Fusing Pole-Mounted Capacitor Banks

How pole banks are built and fused

Pole banks are small, standardized and numerous. One public utility's material standard calls for pre-assembled banks of 300, 450, 600, 900, 1,200 and 1,350 kvar on a 12,470 V four-wire grounded-wye system, with 7.2 kV, 95 kV BIL, all-film units. One capacitor maker's racks hold 3, 6, 9 or 12 single-phase units for systems from 2,400 V to 34,500 V. The RUS bulletin's fusing table covers banks of 150 to 900 kvar at 12,470/7,200 V and 300 to 900 kvar at 24,900/14,400 V.

The RUS bulletin states the fusing practice plainly: capacitor units are not normally fused individually, but by phase, in a bank of one or more capacitors per phase. The fuse is usually an expulsion link in a cutout, which also serves as the bank's switching and isolating point. Note that IEEE C37.99, the bank protection guide, excludes pole-mounted banks on distribution circuits from its scope, so practice here comes from utility standards and manufacturer guidance.

Continuous rating rules for group fuses

Published rules differ in wording but share a target: the link must carry the bank's worst steady current without aging.

• RUS: protection should have a nominal rating of 135% to 140% of the capacitor line current; fusing is normally designed to operate at 120% to 135% of nameplate current, occasionally derated to no less than 108% for multiple units per phase.
• RUS: a link should melt in 5 minutes at 150% to 300% of its rated current; minimum 5 A; in high-lightning areas 15 A or larger, but no greater than 140% of the capacitor's nameplate current. The bulletin adds that the choice should always be based on the particular fuse's time-current characteristics.
• One capacitor maker: continuous capability of at least 135% of rated current for grounded wye banks and 125% for ungrounded wye, because group fuses must withstand normal current including harmonics, inrush and outrush. A municipal utility's protection standard uses the same 135% and 125%.
• Link capability: the same maker says tin T and K links carry 150% of their rating continuously, while silver links carry 100%.
• RUS: sizing must allow for the 135% capability, overvoltage, harmonics and actual kvar on the high side of manufacturing tolerance.

Worked example: checking the RUS T-link table

Per-phase current for a grounded wye bank is (bank kvar / 3) / kV line to neutral. For a 600 kvar bank at 12,470/7,200 V: 200 / 7.2 = 27.8 A. The 135% target is 1.35 x 27.8 = 37.5 A. RUS Table 3 lists a 25T link. A tin 25T carries 1.5 x 25 = 37.5 A continuously, which meets the target exactly.

The table below repeats the RUS 12,470/7,200 V rows and adds the same check for each. Every listed link carries at least 135% of capacitor current continuously when it is a tin link; a silver link of the same number would carry only its rating, so the metal matters as much as the size. The RUS 24,900/14,400 V rows follow the same pattern: 300 kvar at 6.9 A uses an 8T, 450 kvar at 10.4 A a 10T, 600 kvar at 13.9 A a 15T, and 900 kvar at 20.8 A a 20T.

Grounded wye or ungrounded wye

The connection changes both the fault current and the voltage the healthy units see. One capacitor maker's group fusing note says operating a pole bank ungrounded wye limits fault current to about three times line current. The catch is voltage: after one unit shorts, the healthy units in the other phases see line-to-line voltage, 1.73 times their rating. That pushes for a fast-clearing link such as a K, while the transient-current criterion dictates a slow-clearing link such as a T. On a grounded wye bank, the fault current through a shorted unit is limited only by the system, which is what brings the tank rupture curve into play.

Utilities pick one or the other by system. A municipal utility's standards connect banks on its 7.2 kV ungrounded delta system ungrounded wye, and banks on its 14.4 kV solidly grounded wye system grounded wye. The RUS bulletin cautions that floating a wye point to block harmonic ground current is a last resort, because ungrounded cases can become energized to primary voltage, and that with delta or ungrounded wye banks, line voltage can feed back through the capacitor's common connection to an open phase. A relay maker's paper adds a grounded wye drawback: circulating inrush and harmonic currents may cause misoperation of relays and fuses.

The tank rupture limit on link size

Capacitor makers publish tank rupture curves, and one maker's coordination rule is that the fuse maximum clear curve must fall below the tank rupture curve at and below the available fault current, with the rupture curve compensated for asymmetry at high fault currents. Its curves for standard and heavy duty units carry a minimum of 2,000,000 A2s for fault currents of 10,000 A or less, and note a minimum clearing time of 0.8 cycle for safe coordination with expulsion fuses. For scale: 2,000,000 A2s at 10,000 A is 0.020 s, about 1.2 cycles at 60 Hz, and 0.8 cycle is 13.3 ms.

