Full-Range vs Back-Up Current-Limiting Capacitor Fuses

Where the classes are defined

The North American class definitions for high-voltage current-limiting fuses were published in IEEE C37.40, Service Conditions and Definitions for High-Voltage Fuses. IEEE lists the 2003 edition as superseded by IEEE C37.41-2016, so the definitions now sit in the C37.41 document family. IEEE C37.41-2024 is the current design-test standard for high-voltage fuses.

The working group that presented IEEE C37.48.1-2002 described it as the teaching document of the series: C37.40 holds definitions, C37.41 testing, C37.42 to C37.47 ratings and C37.48 the application guide, while C37.48.1 explains how and why the fuses work and how to use them. C37.48.1 is now superseded; IEEE C37.48-2020 is the active guide and tutorial for applying high-voltage fuses. The definitions below are quoted from that working group's tutorial.

IEEE C37.42-2016, also now marked superseded, covered external capacitor fuses rated 1 kV through 38 kV, including expulsion, current-limiting and combination types, for shunt capacitors built to IEEE 18 and NEMA CP 1.

The three current-limiting classes

All three classes are current-limiting at high fault currents. They differ in what happens at currents just above melting, which is exactly the region where many capacitor faults start.

The minimum interrupting current of a back-up fuse

For a back-up fuse, the tutorial defines the minimum interrupting current as the current specified by the manufacturer and marked on the fuse. Between the current that first melts the element and that marked minimum lies a band where the fuse has melted but may not clear. A back-up fuse is safe to apply only if every current that can melt it is either above its marked minimum or interrupted by another device first.

IEC practice uses the same three ideas under IEC 60282-1. One manufacturer's IEC technical guide summarizes them: back-up fuse-links have a minimum breaking current of approximately four times rated current, general-purpose fuse-links break currents that melt them in one hour, and full-range fuse-links break any current that can melt them. IEC 60282-1:2020 is the current edition; its revision adjusted Series II voltages to North American system voltages and clarified fuse-links used above 40°C.

Why low currents matter in capacitor banks

Low-current faults are normal in capacitor work. One manufacturer's capacitor fuse selection guide notes that in ungrounded banks, or banks with more than one series group, faults are capacitively limited and the available fault current is typically less than two or three times the capacitor bank load current. A pole bank fusing guide from a capacitor maker says operating a bank ungrounded wye limits fault current to about three times line current.

Failures also start small. A relay maker's paper describes progressive failure in an externally fused unit: an element failure welds the foils together and shorts the other elements in its group, a second failure raises the voltage on the remaining elements further, and successive failures within the same unit make the fuse operate. Another paper adds that with all-film units the external fuse does not melt with the additional current from the first failures, so the unit stays in service until further series elements fail. The fuse sees a rising current, not a step to full fault current.

The same pole bank guide notes a conflict in ungrounded wye banks: after one unit shorts, the healthy units see line-to-line voltage, 1.73 times their rating, which pushes for a fast-clearing fuse, while the transient-current criterion calls for a slow one. A fuse that can clear anywhere above its melting current handles the low end of that range without help.

Worked example: a capacitively limited fault against a back-up minimum

Take a 600 kvar pole bank of three 200 kvar, 7,200 V units, connected ungrounded wye. The example uses the published 125% group-fusing figure for ungrounded wye banks and the IEC guidance that a back-up fuse-link's minimum breaking current is about four times its rating.

Here the highest fault current the bank can deliver is below the back-up fuse's minimum interrupting current. A current in that band can melt the element over time, but a back-up fuse is not rated to clear it. Either a full-range fuse or a partner device rated for the low end is needed. The numbers are illustrative; the protection study sets the real ones.

Where each class fits in a bank

The bank arrangement decides which end of the current range is the hard one. In inductively limited banks, single series group grounded wye or delta, the fault can be the full system fault current, and the maximum interrupting rating and let-through energy decide. In capacitively limited banks the low end decides.

Metal-enclosed banks commonly use current-limiting fuses on each unit. One metal-enclosed bank maker says each capacitor is individually fused with current-limiting fuses to improve bank reliability and protect against case rupture, and that its harmonic filter banks use 40kA current-limiting fuses with blown-fuse indicators.

Back-up fuses with a partner device

Back-up fuses are not wrong for capacitors; they are used with a partner. Published capacitor fuse guides describe combination designs that pair a current-limiting section with an expulsion section, so the expulsion part clears low currents and the current-limiting part handles high fault currents and parallel energy. One public utility's bill of materials for a 600 kvar pole bank lists an 8.3 kV, 30 A combination expulsion and current-limiting fuse that fits a universal cutout, adopted in 2015 in place of separate expulsion and current-limiting fuses.

One manufacturer also describes a long-element current-limiting design that chops current. It can be used with essentially unlimited parallel energy, but it cannot be used where faults are inductively limited, and its guide says it should not be used on single series group grounded wye or single series group delta banks.

The application rule for any combination is that the partner device must clear every current below the back-up fuse's marked minimum. That is a coordination check, and it belongs in the protection study.

Fuse class is separate from power class and distribution class. The C37.48.1 tutorial describes power class fuses as designed and tested for feeders and locations in or close to the substation, where X/R ratios are high, and distribution class fuses for locations some distance from the substation.

Temperature and the full-range definition

The full-range definition includes a temperature condition: the fuse interrupts any current down to the minimum that can melt the ribbon at the maximum application temperature. A fuse in a hot enclosure melts at a lower current than its catalog rating suggests, so the low end of its range moves too.

DENCO's DNXC catalog (p.74) requires derating above 25°C with AT = A25 [1 - 0.0065 (T - 25)]. At 55°C the factor is 1 - 0.0065 x 30 = 0.805, so a 40 A DNXC8 carries 32.2 A. Apply the derated figure in the continuous-current check and give the protection engineer the maximum ambient with the fuse data.

Where DENCO fuses sit

DENCO's catalog describes the DNXC series as full-range, current-limiting capacitor fuses for bus bar to lead mounting, rated 8.3 kV to 23 kV and 10A to 65A at 50kA. Ratings are 10, 18, 25, 30, 40, 45 and 65 A at 8.3 kV; 10, 12, 18, 25, 30, 35 and 43 A at 15.5 kV; and 12, 18 and 25 A at 23 kV. In the catalog's own words, DNXC fuses were tested at IPH to IEC 60282-1: 2009-10 and IEC 60282-2: 2008-04.

The DNX series is described as current-limiting, clip-style capacitor fuses, rated 2.8 kV to 38 kV. The catalog data used for this guide does not state a class for each DNX rating. If your application depends on interrupting low currents, ask DENCO engineering to confirm the class and, for any back-up rating, the marked minimum interrupting current.

Common mistakes

Class errors usually come from checking only the top of the current range.

• Checking only the maximum interrupting rating and ignoring the low-current end in a capacitively limited bank.
• Applying a back-up fuse without a partner device rated to clear below its marked minimum.
• Treating full-range as a speed rating. It describes the interrupting range; the time-current curve shows speed.
• Assuming every current-limiting capacitor fuse is full-range because one product line is.
• Ignoring enclosure temperature, which moves the melting current the full-range definition refers to.
• Using a current-chopping design on a single series group grounded wye or delta bank.

Frequently asked questions