You open an enclosure and find liquid on the components. Two different failures look alike at first glance: aluminum electrolytic capacitors vented their electrolyte, or water got in from outside. Telling them apart matters, because the cleanup, the repair and the fix are different. In most cases where both are present, the water came first and the capacitors were the casualty.
The diagnosis is mostly visual and takes ten minutes. Where the liquid is, what it left behind, and what it corroded identify the source before you replace anything.
What each symptom points to
Read the enclosure like a failure scene before you touch anything. These signs separate the two causes.
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Localized residue around a capacitor: brownish, oily or crusty deposit on the board, the capacitor can or the parts directly above or below it. The capacitor's top vent is burst or domed. This is electrolyte from a vented can.
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Moisture spread across distant surfaces: droplets or staining on the door, the lid or components with no path back to any capacitor. Electrolyte does not travel that far; water does.
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Standing water in the bottom of the box: a puddle under the lowest point, often below a cable gland. Vented electrolyte is a few millilitres at most; a puddle is ingress or heavy condensation.
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Water tracks or mineral streaks: white or rusty streaks running down from a gland, a corner or a screw. That is an external path with dissolved solids left behind as the water dried.
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Corrosion pattern: electrolyte attacks copper, solder and aluminum near the vent, turning traces green-black and crusty. Ingress water corrodes uniformly and rusts steel hardware and terminal screws everywhere the water touched.
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Dried white or gray crust with no odor, spread widely: consistent with hard water that evaporated, which points again to ingress or a condensation cycle rather than a capacitor.
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Staining only below the capacitors, dry everywhere else: a classic venting event. The can released vapor that condensed on the coolest nearby surface and left electrolyte spatter below.
Causes in order of likelihood
Water ingress first, capacitor failure second. This is the most common sequence and it matches the physics. Moisture on a board or across balancing resistors creates leakage paths. Two series capacitors at 400 V DC with parallel balancing resistors of roughly 100 kΩ only stay balanced if those resistors stay clean; a wet film across one shifts the voltage split and overvolts the other can until it vents. Moisture also corrodes aluminum cases and terminals over time. The capacitors venting is then a symptom, and the liquid you find is a mix of electrolyte and the water that killed them.
A genuine venting event without ingress. Capacitors vent on their own when they are old, run hot, or see ripple current above their rating. Modern "dry" electrolytics hold little water, so a vent releases a small amount of electrolyte and vapor. The vapor condenses on nearby cooler surfaces. The total liquid is small and localized. If your enclosure was sealed and dry everywhere except around the parts, this is your answer.
A condensation cycle, no external leak. A sealed box breathes whenever humid air was trapped inside and temperatures swing. The enclosure walls cool below the dew point at night and water condenses on components, then dries by morning. This looks like water with no entry path. The condensation guide covers the dew point math, and why the rating can't stop condensation explains how it happens even in a tight box.
Cable entries as the path. Most real-world ingress comes through glands: undersized seals on a smaller cable, unsealed unused knockouts, or connectors that were never rated for jetting. Fixing the cable entry and sealing a gland around a smaller cable cover the two most common gland faults.
How to check each cause
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Photograph everything before cleaning. Streaks, drips and corrosion patterns are evidence; you destroy them when you wipe.
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Inspect the vented capacitors. Look for a burst score line or domed top on the can. Note whether residue surrounds only those parts or spreads beyond them.
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Test the residue if you can. Dab a little in distilled water and check with a conductivity meter or a multimeter if you have one. Electrolyte is strongly conductive; clean condensed water is not. A soapy, slick feel also suggests electrolyte.
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Trace every water mark downhill. Water leaves tracks from its entry point. Follow streaks up to a gland, gasket corner, screw or conduit entry. Check under the gasket for a channel or compression loss.
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Check the lowest gland from outside. Remove it and inspect the seal against the cable jacket. Look for water sitting in the thread gap or behind the locknut.
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Look for condensation signatures. Uniform droplets on the lid and door, corrosion on the tops of parts but not the bottoms, and no streaks from any penetration suggest a breathing and condensation cycle rather than a leak.
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Measure the environment. If the box sits outdoors or in a washdown area, compare day and night temperatures. A box that cycles 20 C between day and night in humid air will condense water regularly. The outdoor enclosures guide covers what sun, rain and ice do to a sealed box.
