What you will see
- An arc fault or AFCI message, and the inverter refuses to restart on its own.
- Sometimes a burnt smell, or brown or melted plastic at a connector on the roof.
Why it happens
MC4 connectors join panel cables together. If one is not fully crimped, not fully clicked together, or is a mixed pair from two different makers, it can loosen with daily heating and cooling. Current then jumps the gap as a small arc, which heats and damages the connector.
Modern inverters listen for the electrical noise an arc makes and stop before it causes damage.
Our MC4 connectors and earthing guide explains how connectors should be chosen and crimped. This page is for when one has already failed.
Safety first. Solar panels produce DC voltage whenever there is daylight, even with the inverter and the grid off. A damaged DC joint can arc, and a DC arc does not go out on its own like an AC spark. Never pull apart, tape over, cut or "push back together" a burnt connector. If you see smoke or flame, get everyone away and call Rescue 1122; tell them there is a solar system on site.
MC4 connector burnt: what it means
An MC4 connector should carry the string current with almost no resistance. When the metal contact inside is loose, corroded or badly crimped, resistance rises. Resistance turns current into heat, heat oxidises the metal, and the resistance rises further. Over weeks or months the plastic softens, browns and finally melts or chars. In the worst case the joint opens under load and an arc forms.
Two pieces of evidence point the same way:
- A study by the Dutch research organisation TNO into rooftop PV fires in the Netherlands found that about 70% were linked to faulty connectors, particularly cross-mating: plugging one maker's connector into another's.
- Connector and inverter makers say plugs and sockets must be the same type from the same manufacturer, as the installation standards IEC 62548 and IEC 60364-7-712 require, and that the correct crimping tool must be used.
So a melted connector is nearly always a workmanship or materials problem. Fixing only the plastic without fixing the cause invites the next one.
Burnt MC4 connector
Loose fit
Heat at the joint
Mismatched
Different connector brands mated
Water
Corrosion inside
Quick-check table: MC4 connector melting and DC connector heating
| Symptom | Likely cause | Safe owner check | Call an engineer if... |
|---|---|---|---|
| Brown, black or melted plastic at one joint | High-resistance joint: poor crimp, cross-mated pair, not fully latched | Photograph from a safe distance; do not touch | Always |
| Burning plastic smell on the roof or near the DC isolator | Overheating joint, cable or isolator terminal | Locate the area by smell from a distance; check the app | Always; see our burning smell guide |
| One string producing much less or zero | Joint opened or failing on that string | Compare PV1 and PV2 in the app on a clear day | Drop is sudden or the string reads zero |
| Isolation or earth fault alarm after rain | Water inside a connector, or melted insulation | Note the alarm and the weather | Alarm repeats or stays on |
| Connector lying in water, on a hot roof or hanging by its cable | Poor cable management | Photograph it | Always; it should be clipped and supported |
| Two different-looking halves on one joint | Cross-mating | Photograph both sides of the joint | Always; the pair should be replaced as a matched set |
| DC breaker or fuse tripping | Fault current, or heat at a terminal | Note which breaker and when | It trips again after one reset by the installer's sequence |
Safe owner checks
These use only your eyes, a phone camera and the monitoring app.
- Do not touch. Do not unplug, wiggle, tape or spray anything on the joint. Connectors must never be separated under load.
- Switch off in the manufacturer's order, if it is safe to reach the switches. Many systems use the AC breaker first, then the DC isolator, then the battery breaker, but follow your own inverter manual. Switching off stops current through the joint; it does not remove voltage from the roof wiring in daylight.
- Photograph from a safe distance. Zoom in on the photo, not by leaning in. Capture both halves of the joint and the cable on each side.
- Check the app. Compare string voltages and currents (PV1, PV2) and look for alarms. Our solar monitoring app guide shows where to find them, and one string low output explains what a weak string looks like.
- Look at the other visible joints without touching them.
- Note the history. Was a panel replaced, a cable extended, or work done on the roof recently? Were the panels washed with a pressure jet?
- Keep the area clear of children, pets, cleaners and anyone walking on the roof until an engineer has made it safe.
- Send everything to your installer: photos, app screenshots, the alarm text and the inverter model.
What an engineer will check
A qualified engineer will make the string safe first, then:
- Isolate the string with the DC isolator or fuses and confirm it is de-energised at the joint before touching it, using appropriate PPE.
- Cut back the cable to clean, undamaged copper and check the insulation along the run for heat damage.
- Replace both halves with a matched pair from one manufacturer, rated for the string voltage and current and sized for the cable cross-section, crimped with the correct tool and locked fully.
