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Cables, cables glands and connectors to connect components

Published : 07/11/2018 11:52:41
Categories : Photovoltaic Panels

What differs from common cables to solar cables is that the last ones are expressly made for photovoltaic applications, for this reason they are coated with special insulation miscellaneous that provides to the cable a longer life, and a better resistance to chemical agents, water, corrosion, low temperatures and UV radiations.

A solar cable is composed by:

  • one or more conductors

  • insulator

  • a flame retardant protective sheath (optional).

The sheath preserves from environmental phenomena and chemical and mechanical stress. In some cases you can find a metallic shielding towards electromagnetic field.

Insulation miscellaneous can be subdivided in three categories:

  • thermoplastic (PVC)

  • elastomeric (EPR/HEPR)

  • polyolefin (LSOH).

The first ones get soggy after a certain temperature, the ones in EPR do not get soggy because they are subjected to a cross-linking chemical reaction and the last ones provide a low emission of fumes and toxic and corrosive gas.

On the isolator or on the sheath the following indications must have been reported:

  • producer's name or trade mark

  • HAR identification mark

  • metric progressive marking.

Firstly, there are two criteria to distinguish cables typologies: cables differ by the materials of conductors and by insulation. The two materials mainly used for cables in residential and commercial photovoltaic installations are copper and aluminium. The first one has a better conductivity compared to the second one, but it is more expensive. The second one instead is not permitted inside residences. Isolator has the function to protect against heat, UV light and chemical and atmospheric agents.

Cables used to connect photovoltaic modules must have specific characteristics as described in the CEI 20-91 norm. They must have a copper and tin soul coated with an insulation sheath and a rated voltage of 1,000 V in alternating current and of 1,500 in direct current.

The characteristics of the cable are usually reported in the stamping of the cable, for example the PV20 abbreviation ensures that the cable has a duration of 20,000 hours at 120 degrees, that means 25 years of function in normal conditions.

Another characteristic of cables is their colour that outlines their function and their employment. Normally, red stands for the positive pole, black for negative pole, blue for median pole and yellow or green are for conductors for protection.

ENVIRONMENTAL CONDITIONS THAT AFFECT PHOTOVOLTAIC PLANTS

At the moment of choice of cables, outdoor conditions impose to take note of every environmental phenomena that can affect the duration of the plant. You will need to pay particular attention to the choice and position of cables, in order to preserve required performances for the entire period of activity of the plant, that goes from 20 to 25 years. For example, you should better consider:

  • presence of water: if the cable is on a protective case, water can get into the case even by condensation effect.

  • low temperature: it is recommended not to use a PVC cable at less than 15 degrees, because a too low temperature can cause an hardening of the isolator, making it fragile.

  • High temperatures: one of the peculiarities of the cable is that they link photovoltaic modules and that they must resist at temperatures of 70-80 degrees. For example, for PVC cables, the highest operating temperature is 70 degrees, while for rubber cables the maximum is 90 degrees.

  • Solar radiations and ozone: the solar irradiations determine the deterioration of the sheath and as a consequence the loss of mechanical end electrical characteristics. Furthermore, it is necessary to reduce the quantity of cables exposed to sun at a minimum of 10%.

Cables for photovoltaic systems must also have a minimal section determined by the appropriate dimensioning in order to have an efficient and safe plant. The minimal section must not be inferior to 0,25mm/A for cables up to 50 meters long, following the UNEL 35023 norm.

CABLES DIMENSIONING

Cables dimensioning (CEI 64-8 art. 433.2) is carried out in a way to contain the voltage drop within the predeterminded limits and to guarantee a life span, for insulant conductors influenced by termical effects caused by the current pass in normal operating conditions, that is at least the same of the served photovoltaic plan. In general, the section of a cable must be chosen in a way that it's extent (Iz) is not inferior to the operating current (IB) of the circuit (Iz>IB) and the voltage drop in the tract between the two modules and the inverter shuld be contained within 1%. We recommend to use cables with 4mmq section for short-circuit-current superior to 4 Ampere to cover length inferior to 8 mt. For amperage or longer distances we recommend a minimum section of 6mmq. In order to calculate the necessary section of the cable you can entrust to some online tools, such as oppo.it.