That maker lists the largest links that coordinate with its units and the fault current limits that go with them. A municipal utility's protection standard states the consequence directly: fusing greater than 100K is not allowed due to the capacitor tank rupture curve. Above those fault levels, the maker lists three options: current-limiting fuses, ungrounded wye operation, or moving the rack to a location with less fault current.

Current-limiting and combination fuses on pole banks

Where available fault current exceeds what an expulsion link can coordinate with, the capacitor maker's note lists current-limiting fuses as one remedy. Some utilities have moved that way as standard. One public utility's 600 kvar bank, built from three 200 kvar, 7.2 kV units, uses polymer cutouts rated 100 A, 16 kA asymmetrical, with an 8.3 kV, 30 A combination expulsion and current-limiting fuse that fits a universal cutout; the utility changed to that fuse from separate expulsion and current-limiting fuses in 2015. Checking it with the arithmetic above: 27.8 A x 1.35 = 37.5 A, and the fuse's continuous capability has to be confirmed against that from its maker's data, since the 150% tin-link rule does not apply to current-limiting elements.

DENCO's C-rated capacitor fuses are current-limiting, and DENCO's catalog lists them for indoor use, or outdoor use within an enclosure. The DNX series covers 2.8 kV to 38 kV with a 50 kA asymmetrical interrupting rating; the DNXC series covers 8.3 kV to 23 kV, 10 A to 65 A, 50 kA, for bus bar to lead mounting. For pole-top equipment, send DENCO the bank kvar, unit voltage, available fault current and mounting, and DENCO engineering will confirm what fits.

Transients: inrush, outrush and lightning

Pole banks are usually far from other banks, and the capacitor maker's note observes that back-to-back switching is seldom the case for pole-mounted capacitors. Isolated bank energization still drives inrush through the group fuse, and on grounded wye systems the bank discharges into nearby external faults. The maker lists capacitor bank switching, lightning surges and discharge through external faults as the transient sources group fuses must withstand, and warns that fuses can be damaged by high-magnitude, high-frequency currents.

Lightning gets its own rule in the RUS bulletin: if lightning causes excessive fuse blowing but there is no capacitor damage, replace the fuses with the next larger size, within the 140% of nameplate ceiling for high-lightning areas. Arrester selection supports the fuse. One maker's ordering guide pairs 7.65 kV MCOV arresters with 7,200 V line-to-ground systems and 15.3 kV MCOV with 14,400 V. A capacitor maker's instruction sheet says the arrester protective level for a 1.2 x 50 wave should be less than two-thirds of the capacitor BIL: for a 95 kV BIL unit, below 63.3 kV.

Switching, operation and safety at the pole

• Load-break capability: the RUS bulletin notes a 600 kvar bank at 7,200 V draws about 28 A and cannot be unloaded without load-break cutouts.
• Opening cutouts: a capacitor maker's instructions say cutouts used for switching should be opened as rapidly as possible to minimize arcing.
• Phase balance: the RUS bulletin says to install equal kvar per phase and take the whole bank out when one phase is out.
• Approach: one control maker advises looking for smoke, fire, open fuses, crackling or loud buzzing before approaching a pole bank control, and treating components as if at primary voltage if a fault is suspected.
• Grounding: one public utility's standard warns that capacitors must be grounded after they are de-energized because they stay charged for a long time. OSHA 1910.269(w)(1) requires a wait of at least 5 minutes after disconnection, then short circuiting before work.

Common mistakes

• Using a silver link where a tin link's 150% capability was assumed. The same number carries a third less continuously.
• Going up in link size to stop nuisance blowing without checking the tank rupture limit at the site's fault current.
• Relying on the largest expulsion link where fault current exceeds the limits in the capacitor maker's table. Use current-limiting fuses, ungrounded operation or a different location.
• Sizing a group fuse for an ungrounded wye bank as if it were grounded, or ignoring the line-to-line stress on healthy units after a short.
• Opening an energized bank of 28 A or more with a standard cutout instead of a load-break tool or cutout.
• Leaving a bank on line with one phase fuse blown.

Frequently asked questions