The Simple Pull Test
Why loose connections are the #1 fire hazard in off-grid solar—and how a 3-second habit prevents fires, melted gear, and battery explosions.
Loose wires: a universal problem
Not getting wires truly tight is one of the biggest problems in off-grid solar. It shows up with experienced installers, new techs, and DIY homeowners. A connection can “feel” snug yet still be under-torqued, mis-seated, or loosened later by vibration and thermal cycling. Here on the Big Island, I see this constantly—some installers repeat the same mistake so often it’s practically a habit.
How heat builds in a loose connection
Tight metal-to-metal contact = low resistance → cool.
Loose contact = small contact area → higher resistance → rapid localized heating.
On DC buses carrying hundreds of amps, that heat snowballs. It melts insulation, carbonizes lugs, and can ignite nearby plastics. Sensitive power electronics—inverters and charge controllers—also get damaged by hot lugs and voltage sag.
Crimped lugs: the silent failure
One of the worst offenders is the poorly crimped lug. It can look perfect but be mechanically weak or have inadequate conductor contact. If a crimp isn’t right, the wire will eventually slip or arc under load.
I’ve seen fires inside DC load centers that destroyed the entire interior—wires, breakers, shunts—because one large conductor (like 4/0 at a shunt) wasn’t crimped or tightened correctly.
Battery cables & lead-acid explosions
Battery cables are the most critical connections in the system: highest current, closest to stored energy, and—on lead-acid—near hydrogen gas during charging. A loose battery terminal can overheat, burn the post off, and throw sparks.
Real-world risk: I’ve seen batteries explode—tops blown off, battery compartments scorched, and fires requiring the fire department to foam the area. The cleanup is hazardous and expensive, and the bank is usually a total loss.
DIN-rail breakers & combiner boxes: chronic looseness
Combiner boxes—especially roof-mounted—are repeat offenders. DIN-rail breaker terminals are hard to get truly tight, and years of sun, heat, and cool nights loosen them again. Loose terminations here routinely melt plastic breakers and ignite boxes.
- Install workflow: tighten DIN-rail breaker terminals, wait an hour, then tighten them again.
- Field reality: I frequently open older combiners and can pull conductors out by hand. With today’s high-wattage strings, a loose terminal will melt the breaker and ignite the box.
- Live hazard: Panels are energized whenever there’s light. A melted combiner can leave live conductors exposed—touching the box in daylight can shock you.
How to do the Simple Pull Test
- Torque first: Use the correct tool and spec for lugs, breakers, busbars, and inverter/charger terminals.
- Then pull hard: Grip the insulated conductor close to the termination and pull with intent.
- Watch for slip or movement: any rotation, slide, or click means it was not tight or the crimp is bad—fix it now.
- Repeat on every termination: battery cables, DC bus, shunts, breakers, combiners, inverter/charger studs.
Re-torque schedule & maintenance checklist
Suggested re-torque cadence
- New installs: after 1–2 weeks of operation
- Then: at 3 months
- Ongoing: every 6–12 months (more often for roof combiners)
Quick checklist
- Battery cables (all posts, series/parallel jumpers)
- Shunts and DC bus lugs (large conductors like 1/0–4/0)
- Inverter/charger DC studs
- Charge controller inputs/outputs
- Combiner box DIN-rail breakers (retighten and re-pull)
- Visual scan for discoloration, melted plastic, or odor
Safety notice
Educational content: This page is for safety education and general guidance. Electricity can be hazardous. Use qualified professionals for any work that requires permitting or exceeds your skill level.