Capacitor fuse guide
Capacitor banks are fused for a different reason than transformers or motors: the job is to take a failed capacitor unit out of service before its case ruptures, while riding through the inrush every switching operation brings. These guides cover how that works, how to choose a fuse for it, and how to specify DENCO's DNX and DNXC current-limiting capacitor fuses.
DNX and DNXC fuses Request a quote
Step 1 · Understand
How capacitor fuses work
What a capacitor fuse does, what the C rating means, and why capacitor banks are fused differently from transformers and motors.
What Is a Capacitor Fuse?
A capacitor fuse removes a failed capacitor unit or isolates a bank before the case ruptures. How unit and line fuses work and why banks need them.
What Does C-Rated Mean on a Fuse?
C-rated marks a capacitor fuse. It is a catalog convention, not a standard rating class. How DENCO defines the C number and how C, E and R ratings differ.
Capacitor Case Rupture and How Fusing Prevents It
Why capacitor cans rupture, where the energy comes from, and how to read a case rupture curve against a fuse's total clearing curve, with worked numbers.
Externally Fused vs Internally Fused vs Fuseless Capacitor Banks
Externally fused, internally fused, fuseless and unfused capacitor banks compared: how each handles a failed element, with worked overvoltage numbers.
Current-Limiting vs Expulsion Capacitor Fuses
How current-limiting and expulsion capacitor fuses interrupt, how each handles parallel stored energy and fault current, and where each one fits.
Fusing Power Factor Correction and Harmonic Filter Capacitors
What changes when fusing power factor correction and harmonic filter capacitors: harmonic current, overvoltage limits, enclosure heat and sizing margins.
Capacitor Inrush and Outrush: Why Capacitor Fuses Are Rated Differently
Inrush on energization and outrush between parallel units put short, high-frequency pulses through capacitor fuses. Formulas, worked numbers and I2t.
How Capacitor Banks Work: kvar, Power Factor and Why They Are Installed
How shunt capacitor banks correct power factor and support voltage, with worked kvar, unit current and voltage rise numbers, and where the fuses come in.
How Capacitor Units Fail and What the Fuse Sees
How capacitor elements and units fail, the current and stored energy the fuse sees, and how much the voltage rises on healthy units, with worked numbers.
Capacitor Fuse Glossary: Terms for Capacitor Banks and Fusing
Plain definitions of capacitor bank and capacitor fuse terms, from back-to-back switching and C rating to kOV, parallel energy, sneakout and tank rupture.
Why Capacitor Fuses Blow: Real Failures vs Nuisance Operations
The causes of capacitor fuse operations, from shorted units to inrush, lightning, harmonic resonance and loose connections, and how to tell them apart.
Capacitor Bank Unbalance Protection and How It Works with Fuses
How unbalance relays and unit fuses share capacitor bank protection: methods by bank type, neutral voltage numbers, alarm and trip levels and timing.
Step 2 · Choose
Choosing a capacitor fuse
Voltage, current and interrupting rating, mounting, curves and enclosure temperature: the decisions behind a capacitor fuse specification.
How to Size a Capacitor Fuse
Capacitor fuse sizing step by step: unit current from kvar and kV, published margins, harmonics, ambient derating and outrush checks, with worked numbers.
Capacitor Fuse Voltage Rating: Unit Voltage, System Voltage and Connection
How to pick a capacitor fuse voltage rating: unit vs system voltage, wye and delta banks, the 110% check, class tables and DENCO classes from 1 to 38 kV.
Capacitor Fuse Interrupting Rating and Parallel Stored Energy
How to check a capacitor fuse against system fault current and parallel stored energy: joules per kvar, published limits and worked bank examples.
Full-Range vs Back-Up Current-Limiting Capacitor Fuses
Full-range, general-purpose and back-up current-limiting fuse classes, where IEEE defines them, and why low fault currents decide the class for capacitors.
Clip-Style vs Bus-Mounted Capacitor Fuses
Clip-style vs bus bar to lead capacitor fuse mounting: how each fits a bank, terminals, torque, inspection, and how DENCO DNX and DNXC fuses mount.
Reading Capacitor Fuse Time-Current Curves Against Case Rupture Curves
How to read capacitor fuse melt and clearing curves against tank rupture curves, compare I2t below 0.1 s and check inrush and outrush, with worked numbers.
