Capacitor Fuse Voltage Rating: Unit Voltage, System Voltage and Connection

What the voltage rating of a capacitor fuse means

One manufacturer's capacitor fuse selection guide defines maximum rated voltage as the maximum power system voltage that the fuse can clear against, and notes that for high-voltage capacitor fuses this is generally 8.3, 15.5 or 23 kV, the distribution system maximum voltages. A fuse applied above its rated voltage may not interrupt the arc it creates.

The class name and the rated maximum are not always the same number. The same maker's indoor fuse table, referencing IEEE C37.40, lists the rated maximum voltage as 110% of the class: 5.5 kV class to 6.05 kV, 8.3 kV to 9.13 kV, 15.5 kV to 17.05 kV and 23 kV to 25.3 kV. Check what the maker of your fuse publishes before comparing a contingency voltage with the class name.

Going far above the needed class has a cost too. A current-limiting fuse forces current down by developing a voltage across itself while it interrupts. Writing about low-voltage capacitor fuses, the same guide says the nearest available fuse voltage should be used so that the voltage developed by the fuse during interruption does not damage the system. That is why selection is phrased as the nearest adequate class, not the highest class on the shelf.

From system voltage to capacitor unit voltage

The first step is the voltage across one unit. In a wye bank each unit sits between phase and neutral, so its rating is close to the line-to-line voltage divided by 1.732. In a delta bank each unit sits across two phases and is rated at line-to-line voltage. One capacitor maker's installation instructions state the rule directly: capacitors connected in delta must be rated at the line-to-line system voltage, and capacitors connected in wye at the line-to-neutral voltage.

ANSI C84.1 nominal system voltages, as adopted by a state utility regulator, include 4160Y/2400, 12470Y/7200, 13200Y/7620, 13800Y/7970, 24940Y/14400 and 34500Y/19920. Dividing by 1.732 gives the unit voltages most often seen on nameplates. Utility practice does not always follow the obvious choice: one municipal utility specifies 8,320 V units for its 14.4 kV grounded wye system and 7,200 V units for its 7.2 kV delta system.

Applying line-to-line voltage to a unit rated line-to-neutral is a serious error. RUS Bulletin 1724D-112 notes that 1.732 times rated voltage raises the effective kvar to three times the capacitor's rating.

From unit voltage to fuse voltage class

Published manufacturer guides for medium-voltage unit fuses say these fuses protect individual single-phase capacitor cans, so the fuse voltage rating can be determined from the capacitor unit's voltage rating, and the fuse should not be applied above its rated voltage. Each maker publishes a table for its own fuses. Three examples are compared below as illustrations of the method, not as a DENCO selection table.

Notice the 8,320 V row. One table puts these units on 8.3 kV fuses; the other two put them on 15.5 kV fuses. There is no universal answer for 8,320 V units, so confirm the class with the fuse maker for the exact fuse.

Expulsion capacitor fuses use a different rule. One manufacturer asks for a fuse voltage rating equal to or greater than the capacitor can voltage times 1.1, and assigns its expulsion classes as 9.7 kV for 2.4 to 8.8 kV units, 16.6 kV for 8.3 to 15.1 kV units and 26.2 kV for 15.1 to 23.8 kV units.

Worked example: the 110% contingency check

IEEE's capacitor subcommittee, interpreting IEEE 18-2002 in 2005, said capacitors must operate continuously under contingency conditions up to 110% of rated rms voltage, and that since units are expected to stay in service up to that level, the fuses should be rated to withstand this overvoltage as well. The arithmetic below multiplies common unit voltages by 1.1 and sets the result beside the 110% rated maximums one manufacturer lists for its classes.

The 8,320 V row shows why that unit voltage is the awkward case: 1.1 times the unit voltage lands just above the 9.13 kV maximum one maker lists for its 8.3 kV class, yet another of the published tables still assigns 8,320 V units to 8.3 kV. Compare against the rated maximum the maker of your fuse publishes, and ask DENCO which applies to the DENCO part.

