27 kV and 38 kV Capacitor Fuses

What DENCO lists at 27 kV and 38 kV

DENCO's C-rated family covers AC voltages up to 27 kV and 38 kV, and the DNX series page, catalog p.72, carries both. At 27 kV the listing is DNX27-6C - 50C. At 38 kV the catalog shows a size with the same 6C to 50C range, but its catalog number is still to be confirmed by DENCO, so this guide does not print one. When you request a quote, ask DENCO to state the 38 kV part number in writing.

Both are DNX designs: clip-style current-limiting capacitor fuses, with threaded end studs available and a listed body diameter that is a maximum. DENCO prints the DNX series interrupting rating as 50 kA asymmetrical; confirm it for the exact 27 kV or 38 kV part on the quote. Dimensions are not stated in this guide.

The C number follows DENCO's catalog definition for the DNX: the fuse melts in 1000 seconds at a current of 1.7 to 2.4 times its C number. No standard defines a capacitor fuse C rating. Unlike the lower classes, which split into two or three C ranges, both of these classes have a single range.

Why no unit table points here

The fuse class tables used across this guide all stop at 23.0 kV. A full-range current-limiting table and an indoor combination table end with 19920 V and 21600 V units; an outdoor combination table adds 22800 V. Capacitor units themselves are made from 216 V to 24,940 V, according to a relay maker's paper, so even the top of the unit range is close to the 23 kV class rather than to 27 kV or 38 kV.

Arithmetic shows how close the edge is. For a 22800 V unit, 110% is 25,080 V, the contingency level that IEEE's 2005 interpretation of IEEE 18-2002 says fuses should withstand. One maker's indoor table lists its 23.0 kV class with a 25.3 kV rated maximum. For a unit rated near the 24,940 V top of the range, 110% is 27,434 V. Units at those voltages are where a 23 kV class fuse runs out of room, and where a question about a higher class makes sense. DENCO engineering answers it from the unit data.

When the voltage across a fuse exceeds 23 kV

Most of the time the 27 kV and 38 kV question comes from the circuit, not the unit. Published guidance frames the voltage rating in terms of what the fuse can see. One fuse maker says the rating must be equal to or greater than the maximum open circuit voltage that the system can provide across a fuse that operated clearing the fault. Another maker's low-voltage guidance says that on three-phase capacitors the fuse should be rated at or above the line-to-line voltage.

Bank connection changes that voltage. One capacitor maker's group fusing note says that in an ungrounded wye bank, after one unit shorts, the healthy units see line-to-line voltage, 1.73 times their rating. A relay maker notes that an ungrounded neutral can rise to as much as 2.5 per unit during switching, which is why it asks for neutral voltage transformers rated for full system voltage. An IEC-market fuse guide goes further and says that in capacitor bank applications the fuse may see the bank voltage plus the reversed grid voltage, so its rated voltage should be at least twice the grid voltage. That is guidance for IEC practice, attributed to one maker, and not a North American rule.

IEC 60549 separates unit fuses, which clear a fault inside one unit so the rest of the bank stays in service, from line fuses, which isolate the whole bank. A line fuse sits across system voltage, not unit voltage, which is another way a fuse ends up above 23 kV.

System voltages around 27 kV and 38 kV

ANSI C84.1 nominal three-phase voltages, as adopted by one state regulator, include 22860Y/13200 V, 23000 V, 24940Y/14400 V, 34500 V and 34500Y/19920 V. One capacitor maker's table adds a 26.2 kV wye system for 15125 V units. The table sets each line-to-line value against the DENCO classes. It is a comparison, not a selection: whether a given fuse must withstand line-to-line voltage depends on its position and the bank connection.

Line current for banks at these voltages

A fuse that protects a whole bank or phase carries line current, not unit current. One manufacturer's formula sheet gives three-phase line current as kvar x 1000 divided by the square root of 3 times line-to-line voltage. The table applies it to common bank sizes, with the 1.35 protective margin that published guides use for current-limiting unit fuses shown for comparison only.