Capacitor Fuse Temperature Derating in Enclosures
How to derate a capacitor fuse for enclosure temperature: the DNXC formula, factor and derated-amp tables, worked examples and how to choose the ambient.
Fusing Metal-Enclosed Capacitor Banks
Choosing unit fuses for metal-enclosed capacitor banks: wye vs delta fault current, voltage class, enclosure derating and inrush between steps.
Fusing Pole-Mounted Capacitor Banks
How pole-mounted capacitor banks are group fused: link sizing rules, a worked T-link table, grounded vs ungrounded wye, tank rupture limits and lightning.
Fusing Substation Capacitor Banks: Externally Fused, Internally Fused and Fuseless
How substation capacitor banks are fused: external, internal and fuseless designs, group size and parallel energy limits, unit fuse margins and relays.
Fusing Harmonic Filter Banks
How to fuse harmonic filter capacitor banks: resonance screening, true-rms current, IEEE 18 limits, enclosure derating and the data DENCO needs to quote.
Fusing Industrial Power Factor Correction and Motor-Switched Capacitors
Fusing industrial power factor correction banks and motor capacitors: sizing kvar, NEC Article 460 draft text, fuse margins, derating and discharge rules.
Capacitor Fuse I2t: Melting, Clearing, Outrush and Parallel Energy, With Worked Numbers
Worked capacitor fuse I2t numbers: parallel energy per kvar, n fuses in parallel, melting vs clearing I2t, tank rupture limits and outrush checks.
Coordinating Capacitor Fuses With Unbalance Relays and Upstream Protection
How capacitor unit fuses coordinate with unbalance relays, bank overcurrent, group fuses and breakers, with published delays, pickups and worked numbers.
Inspecting and Maintaining Capacitor Fuses: Indicators, Thermography, Testing and Safety
How to inspect and maintain capacitor fuses: OSHA discharge steps, NETA intervals, thermography, blown-fuse indicators, capacitance checks and storage.
Step 3 · Specify and buy
Specifying and ordering DNX and DNXC fuses
Reading DENCO catalog numbers, choosing between DNX and DNXC, replacing an existing fuse and sending a quote request.
By voltage class
- 2.8 kV capacitor fuses (DNX2)
- 4.3 kV capacitor fuses (DNX4)
- 5.5 kV capacitor fuses (DNX5)
- 8.3 kV capacitor fuses
- 15.5 kV capacitor fuses
- 23 kV capacitor fuses
- 27 kV and 38 kV capacitor fuses
DNX vs DNXC: Which DENCO Capacitor Fuse Fits?
DENCO DNX clip-style vs DNXC full-range bus bar to lead capacitor fuses: voltage classes, C and ampere ratings, derating, mounting and a worked choice.
How to Read a DNX or DNXC Catalog Number
How DENCO DNX and DNXC capacitor fuse catalog numbers work: voltage class digits, DNX C ranges, DNXC ampere ratings, and a worked example by unit voltage.
Replacing Blown Capacitor Fuses Safely
Replacing a blown capacitor fuse: discharge and ground per OSHA, find why it operated, test the unit, check healthy-unit voltage, then match the fuse.
Capacitor Fuse RFQ Checklist: What DENCO Needs to Quote
What to send DENCO for a capacitor fuse quote: unit kvar and voltage, bank connection, fault current, parallel units, harmonics, ambient and mounting.
Buying Capacitor Fuses: Lead Time, Custom Builds and Testing
Buying capacitor fuses from DENCO: 30-day lead time on all products, spares planning, custom designs, in-house testing and IPH Berlin testing on request.
DNXC Selection by Capacitor kvar and Unit Voltage
Calculated DNXC capacitor fuse starting points by unit kvar and voltage at 25, 40 and 55 °C, using kvar / kV, a 1.35 margin and DENCO's derating formula.
Capacitor Fuses from 1 kV to 5.5 kV: DENCO Types 1 to 9
DENCO Types 1 to 9 capacitor protection fuses, 1 kV to 5.5 kV: ratings by voltage and Type, interrupting ratings, voltage class choice and worked sizing.
Replacing an Obsolete or Hard-to-Get Capacitor Fuse
Replacing an obsolete capacitor fuse: what to send DENCO and how voltage class, rating, curve, interrupting rating and mounting are matched, step by step.