Don′t Burry a Ticking Bomb!

Your buried cables will cost you a fortune!

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Burial of string cables directly in the ground is a growing trend among PV installers. This is a practice that comes with significant risk. Let′s explore why this could be an expensive ticking time-bomb for PV.

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Understand The Issue

First, it is important to differentiate between cable types. They are not the same. Main cables - fixed installation cables connecting combiner boxes with central inverters. Those are usually aluminum-conductor cables. We leave those aside for now. Their construction requirements are totally different. Those were designed to be laid directly into the ground.

Our concern today is with string cables which connect individual strings of PV modules with combiner boxes, respectively string inverters. These cables, typically in the cross-sections range up to 10mm2 (in rare cases up to 16mm2), feature thin, two-layer insulation, and are not intended for direct burial.

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Underground Threat

When string cables are buried directly, they face two major threats:

Humidity – with the exception of desert-areas, the underground environment represents 100% humidity. This means that it is similar like if you lay the cable into the water. What does it represent for the PV cable to be water-resistant? We will talk about it in next month′s newsletter. Without water-resistance properties, the cable is at risk of severe damage.

Mechanical load – PV string cable is quite thin and fragile. The insulation his designed with minimum possible thickness. It was never intended to absorb such stress. Underground, they are vulnerable to pressure and movement from soil, stones, and other objects. Not to mention tectonic movememts, which are present all over the world in a certain form/intensity.

The above factors alone should discourage the practice of direct burial for PV string cables.

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What Do Manufacturers Say?

The manufacturers have a tricky face here. Most of them will tell you that direct burial is possible. Period. That is the first side of the story. The second one? Be damn careful while reading the installation instruction manuals or General Terms & Conditions documents. Typically fine printed. Those reveal many limitations and preconditions. If you shall fulfill all of those, you realize that direct burial is basically a science-fiction. Actually, to be more precise, it is possible, but on your own risk. There is a tough competition between the manufacturers, and most of them will promise you just about anything, so you buy their cable. Don′t get yourself trapped in the promisses! If you intend to use the cable for direct burial your own way, make them to clearly declare their responsibility for such.

To protect yourself, demand a certificate! There is a TüV Rheinland Standard 2PFG 2642/11.17 – direct burial cable. Only the cables with this certification can be considered as direct burial ones.

Spoiler alert: no string cable in the world meet this standard′s requirements due to stringent construction requirements.

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The Standards Speak Clearly

Standard speaks very clear language in this case. Standard IEC62930 (Electric cables for PV systems) includes an Annex A – ?Guide To Use“. It is mandatory to use IEC 62440 standard (Guide To Use For Low Voltage Cables). The statement there is very clear:

?General guidance information given in IEC62440 standard (Guide To Use For Low Voltage Cables) must be used.“        

So now we know that besides following the IEC62930, we need to take the IEC62440 into the account while using the PV string cable. And what does the IEC62440 say then?

?Cables shall not be buried directly in the ground“        

It can′t be more definitive! It does not allow any creative interpretation.

To make a long story short – if you bury a cable in the ground, and something goes wrong – YOU are liable! Buried cables will cause extensive damage. Those buried cables might even be a reason for casualties. The legal case will follow. Trial jury will check whether you followed the most recent standards. Once they notice a discrepancy, you are responsible. And guilty.

Attention! Don′t get confused with TüV Rheinland Standard for Direct Burial PV cables 2PFG2642/11.17. This standard is intended to be a truly direct-burial standard, however, it is designed for the main cables, not the string cables!

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Conduit Solves It All. Really?

Yes and No.

Yes, it is a solution to protect the cable against mechanical stress. The cables are protected in a tube, and the surrounding soil is not directly impacting the cables with mechanical load.

No, it is not a solution to protect the cables against humidity. Why? It is very rare that the cables are perfectly tight in the tube. Water often leaks into conduits, creating a waterlogged environment. This means, cable inside of the tube will float in a water environment after some time. This is certainly not acceptable!

Possibly, you are a perfect installer. Possibly, you have the know-how to make the state-of-the-art tubing for your cables. Still trapped though. Still trapped like a water in a tube. The cable has hot-cold cycles periodically. This means the heat emitted by the cables will generate condensation. This condensation does not have any space to escape and will remain in the tube. From that point, the situation is the same as with the previous paragraph. The cable will be soaked in the water permanently.

Are you confused now? How to use the conduit then? The best way is to use the conduit in combination with a water-resistant cable (we talk about it next month). In addition, drill drainage holes into the conduit to allow water to escape, ensuring the cables remain dry and protected.

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Key Takeaways

1. Avoid direct burial of string cables whenever possible.
2. Use conduits or tubes, if burial of the string cables is necessary.
3. Ensure conduits have drainage holes to prevent water accumulation.
4. Opt for water-resistant cables to combat humidity effectively.

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Stay Tuned

Next month′s newsletter will focus on water-resistant/proof cables. Stay informed. Safeguard your PV investment!