5.5 kV Capacitor Fuses (DNX5)

Unit voltages and systems that point to 5.5 kV

Two published tables from one fuse maker agree that 4160 V and 4800 V capacitor units take a 5.5 kV fuse: its outdoor combination current-limiting table and its indoor combination table. The indoor table also places 2400 V and 2770 V units in the 5.5 kV class. A full-range current-limiting table from a different maker, whose product line starts at 8.3 kV, puts all of those units in its 8.3 kV class.

Getting to a 4160 V or 4800 V unit depends on the bank connection. In a delta bank the units carry line-to-line voltage, so a 4160 V delta bank uses 4160 V units and a 4800 V delta bank uses 4800 V units. In a wye bank the units carry line-to-neutral voltage. One capacitor maker's table pairs a 4160 V unit with a 7200 V wye system and a 4800 V unit with an 8320 V wye system; 8320Y/4800 V appears on the ANSI C84.1 nominal list as adopted by one state regulator.

Voltage margin inside the 5.5 kV class

IEEE's 2005 interpretation of IEEE 18-2002 says capacitor units should stay in service for contingencies up to 110% of rated voltage and that their fuses should withstand that overvoltage too. For the two units that define this class: 1.1 x 4160 V is 4576 V and 1.1 x 4800 V is 5280 V. Both sit under 5500 V.

One maker's indoor table also gives each class a rated maximum voltage of 110% of the class figure, referencing IEEE C37.40, which puts its 5.5 kV class at 6.05 kV. That is the published reason 4800 V units fit here comfortably, while at 4.3 kV the same 110% check on a 4160 V unit goes over the class. Ask DENCO to confirm the corresponding figure for the DNX5.

The class also covers wye banks with lower unit voltages in the indoor table, such as 2400 V units on a 4160Y/2400 V system. Another maker's low-voltage guidance prefers the nearest available fuse voltage so the voltage generated while the fuse interrupts does not stress the system. When the unit is well below the class, raise that with DENCO.

DNX5 and the other DENCO 5.5 kV fuses

DENCO's catalog p.72 lists the 5.5 kV DNX in two C ranges. It is a clip-style current-limiting capacitor fuse, with threaded end studs available and a listed body diameter that is a maximum. At the same voltage, DENCO's catalog p.70 lists capacitor protection fuse Types 7, 8 and 9, catalogued in amperes by maximum AC voltage.

The C number is DENCO's convention, since no standard defines a C rating. The catalog says the DNX melts in 1000 seconds at a current of 1.7 to 2.4 times its C number. DENCO maps your required current to a C number; the number itself is not a continuous ampere rating.

• The DNXC is not offered at 5.5 kV. If a specification calls for a full-range, bus bar to lead fuse on 4160 V or 4800 V units, the DENCO DNXC option is the 8.3 kV class.
• With or without a blown fuse indicator is a family-level option for DENCO C-rated fuses; state your choice.

Unit current at 4160 V and 4800 V

For a single-phase unit, current is kvar divided by unit kV. Published guides for medium voltage unit fuses add a 1.35 protective margin; one maker says it covers normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient, and its indoor fuse guide states the margin for a 40 °C ambient instead. The table below uses 1.35 as plain arithmetic.

One capacitor maker lists 200 kvar as its largest standard unit from 2400 V to 4800 V, so the table stops there.

Worked example: 900 kvar on an 8320Y/4800 V system

A 900 kvar grounded wye bank on an 8320Y/4800 V system needs 300 kvar per phase. With 200 kvar as the largest standard 4800 V unit in one maker's catalog, a practical layout is two 150 kvar units in parallel per phase, each with its own fuse.

Each unit draws 150 / 4.8 = 31.3 A. With the 1.35 margin the fuse needs at least 42.2 A of continuous capability. Build the same 300 kvar per phase from three 100 kvar units and each fuse needs only 28.1 A, but there are three fuses per phase, and when one unit fails, two healthy units totaling 200 kvar discharge into it instead of one 150 kvar unit.

Compare a 600 kvar delta stage at 4160 V built from three 200 kvar units: 48.1 A per unit, 64.9 A with the margin. The delta stage needs a larger rating per fuse because each unit is larger and runs at a lower voltage. Both answers are starting points for DENCO, which checks them against the capacitor maker's case rupture curve and the inrush and outrush duty.

Staged metal-enclosed banks at 5 kV class

Much 5.5 kV fusing happens inside metal-enclosed capacitor banks. One bank maker builds them from 2.4 kV through 38 kV with up to 15 capacitor steps, fuses each capacitor individually with current-limiting fuses to improve bank reliability and protect against case rupture, and fits 44 uH transient inrush reactors per stage. It switches stages with vacuum contactors below 6.6 kV.

The makers differ on connection. One maker's manual says its 5 kV class banks are normally connected in delta, while another maker builds its metal-enclosed banks ungrounded wye as standard. The difference changes the unit voltage, so check the drawing rather than assuming.

Stage switching is where back-to-back inrush comes from. One switching maker describes back-to-back inrush as on the order of tens of kiloamperes and tens of kilohertz, which is why staged banks use reactors. A unit fuse must ride through that duty without operating, so send DENCO the step kvar, the reactor value and how often stages switch.

Heat in the enclosure

Enclosed fuses run warmer than the air outside. One bank maker's manual calls for shading where daytime temperatures exceed 40 °C and sets cabinet fans to turn on at about 85 °F. DENCO's catalog prints a temperature derating formula only for its DNXC series. For a DNX5 or a Type 7, 8 or 9 in an enclosure, tell DENCO the highest ambient the fuse will see and let DENCO check the rating.

Information DENCO needs for a 5.5 kV quote

• Unit voltage (4160 V, 4800 V or lower) and kvar for each unit.
• System voltage and connection, delta or wye, grounded or ungrounded.
• Number of steps, kvar per step, units in parallel per phase, and inrush reactor value.
• Available fault current at the bank bus.
• Harmonic content or filter tuning.
• Highest ambient inside the enclosure.
• Clip mounting or threaded end studs, indicator option, quantity.
• Part number of any fuse being replaced, so DENCO engineering can confirm a replacement.

Common mistakes at 5.5 kV

• Assuming a specification for 8.3 kV full-range fuses on 4800 V units is an error. It follows a different maker's table; ask DENCO whether the DNX5 or a DNXC8 suits the bank.
• Using the wye unit voltage on a delta drawing. A 4800 V delta bank uses 4800 V units, not 2770 V units.
• Sizing for steady current only in a staged bank and ignoring back-to-back inrush between steps.
• Mixing up the two DENCO 5.5 kV families. The DNX5 is listed by C range; the Types 7, 8 and 9 by amperes.
• Leaving out the enclosure temperature. DNX derating is not printed, so DENCO needs the number to check it.

Frequently asked questions