Replacing an Obsolete or Hard-to-Get Capacitor Fuse

Why a part-number cross-reference is not enough

Buyers usually arrive with a part number from a fuse that is discontinued, on a long lead time, or simply unknown to the current maintenance team. The quick answer would be a list that says part A equals part B. For capacitor fuses that answer is not safe, which is why this guide does not give one and DENCO does not publish one.

Two fuses can share a voltage class and a current figure and still behave differently. The current figure may not even mean the same thing: DENCO's DNX is rated by C number, which DENCO's catalog defines by a 1000-second melting band, while other capacitor fuses are rated in plain amperes, and expulsion links use their own letter-and-number ratings. The time-current curves, the melting and clearing I2t, whether the fuse is full-range or back-up, how the interrupting rating is stated, and the mounting all decide whether the replacement protects the capacitor the way the original did.

So the replacement is matched to the application, using the original part as evidence of what the bank designer intended. The rest of this article sets out what DENCO looks at and in what order.

What to send

The table lists what DENCO engineering uses to confirm a replacement. The first three rows matter most; with them, most replacements can be reviewed. Photos are better than typed data, because they show markings exactly as printed.

Step 1: voltage class

The replacement must be rated for the voltage the fuse sees when it clears, which depends on the capacitor unit voltage and the bank connection. One capacitor maker's instructions state that units connected in wye are rated at the line-to-neutral system voltage and units in delta at the line-to-line voltage.

Published manufacturer tables map unit voltages to fuse classes, and they do not all agree. For 8320 V units one maker's outdoor table gives the 8.3 kV class while two other tables give 15.5 kV. One maker's indoor table also lists a rated maximum voltage of 110% of each class: 9.13 kV for 8.3 kV, 17.05 kV for 15.5 kV, 25.3 kV for 23 kV. If the original fuse's class differs from what a table suggests, that is a question for DENCO, not a reason to change class silently.

DENCO's capacitor fuse classes run 1 kV to 5.5 kV in the Types 1 to 9 series, 2.8 kV to 38 kV in the DNX and 8.3, 15.5 and 23 kV in the DNXC.

Step 2: current rating and the unit it is expressed in

Next comes the current rating, starting from the capacitor rather than from the old label. Unit current is kvar divided by kV; a 200 kvar, 7.2 kV unit draws 27.8 A. Published practice then applies a margin: application guides for full-range current-limiting capacitor fuses use 1.35, giving 37.5 A here, and other published guidance runs from 1.25 to 1.65 or more. The original fuse rating, divided by the unit current, tells you what margin the original designer used.

Then translate units. A DNXC is rated in amperes. A DNX is rated by C number: DENCO's catalog says a DNX melts in 1000 seconds at 1.7 to 2.4 times its C number, so a 25C DNX melts in 1000 seconds somewhere between 42.5 A and 60 A. Neither figure is directly comparable with an expulsion link rating or another maker's ampere rating; the curve comparison in the next step is what connects them.

Finally, apply the enclosure temperature. DENCO's DNXC catalog derates above 25 °C with AT = A25 [1 - 0.0065 (T - 25)], which is 0.805 at 55 °C. A replacement that matches the old fuse at 25 °C can be too small in a hot enclosure.

Step 3: the curve comparison

The time-current curve is where two fuses are really compared. The IEEE C37.48.1 tutorial notes that fuse curves typically run from 0.01 s to 1000 s. Minimum melting and total clearing curves both matter: the minimum melt curve has to stay clear of inrush, outrush and harmonic duty, and the total clearing curve has to stay below the capacitor's case rupture curve up to the available fault current. One fuse maker's technical guide gives a typical curve tolerance of plus or minus 20%, or 10% with tight manufacturing control, so small differences between curves may be within tolerance and large ones are not.

For fast events, I2t is the better comparison. One manufacturer's capacitor fuse guide says I2t applies to fuse operations faster than about 0.1 or 0.01 seconds, that clearing I2t is always greater than melting I2t, and that total clearing I2t is the I2t the failed capacitor sees. Published tank rupture curves from one capacitor maker set minimum values such as 2,000,000 A2s up to 10,000 A for standard and heavy duty units. The replacement's melting I2t needs to exceed the outrush duty, and its clearing I2t needs to stay under the capacitor's limit.