- Check the panel lead. If the damaged half is the panel's factory connector, they will replace it with a matching type or match the other side to it.
- Check the other joints for cross-mating, poor crimps, loose gland nuts and signs of heat, often with a thermal camera on a sunny day.
- Check the current. Parallel strings joined with branch (Y) connectors, or a design that pushes more current than the connector rating, will heat even a good joint.
- Test insulation resistance on the string and confirm no lasting damage to cables, the DC isolator or the inverter input.
Cable sizing matters too: undersized cable runs hot along its whole length. See our solar cable size guide.
Solar connector burnt: how to prevent it
- One maker per joint. Never cross-mate connectors. If two panel brands are mixed in a system, check that their connectors match or are replaced as pairs; see mixing different solar panels.
- Correct crimping tool, matched to the connector and cable size. Pliers are not a crimping tool.
- Fully latched and sealed. Each joint clicks home, and each gland nut is tightened so water cannot enter.
- Supported and shaded. Connectors clipped to the structure, not lying on hot tiles, in puddles or hanging by their cables.
- Annual inspection of visible joints and a thermal check on larger systems. Our workmanship checklist shows what good DC work looks like.
- Proper protection. DC isolators, fuses and earthing as described in our earthing, surge protection and DC breakers guide.
If a breaker has also been tripping, read solar breaker tripping. If an isolation alarm appears after rain, read inverter isolation fault. For the wider fire picture, read fire safety for rooftop solar.
Is it covered by warranty?
A connector that melted because of a poor crimp or cross-mating is an installation fault, not a product defect, so a panel or inverter manufacturer's warranty is unlikely to cover it. If the inverter input was damaged, the manufacturer decides. Manufacturer warranty terms are given per product,; see our solar warranty guide.
How Solar Works handles connector faults
On systems we install, connectors are crimped with the correct tool, matched at every joint, clipped to the structure, and checked during the tests for safe commissioning. If you are a Solar Works customer, WhatsApp 0336 666 4321 with photos of the joint, the app's string data and the inverter model. Complaints are handled by our own engineering staff, and our 1-year service warranty applies under the applicable project terms. See how after-sales service works.
We can also replace burnt connectors and inspect DC wiring on systems installed by other companies, as a chargeable service.
Frequently asked questions
Why is my MC4 connector burnt?
Usually because the joint has high resistance from a poor crimp, two halves from different makers, a connector not fully latched, or water and dirt inside it. The joint heats under load until the plastic browns and melts. The cause must be found as well as the connector replaced.
Is a melted MC4 connector dangerous?
Yes. A failing DC joint can arc, and DC arcs keep burning. Switch the system off in your manual's order if it is safe, keep away from the wiring, and call a qualified engineer.
Can I unplug a burnt MC4 connector myself?
No. Separating a connector under load can draw an arc, and the roof wiring stays live in daylight even with the inverter off. Leave it to an engineer who can isolate the string first.
Why is my DC connector heating up?
A connector that runs warm under load usually has a loose or poorly crimped contact, a mismatched pair, the wrong cable size, or more current than its rating. Sun on a dark roof adds to the heat. A thermal check on a sunny day finds the hot joints.
Can I mix MC4 connectors from different brands?
It is not recommended. Connector makers and installation standards say mated plugs and sockets should be the same type from the same manufacturer. Cross-mating was the most common cause in a Dutch study of rooftop PV fires.
Does a burnt connector damage the inverter?
Not always, but a DC fault can affect the inverter input, DC isolator or cables. An engineer should test them before restarting.
Do you replace burnt connectors on systems installed by others?
Yes, as a chargeable service across Punjab. Send photos of the joint and your app's string data on WhatsApp 0336 666 4321 first.
Sources
- pv magazine, "Cross-mating of connectors was chief cause of Netherlands PV fires" (12 Apr 2019, on the TNO study)
- Stäubli, "Cross-connection" (connector maker's guidance and standards on mating connectors)
- Hopewind support, "Explanation of damaged DC connector case"
What you can check yourself
- From a safe position, look for discoloured or melted connectors or cables. Do not touch them.
- Note whether anyone has recently walked on or worked near the array.
What not to do
- Do not reset the arc fault repeatedly to get the system running.
- Do not touch, unplug or tape up any connector. They carry DC voltage in daylight and cannot be switched off from the house.
When to call us
- Immediately. An arc fault needs a technician to find and replace the faulty connection.
Urgent solar repairs in Faisalabad and Lahore
An arc fault is one of the few solar problems that should not wait. Call our Faisalabad or Lahore team and keep away from the connectors until a technician has replaced them.
Send us
The fault message and, if safe to take from a distance, photos of any damaged connectors.