Short answer
Most problems arise from connectors of different brands pushed together, from cables chafing over a roof edge and from a DC isolator that has never been checked. Direct current does not extinguish itself at a spark, so a bad connection keeps burning. Have an installation checked after a few years, especially if it was installed by different parties.
Who this article is for Everyone with panels on the roof. You can look yourself; testing and repairing on the DC side is work for an installer.
Why direct current is different
With AC voltage, the voltage passes through zero a hundred times a second. That makes a spark extinguish itself. With direct current that does not happen: an arc is sustained and burns through the material. That is why a poor contact in a DC connection is more dangerous than the same contact in an ordinary circuit.
That is why a poor contact on the DC side is more dangerous than the same contact in an ordinary circuit.
And the DC side is live as soon as light falls on the panels. Even when it is cloudy, even when the inverter is off.
The four places where it goes wrong
1. Connectors of different brands. The familiar plugs look alike and also fit together, but the internal contact surface differs per manufacturer. The connection gets warm, oxidises and eventually catches fire. This is by far the most common cause.
2. Badly crimped connectors. They must be crimped with the right tool. Squeezed with combination pliers, it holds for a few years and then no longer.
3. Chafing cables. A DC cable hanging loose over a roof tile or a sharp edge chafes through its sheath over the years. Cables must be fixed with resistant clips, not with cable ties that cannot withstand UV light.
4. The DC isolator and the roof entry. Where the cables leave the roof there should be a neat watertight entry. And you must be able to reach the DC isolator at the inverter, so something can be switched off in an emergency.
What you can see yourself
- discolouration or brown marks at the inverter or at a junction box
- a burning smell near the inverter, even if it is brief
- cables hanging loose or lying against an edge
- a yield that drops for no reason, which can point to a bad connection
If you smell something, switch off the DC isolator and the circuit and call. Do not go up on the roof to look yourself.
What an inspection involves
I check the installation with a thermal imaging camera, preferably on a sunny day with the installation running. A bad connection then shows up as a warm spot before there is anything to smell. I also measure the insulation resistance of the strings and check the fixings and the roof entries.
If the installation has been in place for more than five years, or was installed by several parties, that is worth the money and the hour. For businesses this is part of the NEN 3140 inspection as standard.
From my own work
During an inspection on a barn roof I found two connectors of different brands joined together. They fit mechanically, but the contact surface is not right. One of them had already turned brown and the cable sheath around it had gone hard. One more summer and that would have been a roof fire. Two new connectors of the same brand, neatly crimped, and the problem was gone.
When to call an electrician
- The installation was installed or extended by several parties
- You smell something or see discolouration at the inverter or on the roof
- The yield has dropped for no clear reason
- The installation is more than five years old and has never been checked
I check the DC side with a thermal imaging camera and test the strings. See also NEN 3140 inspection and the 24/7 fault service.
Frequently asked questions
Are my panels live when the inverter is off?
Yes. As soon as light falls on the panels, the DC side is live, even when it is cloudy.
Can I join connectors of different brands?
No. They do fit together but the contact surface is not right, and that is by far the most common cause of problems on the roof.
How often should an installation be checked?
After about five years, and sooner if the installation was installed or extended by different parties.
Further reading
A dynamic contract and a home battery: is it worth it
With a dynamic contract the battery can charge when electricity is cheap. That works, provided you let it be controlled automatically.
The net metering scheme is ending: what changes for my panels
Your panels keep generating just as much. What changes is the value of the electricity you feed back.
Feed-in charges: why you pay for your own electricity
More and more suppliers charge for feeding your solar power back into the grid. Your bill shows what that costs you.
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