Ground fault on my solar system — help!

Been troubleshooting a ground fault on my narrowboat setup for the past few days and I'm getting nowhere. Hoping someone's seen this before.

I've got a 4kW Renogy array split across the roof, running into a Victron Multiplus II 48/5000 via a Victron MPPT 150/100. System's been solid for about 18 months with no issues.

Yesterday the Multiplus threw a ground fault alarm and won't let the inverter start. I've checked all the obvious stuff:

• Continuity testing between the negative bus and chassis — clean
• Inspected all DC cabling for damage or moisture — nothing obvious
• Tested the PV array in isolation with a multimeter — getting normal voltage readings, no shorts to ground that I can detect

The thing is, we've had a fair bit of damp recently (narrowboat life, innit), and I'm wondering if moisture has worked its way into one of the junction boxes or under the array mounting.

Before I start yanking panels off, is there a methodical way to isolate which section of the array is causing it? I've got the Victron app connected but the ground fault error isn't giving me much detail about where it's detecting the fault.

Also, if it does turn out to be a failed panel or corroded connection, would replacing just the affected panel be straightforward, or am I looking at a full array retest and recertification?

Any guidance appreciated.

Ground faults on boats are a nightmare — water, salt spray, and vibration all having a go at your connections. Have you checked where the array splits? Moisture creeping into the MC4 connectors is usually the culprit on narrowboats, especially if the cables run through any damp sections of the roof.

If it's not the connectors, I'd be checking the actual panel frames for corrosion where they're bolted down. My shepherd's hut nearly had the same issue until I realised one mounting bracket was letting water pool underneath.

What's your Renogy controller showing — is it throwing a specific fault code? That'll narrow it down quick. Also worth confirming your DC earthing setup is proper — some boat installs get creative with grounding and it causes endless grief.

Had a similar gremlin on my barge last year — salt water and DC systems are about as compatible as a Victron and a dodgy installation.

Check your string combiner box first; moisture creeps in through the knockouts even on "sealed" units. Reseal everything with proper silicone, not caulk. If that's not it, inspect every MC4 connector for corrosion — they're cheap to replace and corroded contacts generate resistance faults like clockwork.

Also worth testing each string individually with a multimeter set to resistance mode. Narrowboats vibrate like mad, so connections loosen over time. Tighten everything methodically, especially where the array terminates into your combiner.

What's your DC earthing setup? Floating negative or bonded? That changes how the fault detection system behaves.

Ground faults on boats are absolutely brutal because you've got moisture creeping into everything. Salt spray accelerates corrosion on connectors way faster than on land systems.

My money's on your combiner box or the connections between panels and the main run. Even tiny amounts of salt residue can create a slow ground path. Check:

• Are your MC4 connectors fully seated? Moisture gets in microscopic gaps
• Trace the positive and negative runs for any abraded insulation — vibration on the water does damage you won't spot immediately
• Test each panel string individually with a multimeter set to resistance, grounding to the hull

If you're getting intermittent faults, it's usually corrosion rather than a hard short. I'd suggest replacing connectors rather than trying to clean them — not worth the risk on a boat where you can't easily isolate the system.

What's your combiner box setup? That's often the weak point with marine installations.

@RobBennett93 Narrowboat + 4kW array = moisture's wet dream, literally. Check your DC isolator connectors first — they're usually the culprits on boats where condensation builds up faster than you can say "bilge pump". If you're running ungrounded negative (common mistake), your fault detector will have an absolute meltdown. Get a proper multimeter and test continuity between your negative rail and the chassis — bet you find damp corrosion eating through something you can't see. Salt spray particularly loves MC4 connectors; mine were green and crusty within a season. Swap any dodgy ones out with marine-grade gear and seal everything with dielectric grease. Also worth checking your Renogy's internal fuses aren't weeping moisture into the DC side.

Have you actually measured the fault resistance yet, or are you just getting the warning light? That's where I'd start — my Victron MultiPlus took ages to pinpoint a dodgy connection in the cabin that was only showing 2.8kΩ insulation, not a catastrophic short.

On narrowboats specifically, the real culprit is usually where the array cables pass through the roof gland. Water sits in there, freezes, expands, and eventually breaches the sheathing. I had mine sorted by replacing that entire section with marine-grade UV-resistant cable and a proper IP67 waterproof box inside the cabin.

Also worth checking: are your MC4 connectors the genuine Renogy ones or cheapies? Had a mate use budget connectors once — corroded in about eight months on a boat.

If it's genuinely a narrowboat, you're dealing with constant moisture. Consider running a small desiccant breather on your battery box or wherever your isolator sits. Sounds daft but it genuinely helped mine.

What's your MPPT controller showing for fault codes? That'll narrow it down qu

Got a similar issue in my garden office setup after winter — not quite the same environment as a narrowboat, but moisture is definitely the culprit.

Before you tear everything apart, are you checking the fault with a proper insulation tester? Don't just rely on the inverter's warning. I'd grab a Fluke or similar and test each string separately, then isolate sections of your array. That'll pinpoint whether it's the panels themselves or your cabling run.

One thing the others haven't mentioned — check your weatherproofing around where cables enter your combiner box. That's where I found mine. The grommet had shifted and water was pooling round the terminals. Even a small amount of moisture migration will show up as a fault.

For a narrowboat specifically, you might also want to look at whether your DC isolator is actually rated for marine conditions. Some aren't sealed well enough. Victron's stuff tends to hold up better in damp environments if you're considering a replacement.

What's your actual fault resistance reading showing?

Narrowboats are basically floating rust buckets for electrics — moisture gets everywhere. @ForestJenny's right about measuring the fault resistance first, but on a boat I'd skip straight to the MC4 connectors and any exposed terminals in your combiner box. Saltwater spray and condensation absolutely destroy them.

