PV Cable Standards - no space for creativity!

It is frequently seen in the specifications of the cables, that a questionable terminology is being used. AD8 Waterproof, Direct-Burial, or Water-resistant are common phrases... But did anyone really think what′s behind those statements?

Direct Burial

Let′s start with the easy one. The international standard IEC62930 for cables in the PV system - Annex A "Guide to use" clearly says: "It is mandatory to use IEC62440 (Guide to use for low-voltage cables)". Therefore general guidance given by the IEC62440 must be used.

The paragraph 4 (Safety) of the IEC62440 defines "Cables shall not be buried directly in the ground......"

Due to the enormous cost pressure, PV cables are designed to be with a very thin layers of insulation. Actually, the typical PV cable is at the edge of the minimum requirements of the standard. Thin insulation also means that the cable is fragile in 2 main aspects.

a) mechanical stress - in order to maintain the cable flexible and costs effective, thin and relatively soft insulation is not suitable for direct burial, where mechanical damage could occur easily. Even sand bedding which is sometimes used as an excuse do not guarantee a sufficient protection of the cable against scratches, cuts and other mechanical damages. That is actually the reason, why it is always required to use a tube/conduit if using the cable under-ground. Important to say - this solution has also several traps - moisture condensation, me-chanical damages of the conduit or simply insufficient installation which allows water to penetrate inside can lead into the next serious problem:

b) humidity - respectively long-time water exposure is another factor why the standard intended to avoid a direct burial practice of the DC cable network.

Permanent exposure to the mois-ture (there is always a moisture present in the ground!), and/or water (in the wrongly installed/in-sufficient pipes and conduits) have the ability to penetrate through the insulation directly to the conductor and cause damage. Why? The de-tails will be explained below in the "Waterproof" chapter.

Solution? It exists!

The DC Cable network in a PV array can be split into 2 parts - flexible cables and fixed cables. The flexible ones are typically connecting the modules and strings and require the flexibility for obvious reasons. The fixed cables are typically buried in the ground, and are not a sub-ject of any movements through the lifetime of the installation. Due to the high demand for the description of the requirements, TüV Rheinland introduced the standard 2PfG2642/11.17. Among other requirements, the standard requires a homogenously closed metallic layer which would protect the cable mechanically, but mainly, as a humidity/water protection. Pay attention - wire armoring or metallic tape wrapped around the cable are NOT considered as a closed metal barrier!

Lesson learned: If the PV installation cable layout design requires a direct burial of cables, ask for the 2PfG2642 Standard to be required

Waterproof/AD8! Really?

?Creativity and adopting of the terminology for marketing purposes has no bounds. There is simply no other explanation for the question: “Why some of the cable manufacturers declare their PV String cables as AD8/Waterproof”?

AD8 is a self-declaration state-ment of the cable manufacturer. There is no relevant PV standard which would, at least refer to the physical testing of the AD8 properties. And therefore, there is also no 3rd party laboratory, which would be able to test & grant with a certification.

The reasons are quite obvious – AD8 cables are waterproof sub-sea cables which have a ro-bust metallic layer with the function to prevent the water to penetrate through the insulation layers and damage the core/conductor. It is important to add that the standard 2PfG2642 des-cribed in previous chapter was actually based on the spirit of sub-sea cables.

When wrapping all the above into one package, it is obvious, that it is physically impossible for a thin and relatively simple, flexible PV string cable to meet the requirements for water-proof properties.

?Insulation Properties

?The PV cable related standards are frequently requiring halogen-free materials, which also serve as a flame retardant for the cable insulation. In order to achieve this requirement, the anorganic mineral flame retardant additives need to be added in the raw compounds. The tri-cky point is that those additives are highly hydroscopic. This means the materials are absor-bing the humidity, and in a long-term run, those materials behave literally like a sponge. To conclude once and forever - polymeric materials are NOT humidity resistant. Depending on the structure of the polymers, it is not a question "if", but "when" they are going to fail. And this is also the reason, why the PV cables shall not be used for permanent exposure to the water. Well, nobody could imagine that any sort of long-term water exposure (Floating PV or direct burial, for example) will ever be the reality at the time, when the respective stan-dards were created.

Invisible Danger

The problem with moisture penetrating through the insulation will not appear from one day to another. It is a very slow, silent & asymptomatic process, which will cause a higher resis-tance/lower performance of the array (in an early stage). Frequently, the O&M teams would wrongly indicate it as a faster degradation of the module performance. Once the moisture penetration is already in a late stage and massively spread through the cable network, the panic starts. Massive costs with the complete re-wiring and replacing damaged equipment is on rise.

Solution? 2PfG2750/09.20

TüV Rheinland introduced it′s latest piece of art contributing to the Floating PV, and impro-ved water resistance in general. The 2PfG2750 can be considered as an upgrade of the exis-ting IEC62930 with the intention to focus on a stronger performance in a humid environ-ment, and improved resistance against water.

It is not my intention to describe the mentioned standard to the detail here (I am sure that TüV Rheinland will gladly provide sufficient details), but to mention that the requirements of increased requirements for insulation robustness, change of capacitance based on UL44, UL1581 and UL2556 standards, or higher requirements for long term resistance tests are examples which give the candidates for the certification a significant challenge in meeting the related requirements.

Attention - the purpose of the standard is to grant a significant improvement in terms of the water resistance for Floating PV applications (respectively ground buried conduit applica-tions). The purpose is not to substitute requirements for AD8 cables!

Lesson learned: PV applications in the humid/water areas are showing highly accelerated degradation. If you want to keep your investment safe, pay attention to get cables with 2PfG2750 certification in combination with well-designed cable management and workmanship excellence.

Ending Remark

"Waterproof" and "Water-resis-tant" might sound similar, but actually the difference is huge! Floating PV cables and Direct Burial PV cables have different requirements, different characte-ristics, and therefore different solutions. Do not underestimate the risks and ask for perfection. Ask for undoubtedly certified products.