10.6 Capacitor Banks & Reactors: Inspection, Testing & Acceptance
Key Takeaways
- Domain III subdomain T covers capacitors and reactors together, while NFPA 70 splits them into Article 460 (capacitors) and Article 470 (resistors and reactors).
- NEC 460.6 requires a capacitor rated 1000 V and under to discharge to 50 V or less within one minute; NEC 460.28 allows five minutes for capacitors rated over 1000 V.
- The automatic discharge circuit is the equipment's code-required means of discharge, never the technician's protection — test dead, then short and ground every unit before contact.
- Nameplate capacitance follows from the rating as C = Q / (2 x pi x f x V^2), so each element of a 480 V delta bank carrying 50 kvar should measure about 576 uF.
- Inside a capacitor unit a shorted element group makes the measured capacitance rise, so an increasing capacitance trend is a progressive-failure signature, not a healthy one.
Capacitors and Reactors
Domain III subdomain T is Capacitors and Reactors, and it carries the same four knowledge statements as every other component subdomain: identify properties, types, and applications; apply visual and mechanical inspections; employ methods and procedures for electrical tests; and evaluate test results. It is easy to skip in study because capacitors show up everywhere else in this guide as theory — power factor, charging current, bushing capacitance taps, drive DC buses. On the exam they also appear as apparatus under test, with their own inspection sequence, their own acceptance criteria, and a stored-energy hazard that has killed technicians who treated a de-energized bank as a dead bus.
NFPA 70 splits the two devices into separate articles: Article 460 covers capacitors and Article 470 covers resistors and reactors. ANSI/NETA ATS and MTS keep them together in a single inspection-and-test section, which is why the DCO pairs them.
Types and Applications
| Device | What it does | Where you find it |
|---|---|---|
| Shunt power capacitor | Supplies leading reactive power (kvar) to raise power factor and support voltage | LV motor-control-center and switchgear PF banks, MV substation racks, pole-mounted utility banks |
| Series capacitor | Cancels part of a line's inductive reactance to increase transfer capability | Transmission lines |
| Surge capacitor | Slows the rise time of an incoming surge so it does not concentrate on the first turns of a winding | Motor and generator terminals, usually paired with an arrester |
| Filter capacitor | Tuned with a reactor to a specific harmonic order | Drive, arc-furnace, and rectifier installations |
| Current-limiting reactor | Adds series impedance to hold available fault current below equipment rating | Bus ties and feeders on high-fault-duty buses |
| Neutral grounding reactor | Limits ground-fault current on a resonantly or reactance-grounded system | Generator and transformer neutrals |
| Detuning (filter) reactor | Moves a capacitor bank's resonance away from a significant harmonic | In series with harmonic-filter banks |
The Theory the Exam Actually Tests
The two devices are frequency mirrors of each other:
- Capacitive reactance falls with frequency: Xc = 1 / (2 x pi x f x C)
- Inductive reactance rises with frequency: XL = 2 x pi x f x L
That single contrast explains the most common capacitor-bank field problem. A plant full of variable-frequency drives is injecting 5th and 7th harmonic current. Bolt an undetuned power-factor bank onto that bus and its impedance to those harmonics is low, so it sinks harmonic current it was never sized to carry — and its capacitance in parallel with the source (transformer) inductance forms a parallel resonance that can land near a harmonic the drives are producing, amplifying the very distortion the plant was trying to live with. The cure is a detuning reactor in series with the bank, sized to place the series resonance below the lowest significant harmonic (commonly around the 4.2nd order, safely under the 5th). If you see blown capacitor fuses, bulged cans, and audible bank noise in a drive-heavy plant, harmonics — not the capacitors — are the root cause.
A plant bolts an undetuned 480 V power-factor correction capacitor bank onto a bus that already feeds several variable-frequency drives, and harmonic distortion gets measurably worse. Why?
Stored Energy: The Rule That Keeps You Alive
A capacitor that has been switched out of service can still be holding lethal charge. NFPA 70 therefore requires an automatic means of discharge, and the time limit depends on the voltage rating:
| Capacitor rating | Code section | Required discharge |
|---|---|---|
| 1000 V nominal and under | NEC 460.6 | Residual voltage to 50 V or less within 1 minute |
| Over 1000 V nominal | NEC 460.28 | Residual voltage to 50 V or less within 5 minutes |
Two details of that rule matter in the field. First, the discharge circuit must be permanently connected to the capacitor terminals, or automatically connected after disconnection — a manually operated switch or link is not an acceptable means of discharge. Second, the code discharge is a property of the equipment, not a substitute for your own protection. The internal discharge resistor can fail open, and a can with an open resistor looks exactly like a healthy one while it sits fully charged.
The safe sequence on a bank is therefore:
- Open and lock out the capacitor switching device, then open the isolating means.
- Wait the code discharge time and any longer time the manufacturer states.
- Test dead with a properly rated instrument, using live-dead-live on a known source.
- Short and ground each unit's terminals — terminal to terminal and terminal to case — with a grounding stick before any part of you gets inside the rack.
- Leave the shorting straps in place while you work; re-verify after any test that recharges the unit, because a capacitance or insulation-resistance test puts charge right back on the dielectric.