Grounded wye, ungrounded wye and delta banks

In a grounded wye bank of single-phase units, each unit and its fuse sit between phase and neutral. One fuse maker's capacitor catalog states the rule as a voltage rating equal to or greater than the maximum open-circuit voltage the system can provide across a fuse that has operated, and gives the example of a 13.8 kV grounded wye bank: 13.8 / 1.732 = 7.9 kV, so an 8.3 kV fuse.

Ungrounded wye banks stress the healthy units differently. One manufacturer's pole bank fusing guide explains that after one unit shorts in an ungrounded wye bank, the healthy units see line-to-line voltage, 1.73 times their rating, until the fuse clears. A published protection paper adds that ungrounded wye banks provide no path for zero-sequence currents, and the neutral must be insulated for full line voltage.

Delta banks are built from units rated at line-to-line voltage, so a 4,160 V delta bank uses 4,160 V units and the fuse class follows that rating. A relay maker's paper notes that delta banks have a single series group, so isolating one faulted unit causes no overvoltage on the remaining units. Practice varies by voltage: one metal-enclosed bank maker says its 15 kV class banks are normally ungrounded wye and its 5 kV class banks normally delta.

For three-phase low-voltage capacitors, one manufacturer's guide says the fuse should be rated at or above the line-to-line voltage, and its 4.3/2.5 kV fuse covers 2,500 V single-phase or 4,300 V three-phase use, with two in series for a 4,800 V single-phase capacitor. Beyond statements like these there is no published general rule for delta and ungrounded wye fuse voltage, so have the bank designer state the maximum voltage across an operated fuse.

IEC practice: why some guides double the voltage

IEC-market guidance is more conservative. One manufacturer's IEC technical guide says the fuse rated voltage shall be equal to or higher than the highest voltage that may occur during a disturbance, and that for capacitor banks this may be doubled, because the fuse-link may see the full voltage on the charged bank plus the reversely polarized grid voltage. Its conclusion is that the rated voltage should be at least twice the grid voltage.

That conflicts with the North American unit-fuse tables, which place a 7,200 V unit on an 8.3 kV fuse. Both are published practice for their own products and markets. IEC 60549:2013 covers fuses for the external protection of shunt capacitors and refers to capacitors built to IEC 60871-1. Pick one method and record it in the design file.

DENCO capacitor fuse voltage classes

DENCO's catalog lists C-rated capacitor fuses at 1000 V, 1.5, 2.5, 3, 4.3, 5.5, 8.3, 15.5, 23, 27 and 38 kV AC. Up to 5.5 kV the Types 1 to 9 are catalogued by maximum AC voltage and Type number. The DNX clip-style series runs from 2.8 kV to 38 kV, and the DNXC bus-mounted, full-range series covers 8.3, 15.5 and 23 kV. Each class has its own page in this guide.

When asking DENCO to confirm a class, send the unit nameplate voltage and kvar, the system voltage, whether the bank is grounded wye, ungrounded wye or delta, the number of series groups, and the part number of any fuse being replaced. For 8,320 V units, and for any unit whose 110% voltage comes close to a class limit, say so in the request so the rated maximum voltage of the DENCO part can be checked against it. A photograph of the existing mounting also helps, because a change of class can change the fuse length.

Common mistakes

Most voltage-class errors come from using the wrong voltage as the starting point.

• Taking the fuse class from the bus voltage instead of the unit voltage, which puts a 7,200 V unit on a 15.5 kV fuse it does not need.
• Assuming every 8,320 V unit goes on an 8.3 kV fuse, or on a 15.5 kV fuse. Published tables differ; confirm with the fuse maker.
• Using a wye unit rating for a delta bank. Delta units are rated line-to-line.
• Comparing the 110% contingency voltage with the class name rather than the rated maximum voltage the fuse maker publishes.
• Mixing the IEC twice-the-grid-voltage rule with a North American unit-fuse table in the same design.
• Stepping up two classes for safety without checking the mounting length and the voltage the fuse develops while clearing.

Frequently asked questions