Replacing Blown Capacitor Fuses Safely

Make the bank safe before anything else

A capacitor that has been disconnected can still hold a dangerous charge. OSHA 29 CFR 1910.269(w)(1) sets the sequence for work on capacitors. Follow it, together with NFPA 70E and your own site procedure, before you open an enclosure or reach for a fuse.

Two further points come from the USDA Rural Utilities Service capacitor bulletin. It is not good practice to rely on a capacitor's internal discharging resistors, and units removed from service should stay shorted to their cases because capacitors tend to accumulate a residual charge. NFPA 70E has included requirements for work on capacitors in Article 360 since its 2021 edition.

For scale, a single 50 kvar, 7200 V unit has a capacitance of about 2.56 microfarads. Charged to the crest of its rated voltage, about 10,182 V, it stores 0.5 x 2.56 x 10^-6 x 10,182^2, or roughly 133 joules. A group of units in parallel stores many times that.

• Disconnect the capacitors from energized sources and short circuit them. The employee applying the short circuit waits at least 5 minutes from the time of disconnection (OSHA 1910.269(w)(1)(i)).
• Before handling units, short circuit each unit in series-parallel banks between all terminals and the capacitor case or its rack. Bond ungrounded substation racks to ground (OSHA 1910.269(w)(1)(ii)).
• Short circuit any line connected to capacitors before treating it as deenergized (OSHA 1910.269(w)(1)(iii)).
• In California, Title 8 Section 2940.17 requires a means to reduce residual voltage to 50 volts or less within 5 minutes, and states that the internal discharge device is not a substitute for externally short circuiting and grounding.
• Treat a unit behind a blown fuse with extra care. The fuse may have isolated it from the bank's discharge path, and a bulged unit should be left to cool before handling, as one capacitor maker's instructions advise.

Find out why the fuse operated

In an externally fused bank the unit fuse is the first line of protection. IEC 60549 describes unit fuses as clearing a fault inside a capacitor unit so the rest of the bank can stay in service. A blown unit fuse therefore usually means the unit behind it failed, but not always. The table groups the causes named in utility and manufacturer literature with the check each one suggests.

One capacitor maker's troubleshooting table puts it in three lines for a blown capacitor fuse: check for a shorted capacitor unit, check for a short circuit, and treat frequent unexplained fuse blowing as a possible sign of parallel resonance that calls for a harmonic analysis.

Test the capacitor unit, not only the fuse

The USDA RUS bulletin says capacitors suspected of damage should be tested with a commercially available capacitor checker, and that a unit whose measured capacitance varies significantly from the rated value should be retired. It also notes that slight bulging or blooming of a tank is not necessarily a sign of failure; failure usually shows as a severely bulging or ruptured case, or a blown fuse.

A relay maker's paper co-written with a utility adds a caution that runs both ways: intact fuses do not necessarily mean the capacitor units are in perfect condition, and a failed fuse does not necessarily mean the unit has failed. Finding partly failed units takes a capacitance measurement of all units, not just the one behind the blown fuse.

To check a reading, compute the rated capacitance from the nameplate. For a single-phase unit, C in microfarads equals kvar x 1000 divided by (2 x pi x f x kV squared). A 200 kvar, 7200 V unit at 60 Hz works out to 200,000 / (377 x 7.2^2), about 10.23 microfarads. Compare your measurement with that figure and with the capacitor manufacturer's tolerance.

Check what the open fuse did to the rest of the bank

When a fuse removes a unit, the remaining units in that parallel group carry a larger share of the group voltage. A relay maker's unbalance paper gives the factor for a grounded wye, externally fused bank as kOV = S x P / (S x P - F x (S - 1)), where S is the number of series groups, P the units in parallel per group and F the failed units in one group.

With 4 series groups of 10 units, one open fuse gives 40 / (40 - 3) = 1.081, an 8.1% rise on the healthy units in that group. A second failure in the same group gives 40 / (40 - 6) = 1.176, which is above the 110% figure that IEEE's capacitor subcommittee described as a contingency limit for IEEE 18-2002 units. Typical practice in an informal subcommittee survey was to alarm at 105% of rated voltage and trip above 110%.