Bonding matters too: capacitor cases and racks must be bonded, and a floating case in an MV rack can sit at a hazardous potential through capacitive coupling even with the bank isolated.
Per NFPA 70, a capacitor rated over 1000 volts must have a means of discharge that reduces the residual voltage to:
Visual and Mechanical Inspection
The inspection list is short and high-yield:
- Compare nameplate data — kvar, voltage, frequency, number of phases, connection (wye or delta), and whether internal fusing is present — against the drawings and the actual installation.
- Look for bulged or leaking cases. A swollen can means internal element failure has generated gas; that unit is done and may fail violently.
- Check fuses: expulsion fuse links on externally fused units, and blown-fuse indicators. A blown fuse on one can unbalances the bank and overvoltages the survivors.
- Verify bolted-connection torque to the manufacturer's values, and check that the case bonding and rack grounding are intact.
- Verify the discharge device and its wiring are present and connected — this is a code item, not a preference.
- On reactors, confirm coil spacing, bracing, and ventilation clearance are as built, that insulators and creepage surfaces are clean, and that the manufacturer's magnetic clearance envelope around an air-core unit is respected. An air-core reactor's external field induces circulating current in any closed metallic loop nearby, so new conduit, rebar, or a fence run inside the keep-out zone will overheat.
Electrical Tests and Acceptance
| Test | What it proves | How to judge it |
|---|---|---|
| Capacitance measurement, terminal to terminal | The dielectric element groups are intact | Compare to nameplate (or calculated) capacitance and to the other identical units in the bank |
| Discharge-resistor resistance, terminal to terminal with an ohmmeter | The automatic means of discharge works | Reads the resistor value — megohms typically; infinite = open resistor = defective, charged-can hazard |
| Insulation resistance, terminals tied together to case | The unit-to-ground insulation is sound | Manufacturer's value; compare across units |
| Bolted-connection resistance (ductor) | Joints in the bank and rack | Comparative across phases, per the torque and resistance rules in section 9.1 |
| Thermographic survey, energized | Loose joints, unbalanced current, failing cans | Follow the delta-T criteria in section 5.4 |
Calculating What the Capacitance Should Be
You can derive nameplate capacitance from the rating, which is how you sanity-check a meter reading in the field:
C = Q / (2 x pi x f x V^2)
where Q is the reactive power across that individual capacitor and V is the voltage across that individual capacitor.
Worked example. A 480 V, 60 Hz, three-phase, 150 kvar delta-connected bank. Each of the three elements sits across the full 480 V line-to-line voltage and carries one third of the total:
- Per-element reactive power: 150 kvar / 3 = 50 kvar
- C = 50,000 / (2 x pi x 60 x 480^2) = 50,000 / 86,858,000 = 0.000576 F = 576 uF
- Cross-check by current: I = 50,000 / 480 = 104 A, so Xc = 480 / 104 = 4.61 ohms, and C = 1 / (2 x pi x 60 x 4.61) = 576 uF. The two routes agree.
Magnitudes are worth memorizing because they catch meter-range mistakes: a low-voltage bank element is hundreds of microfarads, while a medium-voltage can is around ten — a 7,960 V, 300 kvar unit works out to about 12.6 uF.
Evaluating the Results
- Compare within the bank first. Identical units on the same bus should measure within a tight band of each other. One outlier is worth more than any absolute number.
- Know which direction failure moves the reading. Inside a capacitor unit, element groups are connected in series-parallel. When a group shorts, the remaining groups take a higher share of the voltage and the measured capacitance rises. Rising capacitance is a progressive-failure signature, not a healthy trend, and it accelerates because each failure overstresses what is left.
- Judge against the manufacturer's tolerance. Nameplate capacitance is published with a tolerance band; the manufacturer's value is the pass/fail number, and NETA's specifications defer to it.
- Trend it. A capacitance value recorded at acceptance is the baseline every later maintenance round is measured against.
Reactors
Dry-type air-core reactors are tested like an air-insulated winding:
- Insulation resistance from the winding to the mounting structure and ground.
- Winding resistance, compared phase to phase and to the nameplate or baseline. A rising value points at a loose or corroded joint; a falling value can indicate shorted turns.
- Inductance or impedance measurement, compared to nameplate. This is the reactor equivalent of the capacitance check, and it is how you prove a current-limiting reactor still limits.
- Thermography under load, since an overheating reactor is often the first sign of a shorted turn or an induced-current problem in surrounding steel.
Oil-filled reactors are treated as transformer windings: insulation resistance, winding resistance, power factor, and the insulating-liquid tests covered in section 6.4.
Exam Traps
- The internal discharge resistor is not personal protection. Grounds go on before hands go in.
- An ohmmeter across a capacitor's terminals is not an insulation test. It reads the discharge resistor.
- A blown capacitor fuse is not a nuisance trip. It unbalances the bank and overvoltages the remaining units.
- "Add capacitors to fix power factor" is wrong when drives are present unless the bank is detuned.
- Rising capacitance is bad. Candidates instinctively read "higher" as "healthier"; on a capacitor unit it means shorted element groups.
Official Resources
- NETA Exam Preparation — The Level 2 Detailed Content Outline that defines subdomain III.T.
A technician places an ohmmeter across the two bushings of a single-phase power capacitor unit and reads about 5 megohms. The most likely interpretation is: