2.8 kV Capacitor Fuses (DNX2)

Which capacitors land in the 2.8 kV class

The 2.8 kV class is the lowest voltage class in DENCO's DNX series. It covers 2400 V and 2770 V single-phase capacitor units, the two unit voltages that one published manufacturer table for outdoor combination current-limiting capacitor fuses assigns to a 2.8 kV fuse.

One capacitor maker's catalog pairs a 2400 V unit with a 4160 V wye system and a 2770 V unit with a 4800 V wye system; 4.16 kV and 4.8 kV divided by the square root of 3 give 2.40 kV and 2.77 kV. In a delta bank the units are rated at line-to-line voltage, so a delta bank on a 2400 V system also uses 2400 V units. Both 2400 V and 4160Y/2400 V are on the ANSI C84.1 nominal list as adopted by one state utility regulator.

Why the tables disagree for 2400 V and 2770 V units

Three published tables give three classes for the same two unit voltages. The outdoor combination table uses 2.8 kV. An indoor combination table from the same maker uses 5.5 kV for every unit from 2400 V to 4800 V. A full-range current-limiting table from a different maker puts everything from 2400 V to 7960 V in its 8.3 kV class, because that product line starts at 8.3 kV.

Each table reflects one product range, so the class depends on the fuse family as much as on the capacitor. One maker's guide says a unit fuse's voltage rating can be determined from the capacitor unit's voltage rating, and warns against applying the fuse above its rated voltage.

For DENCO parts, a 2400 V or 2770 V unit points to the DNX2 for a clip-style fuse. For the full-range, bus bar to lead DNXC, the lowest class is 8.3 kV, so the conversation moves to DNXC8.

The 110 percent check at 2770 V

IEEE's 2005 interpretation of IEEE 18-2002 says capacitor units are expected to stay in service for bank contingencies up to 110% of rated voltage, and that the fuses should be rated to withstand this overvoltage as well. Run that number for both unit voltages: 1.1 x 2400 V is 2640 V, and 1.1 x 2770 V is 3047 V.

2640 V sits under 2.8 kV. 3047 V does not. One maker's indoor table lists each fuse class with a rated maximum voltage 10% above the class figure, for example 5.5 kV to 6.05 kV, citing IEEE C37.40. If the same relationship applied at 2.8 kV, the maximum would be 3.08 kV and would cover 3047 V. Whether DENCO's 2.8 kV figure is a class name with a higher maximum, or the maximum itself, is a question to put to DENCO before you fix the order. It matters most for 2770 V units on 4800 V systems.

DNX2 catalog numbers and what they mean

DENCO lists the 2.8 kV DNX in two C ranges on catalog p.72. The DNX is a clip-style current-limiting capacitor fuse; threaded end studs are also available, and the listed body diameter is a maximum. DENCO's family listing offers C-rated fuses with or without a blown fuse indicator.

No standard defines a C rating for capacitor fuses. It is a catalog convention, and DENCO's catalog defines it for the DNX: the fuse melts in 1000 seconds at a current between 1.7 and 2.4 times its C number. DENCO's family range is written 1C to 200C (1A to 200A), so a C number corresponds to an ampere value. That is a melting definition, not a continuous rating, so DENCO converts your required current into a C number.

• Other DENCO capacitor protection fuses near this voltage are catalogued in amperes by maximum AC voltage and Type number: Types 1, 3 and 5 at 2.5 kV (25 A to 200 A, 50 kA symmetrical) and Types 1, 2 and 6 at 3 kV (50 A to 175 A, 50 kA symmetrical).
• Which DENCO series suits a 2400 V or 2770 V unit depends on mounting, current and the bank design. DENCO will say which one to quote.

Unit current at 2400 V and 2770 V

Unit current is kvar divided by unit kV. Published manufacturer guides for medium voltage unit fuses then apply a 1.35 protective margin, which one maker says accounts for normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient. One maker's guidance for low-voltage capacitors from 1.2 to 3 kV uses 1.65 times capacitor current to cover harmonics and switching currents. Both figures are maker guidance, not a standard, and 2400 V and 2770 V units sit inside that 1.2 to 3 kV band, so the table shows both.