Let us suppose that a cable has a 1,2 mmq section (that means a resistance of 0,012 ohm/meter) and a 2mt length (1mt for postitive cable and 1mt for negative cable). The total resistance of the cable will be: 2x0.012=0.024 Ohm. If now we suppose that in the cable flows direct current of 20A, in this way we can apply the Ohm law to calculate the voltage drop along the cable. According to Ohm law: V=IxR = 2x0.024=0.048 V. if your system works at 12V, the voltage drop will result 0.4%, which is an acceptable value. With the dubling of circulating current, the voltage dop will obviously double, and the same happens with a double length of the cable, while if the system works at 24V, the voltage drop will result halved.

Wiring operation is not subject to particular precaution, we just remind you to pay attention in linking cables and terminal boxes in order to avoid dispersions.

TECHNICAL TABLES OF PFOTOVOLTAIC CABLES

Cores' number x section

Approx conductor diameter

Minimum insulation thickness

Maximum external diameter

Maximum electric resistance 20°C

Current carrying 60°C

(N° x mm2)

(mm)

(mm)

(mm)

(Ohm/km)

(A)

1 x 1.5

1.5

0.7

5.1

13.7

30

1 x 2.5

2.0

0.7

5.7

8.21

40

1x 4

2.5

0.7

6.2

5.09

55

1x 6

3.0

0.7

6.9

3.39

70

1 x 10

3.9

0.7

8.2

1.95

95

1 x 16

5.0

0.7

9.3

1.24

130

1 x 25

6.4

0.9

11.4

0.795

180

1 x 35

7.7

0.9

12.8

0.565

220

1 x 50

9.2

1.0

14.8

0.393

280

1 x 70

11.0

1.1

16.9

0.277

350

CONNECTORS

The majority of photovoltaic panels, especially if they dispose of a certain power, are equipped with a junction box on their back, from which two cables come out, which end with the respective plug-and-socket connectors. This is about specific connectors with locking system, generally a MC one, with a maximum voltage of 1.000 DC V. Multicontact connectors MC3 and MC4 (respectively with a 3mm and 4mm diameter, both with a maximum admitted power of 20A for section cables larger than 4mmq and of 30A for 6 mmq cables) are always the most known standard on the market. Other common typologies of connectors are the Tyco, adaptable to MC standard through dedicated adapters.

In order to connect panels between them or to link strings to inverters, you can use particular watertigh connectors (that means that they have a IP65 protection class in the case of MC3 connectors and IP6 in the case of MC4 connectors) which have the characteristic to resist against bad weather without getting damaged for a 20 years duration end without letting water and dust penetrating in the inside. Like cables, also connectors have to resist to temperatures from -40 to 120 degrees.

In case you need to install two panels, the linking will be carried out through MC4 splitters or parallels connectors. Such as Mulicontact splitters MC4T that are especially designed to link renewable energy plants, in order to link parallel groups of panels or strings of panels.

CABLE GLANDS

Cable glands are essential for a photovoltaic plant installation because they represent a jonction point between the panel and the cockpit. The two-way cable gald can be find at the same point where the caravans roof gets drilled in order to let cables pass from the outside to the internal part. It can get easily glued to the roof surface with resins, glues or silicon on clean surfaces in order to make it waterproof and safe from water and damp sources. This represents a safe system to allow the passage of the cable in the internal part wihout risking water infiltrations. Cable glands are often also resistant to UV lights, and provide a resistsant plasic stucture and two routes for cables including O-ring to make the product watertight. We recommend to use a cable gland such as the following one in order to guaratee de product impermeability.

Generally, MC4 connectors result incompatible with spaces internal to the cable gland, that is why it is better remove them and execute the linking with a terminal box internally to the cable gland. This allows to insert cables coming from the panel right into the cable gland and, once inside the cell, to connect them through a terminal box to cables that arrive from the regulator. In this way the link will be easy to dissect and, at the same time, it will find itself in a zone safe from water and others incidental breakages caused by impacts.

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