DENCO publishes DNXC curves on catalog p.73 and can supply the curve for any DNX rating proposed. If you have the original fuse's curve, send it; the comparison is then direct.

Step 4: interrupting rating and fuse class

Compare interrupting ratings on the same basis. DENCO lists the DNX at 50 kA asymmetrical, the DNXC at 50 kA, and Types 1 to 9 at 40 to 65 kA symmetrical. The IEEE C37.48.1 tutorial notes that a fully asymmetrical crest can be about 2.8 times the rms symmetrical value against 1.414 for a symmetrical one, so a symmetrical and an asymmetrical figure are not interchangeable. If the old fuse or the fault study does not say which basis it uses, ask.

Fuse class matters as much. A full-range current-limiting fuse, by the C37.48.1 tutorial's definition, interrupts any current from the minimum that melts it up to its maximum interrupting rating; a back-up fuse needs another device below its minimum interrupting current. Replacing a full-range fuse with a back-up fuse in a bank whose faults are limited by the capacitors to a few times load current would leave a gap. DENCO's catalog calls the DNXC full-range; for any DNX rating, ask DENCO which class applies.

Step 5: mounting and physical fit

Only after the electrical match comes the mechanical one, though in practice it is often the hardest constraint. The DNX is clip style, with threaded end studs available; the DNXC mounts from a bus bar to the capacitor lead. Clip spacing, ferrule diameter, stud thread, length and the clearance to neighboring fuses all need to fit the existing enclosure or rack, and DENCO confirms dimensions for the specific part rather than from a catalog table.

Check the indicator as well. If the maintenance routine depends on seeing which fuse has operated, the replacement needs an indicator; DENCO's C-rated fuses are offered with or without one. Where no listed part fits the hardware, adapting the mounting and a custom fuse are both options DENCO engineering can price.

Old drawings and superseded standards

Replacement requests often come with a specification written years ago, and the standards it cites may have moved. IEEE C37.43-2008, which covered expulsion, current-limiting and combination-type external capacitor fuses from 1 kV to 38 kV, was superseded by IEEE C37.42-2016, which also absorbed C37.46 and C37.47. C37.42-2016 is itself now marked superseded. The fuse definitions once in IEEE C37.40 now live in the C37.41 family, whose current edition is C37.41-2024.

Capacitor limits have moved too. IEEE's capacitor subcommittee explained that the continuous current capability of 180% in earlier editions of IEEE 18 was changed to 135% in IEEE 18-2002; IEEE 18-2025 is the current edition. NEMA CP 1-2000 (reaffirmed 2008) is still cited on many drawings. When a spec cites an old edition, send it anyway; DENCO engineering will tell you what it can test or confirm against.

What DENCO sends back

The outcome of a review is one of three answers. The first is a listed DENCO part, with the reasoning: the class, the rating against the calculated minimum and the ambient, and the curve to plot against the capacitor's case rupture curve. The second is a listed part with a condition, such as a mounting adapter or a note that the protection engineer should confirm a coordination margin. The third is a custom design, where no listed part meets the electrical and mechanical constraints together.

In each case the final decision stays with the bank owner and the engineer who signs the protection study. DENCO engineering can work with that engineer directly on curves and ratings, and the 30-day lead time applies once the part is agreed.

Common mistakes when replacing an obsolete capacitor fuse

These are the errors that turn a quick replacement into a repeat failure.

• Matching voltage and current figures from the label and skipping the curve comparison.
• Treating a C number, an ampere rating and an expulsion link rating as the same scale.
• Replacing a full-range fuse with a back-up fuse, or the reverse, without noticing.
• Comparing an asymmetrical interrupting rating with a symmetrical fault current.
• Copying the original rating into a hotter enclosure without derating.
• Changing voltage class to suit an available part without checking the voltage the fuse sees when it clears.

Frequently asked questions