Buying Capacitor Fuses: Lead Time, Custom Builds and Testing

Who you are buying from

DENCO Fuses, Inc. is based in Miami, Florida. It was incorporated in Florida in 1988 as DenMar Transit Engineering, later DenMar Fuses. The team has over 100 years of combined experience, solely in manufacturing electrical fuses.

DENCO serves utilities and power companies, OEMs and specialty custom design customers, electrical contractors and transit. Its C-rated capacitor fuse family covers capacitor and capacitor bank protection, power factor correction equipment, harmonic filtering equipment, induction heating systems, and high power drives and welders, for use indoors or outdoors within an enclosure. The family is current-limiting with silent operation and no discharge, and is offered with or without a blown fuse indicator.

For a buyer, the practical points are what DENCO can take on and how to reach the people who answer technical questions: catalog capacitor fuses from 1 kV to 38 kV, custom designs for OEM and specialty work, in-house testing, and third-party testing on request. The sections below take each in turn, then cover spares, purchase order wording and the mistakes that slow orders down.

What the catalog covers

Before asking for a custom build, check whether a catalog series already fits. The table summarizes DENCO's capacitor fuse series as printed in the catalog. Ratings outside these rows, or a mounting the catalog does not show, are the starting point for a custom request.

Lead time and planning

DENCO quotes a 30-day lead time on all products. Lead times for medium voltage fuses are commonly longer, which matters most when a bank is running on its contingency margin with fuses out, or when an OEM build is waiting on one line item.

Plan around the 30 days rather than assuming stock. The cost of waiting is easiest to see from the bank side. A relay maker's unbalance paper gives the voltage rise on healthy units in a grounded wye, externally fused bank after F units in one group are lost as S x P / (S x P - F x (S - 1)). With 3 series groups of 8 units, one open fuse puts the healthy units in that group at 24 / (24 - 2) = 1.091 times their normal voltage. A second open fuse in the same group gives 24 / 20 = 1.20, above the 110% contingency figure IEEE's capacitor subcommittee described for IEEE 18-2002 units. A bank in that state usually trips, and stays out until fuses arrive.

If the request is a custom design, the schedule is set during quoting, so say when you need the fuses.

Planning spares: a worked inventory

No standard sets a spares quantity for capacitor fuses; it is the owner's decision. What helps is a clear count of positions by rating, because a substation or plant often uses several ratings that look alike. The example below is illustrative.

Take a site with two banks. Bank 1 is a 12.47 kV grounded wye bank of 7200 V, 200 kvar units, 4 per phase, in an enclosure that reaches 40 °C. Bank 2 is a 24.94 kV grounded wye bank of 14400 V, 300 kvar units, 3 per phase, outdoors at a 40 °C maximum. Working through the DNXC selection by kvar method in this guide, Bank 1 lands on DNXC8-45 (27.8 A unit current, 37.5 A minimum, 45 x 0.9025 = 40.6 A) and Bank 2 on DNXC15-35 (20.8 A, 28.1 A minimum, 35 x 0.9025 = 31.6 A). That is 12 positions of one part and 9 of another, so spares have to be held for both. Both results are calculated starting points that DENCO and the bank's protection engineer still confirm.

Order the spares with the first lot so they come from the same build and paperwork. Store them as the fuse maker advises: one fuse maker's instructions call for a clean, dry storage area and an inspection for damage and missing parts on receipt.

Custom designs

Not every bank fits a catalog row. A voltage class, current rating, mounting or end connection that the catalog does not list can be reviewed as a custom build. DENCO's catalog pages for its 1 kV to 5.5 kV capacitor protection fuses (Types 1 to 9) note that they can be designed to customer specifications, and the DNX catalog notes threaded end studs as an alternative to clip ends.

For a custom request, send the same application data you would for a catalog part, plus what the listed parts do not meet: physical envelope, terminal style, rating, or test requirement. Common triggers are a calculated DNXC requirement above the top rating in a class (65 A at 8.3 kV, 43 A at 15.5 kV, 25 A at 23 kV), a bank with an unusual unit voltage, or an existing enclosure whose clips or bus spacing no current part fits.

Testing

DENCO tests fuses in house at its factory lab. When a project requires independent results, DENCO can arrange third-party testing at IPH Berlin in Germany on client request. Say so in the RFQ, with the test standard and the tests you need, so the testing is scoped into the quote and schedule.

For the DNXC series, the DENCO catalog states the fuses were tested at IPH to IEC 60282-1: 2009-10 and IEC 60282-2: 2008-04. That is the catalog's own wording. For other ratings or for test reports, ask DENCO what exists and what would need to be run.

Routine production testing is a separate question from design testing. One fuse maker describes pulsing each current-limiting fuse twice with a pulse calibrated to 95% of its published minimum melt I2t, and notes that a resistance check alone cannot find a damaged element. If your specification needs routine test records for each fuse, say so in the RFQ and ask DENCO what its factory lab performs and documents.

It helps to name the right standard. IEC 60282-1 covers high-voltage current-limiting fuses, and IEC 60549 covers fuses for the external protection of shunt capacitors, with capacitor fuse-links expected to meet IEC 60282-1 plus the additional tests of IEC 60549. In North America, IEEE C37.41 sets design tests for high-voltage fuses, including interrupting, temperature-rise and time-current tests. Write the edition you need into the purchase order.

Quality system wording

DENCO fuses are manufactured in an ISO 9001:2015 certified facility. If your supplier qualification form asks for quality system details, ask DENCO for the documents your form requires.

For capacitor fuses in the Types 1 to 9 series, the catalog states they are designed to ANSI C37-41 specs. That is the catalog's wording, not a listing or certification claim.

Replacement orders vs new designs

A replacement order for an existing bank and a specification for new equipment need different information, and it helps to say which one you are sending.

For a replacement, the most useful items are the existing part number exactly as marked, photos of the fuse label and mounting, the capacitor nameplate, and the number of positions. DENCO engineering will confirm a replacement from that data. No cross-reference is assumed from a part number alone.

For new equipment, start from the bank design: capacitor unit voltage and kvar, connection and grounding, units in parallel, available fault current, harmonic and switching duty, enclosure ambient and the mounting you intend to use. In both cases, name the person who signs off the protection study, so DENCO engineering can work directly with that engineer on curves and ratings.

Writing the purchase order

A capacitor fuse purchase order that names the application as well as the part avoids most disputes at receipt. The items below are a practical minimum; your own procurement rules may add more.

Where the order is for a replacement in a bank with a history of fuse operations, add a note of the cause found, so the supplier knows whether the duty has changed since the original fuses were chosen.

• Full catalog number for each position, with the exact C number for any DNX row.
• Capacitor unit voltage and kvar the fuse is for, and the maximum enclosure ambient used in sizing.
• Interrupting rating required and its basis, symmetrical or asymmetrical.
• Mounting style and end connections, and whether a blown fuse indicator is required.
• Quantity per rating, spares, and the delivery date.
• Any test standard, edition and witness or third-party testing requirement.
• Documents required at shipment, such as curves or test records, agreed during quoting.

Common mistakes when buying capacitor fuses

These cost the most time after the order is placed.

• Ordering exact quantities with no spares, then waiting a full lead time after the first failure.
• Writing "ISO certified" or a listing claim into a specification from a catalog phrase. Quote the catalog wording as written.
• Asking for third-party testing after the order instead of in the RFQ.
• Treating two ratings that look alike as one stock item across banks.
• Specifying a superseded standard edition from an old drawing without checking the current one.
• Leaving the enclosure ambient off a DNXC order, so the rating is chosen for 25 °C.

Frequently asked questions