The kvar range stops at 200 because one capacitor maker's catalog lists 200 kvar as its largest standard unit from 2400 V to 4800 V, and offers 2400 V and 2770 V units only in its extreme-duty line. A rural utility bulletin notes that the smallest standard unit for primary use is now 50 kvar.

Worked example: a 600 kvar bank on a 4160Y/2400 V system

Take a 600 kvar grounded wye bank on a 4160Y/2400 V system with one 200 kvar, 2400 V unit per phase. Each unit draws 200 / 2.4 = 83.3 A. With the 1.35 margin the fuse needs at least 112.5 A of continuous capability; with the 1.65 low-voltage figure it is 137.5 A.

Move the same 600 kvar to a 4800 V wye system with 2770 V units and the current falls to 72.2 A per unit, or 97.5 A with the 1.35 margin. That 13% drop comes only from the higher unit voltage.

Currents this large are the reason to involve DENCO at the start. The DNX2 listing tops out at 100C, while DENCO's 2.5 kV Types reach 200 A and its 3 kV Types reach 175 A. Which design carries the duty, and at what C number or ampere rating, is DENCO's call against the capacitor maker's case rupture curve and a protection study.

Interrupting rating and fault current

DENCO prints the DNX interrupting rating as 50 kA asymmetrical and the 2.5 kV and 3 kV Types as 50 kA symmetrical. Do not convert one to the other yourself: the IEEE fuse working group tutorial notes that a symmetrical crest is 1.414 times the rms value, while a fully asymmetrical crest can reach about 2.8 times the rms symmetrical value depending on X/R.

The bank connection changes what the fuse sees. One manufacturer's guide says faults in single series group grounded wye or delta banks are inductively limited, so only the available system fault current limits them, while ungrounded wye or multi-series-group banks are capacitively limited, typically to less than two or three times bank load current. Give DENCO both the connection and the available fault current.

Where 2.8 kV capacitor fuses are used

DENCO lists its C-rated fuses for 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. At 2400 V and 2770 V most of that work is in plant equipment rather than on distribution poles, and one capacitor bank maker's manual notes that its 5 kV class metal-enclosed banks are normally delta connected.

DENCO's catalog prints a temperature derating formula for the DNXC only, so for a DNX2 in a warm enclosure, give DENCO the maximum ambient and let it check the rating.

What to send DENCO for a 2.8 kV quote

A complete request lets DENCO engineering choose between the DNX2 and the 2.5 kV or 3 kV Types in one pass.

• Capacitor unit voltage (2400 V or 2770 V) and kvar, plus the capacitor maker and model if known.
• System voltage and bank connection: grounded wye, ungrounded wye or delta.
• Units per phase, and whether each unit has its own fuse or a fuse protects a group.
• Available fault current at the bank, and its basis (symmetrical or asymmetrical) if known.
• Harmonic content or filter tuning, and how often the bank switches.
• Maximum ambient at the fuse, and whether it is indoors or outdoors within an enclosure.
• Mounting (clip or threaded end studs), indicator option, and quantity.
• The old fuse's part number if this is a replacement. DENCO engineering will confirm a replacement from it.

Common mistakes at 2.8 kV

• Treating 2.8 kV as the only correct class for 2400 V units. Published tables use 2.8, 5.5 and 8.3 kV for the same unit.
• Skipping the 110% check on 2770 V units. 1.1 x 2770 V is above 2.8 kV, so ask DENCO what the class maximum is.
• Reading the C number as a continuous ampere rating. DENCO defines it by melting time, and DENCO sets it from your current.
• Ordering a DNXC at 2.8 kV. The DNXC starts at 8.3 kV; at 2.8 kV the DENCO choices are the DNX2 and the low-voltage Types.
• Comparing a 50 kA asymmetrical rating with a symmetrical fault study figure without asking how the two relate for your X/R.

Frequently asked questions