DENCO's 27 kV and 38 kV DNX sizes run from 6C to 50C. How a required current maps to a C number is DENCO's call, because the C number describes melting at 1000 seconds rather than a continuous rating, but the table makes it clear that large banks at 24.94 kV can call for currents well past what a small C number suggests. Send DENCO the bank kvar and voltage.

Interrupting duty at 27 kV and 38 kV

DENCO's catalog gives one interrupting figure for the DNX series, 50 kA asymmetrical, and DENCO's family listing spans 40 kA to 80 kA across all its C-rated fuses. For a part at these voltages, ask DENCO to state the interrupting rating for that exact catalog number, with its basis.

The basis is not a formality. The IEEE fuse working group tutorial on current-limiting fuses explains that the crest of a symmetrical fault is 1.414 times its rms value, while a fully asymmetrical crest can reach about 2.8 times the rms symmetrical value, depending on the circuit X/R. A fault study usually reports rms symmetrical current, so give DENCO the study value and X/R and let DENCO compare.

What the fuse must interrupt also depends on where it sits. One manufacturer's guide says faults inside ungrounded or multi-series-group banks are capacitively limited, typically to less than two or three times bank load current, while single series group grounded wye and delta banks are inductively limited and see the available system fault current. A line fuse ahead of the bank sees system fault current regardless.

Where 27 kV and 38 kV capacitor equipment shows up

Metal-enclosed capacitor banks reach these voltages. One bank maker builds them from 2.4 kV through 38 kV, with reactive power ratings to 40,000 kvar in a single enclosure, and fuses each capacitor individually with current-limiting fuses. It switches stages with SF6 switches at 25 kV and up, and shows a 60 Mvar, 38 kV, five-stage metal-enclosed bank being installed at a solar plant. The same maker shows a 34.5 kV, single-step, 7 Mvar fifth harmonic filter bank. IEEE 1531-2020 is the guide for specifying components, protection and control of harmonic filters.

On distribution lines, one maker's pole racks are built for systems up to 34,500 V, and its pole banks are offered for 24,940 V systems at 125 kV BIL. Those use 19.92 kV and 14.4 kV units with 23 kV and 15.5 kV class unit fuses; the higher classes come in only where a fuse sits across more than the unit voltage.

DENCO lists its C-rated fuses for capacitor and capacitor bank protection, power factor correction and harmonic filtering equipment, induction heating and high power drives, indoors or outdoors within an enclosure. At 27 kV and 38 kV, expect the conversation to be about a specific piece of equipment.

Buying a 27 kV or 38 kV capacitor fuse from DENCO

Parts in these classes are less common, so the details of the order matter more. DENCO quotes a 30-day lead time on all products, compared with lead times that are commonly longer elsewhere in the market. DENCO tests in its own factory lab and, on request, arranges third-party testing at IPH Berlin. DENCO serves OEM and specialty custom design work as well as utilities and contractors, which is the route for a 38 kV size or any variant not covered by the catalog listing.

• Equipment one-line or bank drawing, with the bank connection and grounding.
• Where the fuse sits: on a unit, on a phase or group, or on the line feeding the bank.
• System voltage, unit voltage and kvar, and bank kvar.
• Available fault current and its basis.
• Maximum ambient at the fuse and whether it is in an enclosure.
• Mounting (clip or threaded end studs), indicator option and quantity.
• For the 38 kV size, a request for the catalog number and dimensions in the quote.
• Any existing fuse part number. DENCO engineering will confirm a replacement from it.
• Whether you need third-party test documentation from IPH.

Common mistakes above 23 kV

• Picking a 38 kV fuse for every 34.5 kV job. Unit fuses in a 34500Y/19920 V wye bank see 19.92 kV and are 23 kV class.
• Picking a 23 kV fuse for a fuse that sits across line-to-line voltage on a 24.94 kV or 34.5 kV system.
• Ordering a DNXC at 27 kV or 38 kV. The DNXC stops at 23 kV.
• Quoting a 38 kV DNX catalog number from an old document. Get the number from DENCO.
• Reading 6C to 50C as a continuous ampere range. It is a C range, and DENCO maps your current to it.
• Applying IEC-market doubling rules to a North American bank, or the reverse, without saying which practice the specification follows.

Frequently asked questions