Clip-Style vs Bus-Mounted Capacitor Fuses

Two ways to put a fuse on a capacitor

Capacitor fuses reach the circuit in one of two ways. A clip-style fuse has cylindrical end ferrules that press into spring clips on an insulated mounting, the way many indoor current-limiting fuses mount in switchgear. The mounting is wired to the capacitor terminal and the bus. A bus-mounted fuse has terminals on its end caps that bolt to the bank's bus bar at one end and to the capacitor terminal lead at the other, so the fuse is a structural part of the connection.

In externally fused substation banks, a relay maker's paper describes the arrangement: an individual fuse, externally mounted between the capacitor unit and the capacitor bank fuse bus, typically protects each unit. That is the setting bus-mounted capacitor fuses are built for.

Both approaches appear in DENCO's catalog. The DNX series is listed as clip-style capacitor fuses from 2.8 kV to 38 kV, with threaded end studs also available. The DNXC series is listed for bus bar to lead mounting from 8.3 kV to 23 kV, and the catalog includes a DNXC dimension drawing for that arrangement on p.74.

Clip-style mounting: what has to match

With a clip-style fuse, the fuse and mounting must match on more than voltage and current. One manufacturer's clip-style current-limiting fuse catalog explains that each fuse is marked with a mounting code number, that the code defines the mounting's insulation level, contact spacing and contact type, and that the code on the fuse and the mount must be the same. It also offers extenders to fit a shorter fuse into a longer mounting. That shows what a replacement has to reproduce.

DENCO's catalog lists a maximum body diameter for each DNX size, so check clearances between adjacent fuses and to grounded metal using the maximum, not a nominal figure. Where existing clips are worn, discolored or loose, service or replace them with the fuse; a poor clip contact runs hot.

• Ferrule diameter and overall length
• Clip center-to-center spacing
• Insulation level of the mounting
• Clearance to adjacent phases and grounded metal, using the maximum body diameter
• Whether the mounting needs threaded studs instead of ferrules

Bus bar to lead mounting: what has to match

A bus-mounted fuse replaces a length of conductor, so the fit questions are mechanical and electrical at once: the bolt size and thread at each end, the overall length between the bus bar and the capacitor terminal, and the spacing to the next fuse in the row. Published bus-mounted capacitor fuses are aimed at fusing individual capacitor units in outdoor and enclosed banks, each unit carrying its own fuse from a common bus.

Published manufacturer drawings for bus-mounted capacitor fuses show a recommended minimum spacing between fuses and a specified bolt thread. Spacing is not only a clearance question: one maker rates the parallel-energy capability of its capacitor expulsion fuse tubes for use with the unit spacing recommended per IEEE Std 18-2012. Use DENCO's DNXC drawing for DENCO fuses, and confirm dimensions with DENCO engineering, because extracted DNXC dimensions have not been verified for this guide.

One capacitor fuse maker notes that its fuse type is typically mounted directly on the capacitor, and that its capacitor fuses are non-disconnecting, requiring a separate disconnect switch. Whatever the mounting, a capacitor fuse is not a switch unless its catalog says so.

Single-bushing and two-bushing units

The capacitor itself shapes the mounting. Single-phase units come with one or two bushings. A municipal utility's purchase specification asks for a single bushing with a ground stud for use on its 14.4 kV wye system, or two bushings for use on its 7.2 kV delta system.

Two-bushing units on delta or ungrounded wye systems raise a question the mounting must answer. One capacitor maker's instructions warn that for single-phase two-bushing capacitors fused on only one terminal, and applied on delta or ungrounded wye systems, an internal ground fault from the unfused phase to case might result in case rupture. The same warning covers three-phase capacitors fused on only two terminals. Whether to fuse both terminals is a bank design decision; raise it with the bank designer before choosing hardware.

The mounting's insulation level must suit the unit too. One maker catalogs its units from 2,400 V through 14,400 V at 95 kV BIL, with 125 and 150 kV BIL options at 11,400 to 14,400 V. Clip mountings and bus supports need clearances that match.