So before re-energizing, read the unbalance relay record. If more than one fuse in a group has operated, the units that stayed in service may have spent time above their rating, and that is a reason to test them as well.

What the replacement has to match

Start from the original fuse specification and the bank design, not just the label on the blown fuse. If the original sizing basis is not on file, rebuild it before ordering. The checklist below is the minimum.

• Voltage class, for example 8.3, 15.5 or 23 kV, chosen against the capacitor unit voltage and the bank connection.
• Current rating in the same terms as the original: a C number for C-rated fuses such as the DNX, or amperes for fuses such as the DNXC.
• Interrupting rating, and whether it is stated as symmetrical or asymmetrical. Do not convert between the two.
• Fuse class: full-range or back-up current-limiting, or expulsion. A back-up fuse relies on another device for low-current faults.
• Mounting: clip style, bus bar to lead, or end studs, plus the clearances the enclosure allows.
• Blown fuse indicator, if the bank's inspection routine depends on one.
• Enclosure ambient. DNXC fuses, for example, are derated above 25 °C with the DENCO catalog formula AT = A25 [1 - 0.0065 (T - 25)].

The fuses that did not operate

When a unit fails, the healthy units in parallel discharge into it through their own fuses. A widely cited protection paper states that the fuses on healthy units must not blow when discharging into an adjacent faulted capacitor. One manufacturer's capacitor fuse selection guide adds that each time a fuse element is heated near its melting point, the fuse deteriorates slightly.

That damage is hard to see. One fuse maker's note on I2t testing reports that a nick halfway from the element edge to a hole cut minimum melt I2t by 44% while changing resistance only 0.75%, so a resistance check alone will not find it. The same maker offers an inrush withstand test for suspect current-limiting fuses.

Whether to replace neighboring fuses after an event is a judgment for the bank owner, working with the capacitor and fuse manufacturers. This guide does not give a rule for it. Record which positions saw the event so the decision can be made with the data.

Replacing another manufacturer's fuse with a DENCO fuse

Do not assume that a DENCO part is equivalent to another maker's part because the voltage and current figures look alike. Melting curves, clearing behavior, fuse class and mounting can all differ.

Send DENCO the existing part number exactly as marked, photos of the label and mounting, the capacitor unit voltage and kvar, the bank connection, the available fault current and the enclosure ambient. DENCO engineering will review the data and confirm a replacement, or tell you what else is needed. The cross-reference and replacement article in this guide sets out how that review works.

A replacement sequence, step by step

Put together, a replacement usually runs in this order. Adapt it to your site procedure; it does not replace one.

• Disconnect, lock out, wait at least 5 minutes, then short circuit and ground the bank as OSHA 1910.269(w)(1) describes.
• Record which positions operated, the indicator state of each fuse and any visible damage, with photos.
• Test the capacitance of the unit behind each blown fuse and of the other units in the same group.
• Read the unbalance relay record and work out how long the healthy units ran with fuses open.
• Find the cause: failed unit, lightning, resonance, loose connection or switching duty.
• Confirm the replacement against the original specification, the enclosure ambient and the case rupture curve.
• Torque connections to the capacitor maker's values, clear the grounds under your procedure and re-energize.
• Recheck phase currents and the unbalance signal after the bank returns to service.

Common mistakes when replacing capacitor fuses

Most repeat failures after a fuse change trace back to one of these.

• Fitting a new fuse on a failed unit, which exposes the new fuse and the bank to the same fault.
• Relying on the capacitor's internal discharge resistor instead of shorting and grounding.
• Fitting a larger fuse to stop repeat operations without finding the cause or rechecking the case rupture curve.
• Replacing a C-rated fuse with an ampere-rated fuse of the same number, or the reverse, without comparing curves.
• Returning a bank to service without reading the unbalance relay record or checking how many fuses in the group operated.
• Ignoring the enclosure temperature when the original fuse was sized for 25 °C.

Frequently asked questions