What I'd do: disconnect each string one at a time and see if the fault clears. If it does, you've isolated which array section's affected. Then grab a multimeter set to resistance mode and probe around those connectors — you're looking for anything under a few megohms between live and frame.

Had similar on my previous setup — turned out the isolator itself had corroded internally. Switched to a Victron DC disconnect with better sealing and never looked back. Victron gear's pricey but it actually survives on the water.

Also check your earthing system isn't compromised. Boats move, vibrate, connectors come loose. Dead simple but easily overlooked.

Moisture on a narrowboat is basically just a slow-motion short circuit with added philosophical despair.

Before you start replacing kit, grab a multimeter and check your actual fault resistance — @ForestJenny's spot on there. If it's reading low (under 1MΩ), you've got water ingress somewhere nasty. The Renogy combiner boxes are decent but the real culprit's usually the MC4 connectors or junction boxes — water loves a tight seal that's slightly loose.

Quick wins: unplug each string one at a time and retest. Isolate which array section's leaking. Then it's just methodical — connectors first, then work backwards through your DC runs. Use the boat's motion to your advantage actually — tilt the panels different angles and watch if the fault changes.

Had this exact dance with my van conversion setup last spring. Turned out a single dodgy connector I'd forgotten about. Cost me about 15 quid and three days of cursing.

What's your fault resistance reading showing?

Right, I've been through this exact nightmare on my converted horsebox. The moisture issue is real enough, but on a narrowboat you've got it worse than most of us.

Here's what actually worked for me: don't just look at the obvious damp patches. Check your MC4 connectors first—water creeps in there and sits. I found corrosion inside the connectors themselves before I found anything on the array. Unplug them, dry thoroughly, and consider upgrading to sealed connectors if yours are older.

Second thing: your combiner box is probably the culprit. If it's not IP65 rated or higher, moisture condenses inside during temperature swings (happens constantly on water). I moved mine into a ventilated enclosure and the fault cleared.

The ground fault detector on your Victron or Renogy kit should tell you which string is faulting—have you narrowed it down that far yet? If you're getting faults across the board it's likely the combiner or wiring loom, not individual panels.

What's your combiner box situation? And are you reading specific strings or just the overall

Narrowboats + electrics = nature's way of testing your patience, innit. The moisture thing is real but here's what actually sorted mine when I had a similar fault on my system:

Get a proper insulation tester — not just relying on your inverter's built-in fault detection. They're cheap enough and'll tell you exactly which string's le

@RobBennett93 - Mate, narrowboats are absolutely brilliant until the damp gets involved. Since @T5Project and the others have flagged the moisture angle (which is spot on), here's what I'd add:

Have you actually located which circuit the fault is on yet? Before chasing your tail further, grab a multimeter and test each string separately—isolate them from your charge controller one at a time. That'll narrow it down fast.

Also worth checking: your DC cabling runs. On a boat, cables can get pinched or abraded against fittings, and the salt air absolutely hammers the insulation over time. Even a tiny nick that's been weeping moisture for months will eventually trigger a ground fault.

If it's intermittent, it's almost certainly moisture-related rather than a hard short. Try running a dehumidifier in the engine room or wherever your main kit is for a week and see if it clears. I know that sounds daft, but I've seen it work surprisingly often.

What's your actual fault code showing, if your controller's displaying one?

Ground faults on boats are a proper pain — the salt spray and constant moisture will find every weak spot.

Have you checked your DC isolator and breaker connections? Mine corroded something awful after six months on the water. Cleaned them up with a wire brush and a tiny bit of dielectric grease, sorted it.

Also worth checking if water's gotten into your combiner box or the back of your array junction boxes. Even tiny amounts of condensation can cause creeping faults that take forever to track down. Pop the covers off in dry weather and have a look.

If it's definitely a ground fault (assuming you've got a Victron BMV or similar monitoring it), try isolating each string one at a time to narrow down which panel run's problematic. Could be a single dodgy connector rather than a full string failure.

The damp on narrowboats is relentless though — make sure everything's sealed properly and consider running silica canisters in your battery box during winter. Saved me endless grief.

I had exactly this on my setup last winter. The thing that caught me out was the cable glands where the array feeds down into the cabin — moisture creeps in there even when you think they're sealed tight.

Worth checking:

• Cable integrity under the panels. UV damage + flex from boat movement = tiny cracks in the insulation you won't spot visually
• Your DC isolator housing. Water pools there something chronic on narrowboats, especially if it's mounted anywhere near the gunwale
• Continuity between your frame and the boat's hull. I had a corroded bolt that wasn't making proper contact — Victron's fault detection went mental until I cleaned it back to bare metal and used some dielectric grease

The Renogy gear is solid, but the installation environment on a boat is brutal. I'd grab a decent multimeter and test each string individually disconnected from the charge controller. That'll narrow down whether it's array-side or your balance-of-system.

Also, how old are your cables? Narrowboat movement + temperature swings degrade them faster than stationary setups.

Ground faults on boats are grim. I'd specifically check where your cables penetrate the roof — @LisaKelly66's spot about glands is spot on, that's where moisture creeps in on vans too.

Also worth testing each string separately with a multimeter to isolate which one's faulting. Renogy gear's generally solid but the connectors can corrode if they're not properly sealed. MC4s especially — even a tiny bit of salt spray will do it.

How old's your array? If the fault's intermittent it might be a damp-related resistance issue rather than actual damage. Try running it on a dry day and see if the fault clears — that'll tell you if it's moisture or a genuine break.

What's your MPPT doing — cutting out or throwing an actual ground fault code?