Worked example: unit fusing vs group fusing on a 1,200 kvar bank

Mounting style follows the fusing scheme. USDA RUS Bulletin 1724D-112 notes that pole-mounted units are not normally fused individually but by phase, with group fuses in cutouts. Published metal-enclosed designs and externally fused substation banks put a fuse on each unit. One utility's pole bank standard builds a 1,200 kvar bank on its 12,470 V grounded wye system from two 200 kvar, 7.2 kV units per phase.

The comparison uses the published 135% group-fusing figure for grounded wye pole banks and the 1.35 unit-fuse margin from published manufacturer guides, both at 25°C. Unit fusing doubles the fuse count and puts one fuse on every unit, which is what lets the operated fuse identify the failed unit. Group fusing uses the cutouts the pole already needs. Neither choice changes the case rupture and outrush checks, which still apply to each fuse.

Connections, torque and hot spots

A capacitor fuse carries load current continuously, so its connections matter as much as its rating. A maker of switching and protection equipment explains that loose button-head, cap or rod connections create a hot spot that makes a fuse link behave as though it is heavily preloaded, so a transient current that would not normally affect the link causes it to blow unexpectedly. It adds that this is most common on capacitor bank fuses, because their continuous currents are higher.

Torque values come from the equipment documentation. One metal-enclosed bank maker's torque table lists 17 lb-ft for capacitor unit bushing terminals and 20 lb-ft for power fuse terminals. A capacitor maker's instructions say tightening terminal nuts to more than 25 foot-pounds is not required or recommended. Another capacitor maker recommends checking connection torque after the first four weeks and every 24 months after that.

The same documents call for periodic bushing cleaning in contaminated atmospheres to prevent arc-over. Use the torque values the bank and fuse makers publish for your hardware; do not carry a figure from one product to another.

Indication and inspection

Mounting style affects how a blown fuse is found. DENCO's catalog offers its C-rated fuses with or without a blown fuse indicator. Published designs use an indicator button that protrudes when the fuse has operated, or an ejector spring that throws the link's leader out to give a position indication; a relay maker's paper notes that in an externally fused bank the spring-loaded fuse pops out when a failure occurs.

Thermography finds what eyes miss. A test-equipment maker advises energizing the bank for at least an hour before an infrared survey, notes that a blown fuse shows as a temperature difference and that some units have indicators while others do not, and gives twice a year as a typical preventive maintenance interval. A relay maker's paper adds a caution: intact fuses do not necessarily mean the units are healthy, and a failed fuse does not necessarily mean the unit has failed, so capacitance measurement is the full check.

Consider who inspects the bank and from where. An indicator visible from outside the enclosure or from the ground saves a shutdown just to find the operated fuse.

Choosing between clip and bus mounting

In replacement work the existing equipment has usually made the choice: replace like with like and match the fit. In new equipment, the table lists the practical differences. DENCO's catalog lists its C-rated fuses for indoor use, or outdoor use within an enclosure.

Mounting does not change the electrical selection. Voltage class, current rating, interrupting rating and the outrush and case rupture checks are the same whichever way the fuse is held. Settle the electrical rating first, then pick the mounting that fits the equipment, and send DENCO a drawing or photographs of the existing arrangement with the request for quote.

Common mistakes

Mounting errors tend to show up months later as hot connections or nuisance operations.

• Matching voltage and current but not ferrule size, length, clip spacing or insulation level.
• Reusing worn or discolored clips with a new fuse.
• Carrying a torque value from one product's manual to another's hardware, or over-tightening capacitor bushings.
• Fusing one terminal of a two-bushing unit on a delta or ungrounded wye bank without the bank designer's review.
• Treating a non-disconnecting capacitor fuse as a means of isolation.
• Taking dimensions from another manufacturer's catalog instead of the installed equipment and DENCO's drawing.

Frequently asked questions