The fix
For vented capacitors: replace the cans, and replace any that share the bank, because series or parallel cans that vented together aged together. Clean all electrolyte off the board with 90%+ isopropyl alcohol and a lint-free wipe. Electrolyte is conductive and corrosive; any crust left behind keeps corroding traces and terminals after the board is dry. Inspect and touch up corroded traces and terminals. Then ask why they vented: heat, ripple current, overvoltage from imbalanced sharing, or age.
For water ingress: find and seal the actual path. Rework glands so the seal matches the cable outer diameter. Replace a compressed or cut door gasket. Seal unused openings. If the box was a low-IP economy enclosure in a wet location, re-specify it — a manufacturer-rated NEMA 4X / IP66 enclosure resists hose-directed water and washdown when every penetration is sealed correctly; it is rated by the manufacturer and not UL listed.
For condensation: stop the box from breathing wet air. Seal entries properly, dry the interior before closing it, and add an anti-condensation heater if the temperature cycles cross the dew point. When an outdoor enclosure needs an anti-condensation heater gives the sizing logic.
How to stop it coming back
- Seal every cable entry with a gland matched to the cable's outer diameter, torqued to spec, and one cable per gland.
- Keep unused openings plugged with sealed, rated blanks, not tape.
- Mount the box so the door faces away from hose spray, and never aim washdown jets at the gasket line. Hose-down mounting practice is its own subject; the details of drain placement, sloped tops and hinge side are covered in sealing an outdoor junction box against running water.
- Replace door gaskets on schedule rather than when they leak; compression set is invisible until it rains.
- If capacitors run hot, address the heat — larger cans, better layout, or more enclosure volume. Component layout inside a sealed enclosure to manage heat explains how placement changes component temperature in a closed box.
- For corrosive or wet environments, choose 316L stainless so that any moisture event does not turn into a rust event on top of the electrical one.
When to re-specify the enclosure
Replace or re-specify the enclosure rather than repairing it when any of these hold:
- The box never had a rating suited to its location, such as a low-IP enclosure in a washdown or rain-exposed spot.
- The gasket seat or door is deformed, so no gasket will seal it again.
- Corrosion has pitted the enclosure walls or the mounting panel through any coating.
- The water ingress came from the mounting method itself, such as screws through unsealed holes — see mounting screws through an outdoor enclosure without leaks.
- The internal heat load plus an anti-condensation heater needs more volume than the current box has.
When you re-specify, a wall-mount stainless enclosure rated NEMA 4X / IP66 by the manufacturer (not UL listed) covers rain, hose-down and dust in one rating. If you are unsure which box fits the environment and the panel, the enclosure selector filters by environment, listing requirement and inside dimensions. Custom enclosures are built to your cutouts and delivered in 10 business days, counted from your approval of the drawing.
The key habit is to diagnose before you clean. A wiped enclosure tells you nothing, and the next failure will look exactly like the last one.
Questions
Can a vented capacitor produce enough liquid to look like condensation?
A vented aluminum electrolytic capacitor releases electrolyte and vapor, and the vapor can condense on nearby surfaces. A single venting event leaves localized residue near the part, not pooled water across the whole enclosure. If you find widespread moisture, standing water at the bottom, or corrosion on parts far from the capacitors, an external leak or condensation cycle is the more likely source, and the capacitors are probably victims rather than the cause.
Is electrolyte conductive and does it have to be cleaned off?
Yes. Electrolyte is far more conductive than clean water and it is corrosive to copper, solder and aluminum. Left in place it causes leakage paths and ongoing corrosion even after the enclosure dries out. Clean affected boards and the enclosure interior with isopropyl alcohol, let it dry fully, and inspect traces and terminals for damage before re-energizing anything.
How do I check whether water is getting in from outside?
Look first at the cable entries, the door gasket and any screws through the walls, because those are the common paths. Water runs downhill: streak marks from a gland or a corner, rust or staining below a fastener, and pooled water under the lowest entry all point to an external path. If everything external is dry and clean, and the moisture is concentrated around the failed capacitors, the venting itself is the likely source.
What rating stops water ingress in the first place?
A manufacturer-rated NEMA 4X / IP66 enclosure, with properly sized and tightened cable glands and an intact door gasket, resists hose-directed water and heavy rain. Our enclosures are rated by the manufacturer and not UL listed. The rating only holds if every penetration is sealed, so the glands, gasket and mounting method matter as much as the box itself.
