reactors.
• Harmonic data: drive and rectifier load as a share of transformer capacity, or a power quality survey.
• Available fault current at the capacitor and its basis, symmetrical or asymmetrical.
• Maximum ambient inside the enclosure, mounting, and whether a blown fuse indicator is required.
• Existing fuse part number and photos, for a replacement.

Common mistakes with 1 kV to 5.5 kV capacitor fuses

These come up most often in plant power factor correction and drive applications.

• Choosing the fuse class from the phase-to-neutral voltage on a three-phase capacitor instead of the line-to-line voltage.
• Applying the medium voltage 1.35 margin to a low-voltage, multi-step bank without checking inrush.
• Assuming the same Type number means the same design at different voltages.
• Comparing a symmetrical interrupting rating with an asymmetrical fault figure.
• Leaving parallel capacitors on one group fuse, so a failed unit takes the discharge of its neighbors.
• Relying on discharge resistors instead of shorting and grounding before work.

Frequently asked questions