By Morgan Havire, AMNS Consulting Engineers
The system of cables used for the transmission of electricity are typically outdoor and are subjected to the harsh environmental factors which means that they require protection to guarantee proper functioning over a prolonged period and safety. The factors that affect cables can be categorised into ‘Thermal’ and ‘External Influences’.
External influences: Any influences that are external to the cables which can affect the safe operation of the cable and also the design. These include factors such as:
• Ambient temperature – temperature of air or any other medium where the cables are installed
• Direct sunlight
• Animals and plants
• Rain or water vapour or water accumulation
• Mechanical damage
Thermal effects: Thermal effects arise due to the operating conditions of cables such as:
• Method of installation
• Cable grouping
• Protective measures
• Current, and accessibility
Additional thermal effects could be due to over-current (overloads and short circuits).
Protection of cables from sunlight and heat
Cables that are used in PV installations are rated as Ultra Violet (UV) resistant or simply that they can withstand high temperature ranges from as low as -60°C up to as high as 90°C. Although the insulation and jacket materials that surround the conductors are extremely resistant to UV radiation, the underlying fact is that they must be kept from direct sunlight exposure as much as possible.
With continuous direct sunlight exposure the cables suffer degradation causing damage. By also selecting the correct colour for the PV outdoor cable which is ‘Black’ (containing carbon black in its insulation), the cable will be provided with additional UV resistance. Unprotected PV cables from direct sunlight or high ambient temperature results in the cable becoming brittle or breakable.
Protection of cables from direct sunlight can be achieved by implementing a safe, reliable and code compliant wire management system. By conveniently using the available shade of the modules and other structural members, one can improve on the life of PV cables.
Protection of PV cables from heat sources can be achieved by not fixing the cables within six inches from the heat source. Another method to protect the PV cables from such a heat source is by adequately shielding the cable and its protective sheathing or properly selecting a PV cable of special design (heat resistant cable) to withstand the effects of such exposure. Cable trays’ systems can also be used in lengths of the panel arrangements where PV cables run along the trays under the modules.
Protection of PV cables from water
PV cables should not be in constant contact with water as this may cause the cables to be saturated with water. Persistent water saturation can lead to insulation resistance damage and even rotting of cables causing short circuits and earth leakages. Cables which can be subjected to such conditions must be carefully selected in accordance with the acceptable standards.
The cable selection must ensure that the cable used is for outdoor use and can be used in wet conditions. In order to prevent or protect cables from water damage due to constant water logging, shading and elevating of PV cables in sealed channels and conduits.
For PV cables underground the same principle of sealing exposed ends of conduits or sleeves will alleviate the saturation of cables with water. To protect the PV cables from water in general they shall not be fixed (if possible) where they will be exposed to the accumulation of water which might damage the conductor or its protective cover unless the cable or its protective sheathing are adequately shielded or of a special design to withstand the effects of such exposure.
Protection of cables from physical damage
TUV approved PV cables are manufactured with mechanical robustness. These solar cable are insulated twice with an insulation around the conductor and an insulation outer jacket. If the outer jacket is damaged or cut, there is another layer providing the necessary protection for the conductor. The insulation for PV cables is also manufactured by using the electronic beam cross linking procedure which improves the shear and impact strength of the cable thus protecting the cable more effectively.
Notwithstanding the above, damage can still occur during PV cable installation if sharp edges and corners exist. Moreover, PV cables are installed through openings drilled in the structural metal work of the PV system creating sharp holes that can cut through the insulation of the cable. PV cables and their insulation shall be protected by ensuring that every hole is bushed so as to prevent abrasion of the cable insulation or even compressed under the weight of modules.
PV cables must be protected from possible movement by the fastening of cable ties, clips and other attachment tools ensuring that the electrical properties of the cables is not in any way compromised.
Protection of PV cables against corrosion
The insulation for PV cables is manufactured by using the electronic beam cross linking procedure. These cross linked insulation materials greatly improve the chemical resistance of the cable thus protecting the cable more effectively.
The type of PV cable insulation must be selected to be able to protect the cable from corrosive chemicals. The protection of PV cable can be provided by fixing the cable in positions where the cable or its protective insulation is adequately shielded or exposed to corrosive chemicals or as mentioned, of special design to withstand the effects of such exposure.
PV cables that are likely to be exposed to chemicals should have an insulation cover that has a high degree of resistance to chemicals. The PV cables can be afforded protection by being kept or shall not be fixed within six inches from such corrosive materials the contents of which might damage the cable or its insulation unless the cable is adequately shielded or of a special design to withstand the effects of such chemicals.
Protection of cables from animals
Pests like rodents feed on PV cables resulting in a loss of production and extensive repair works if the issue is not addressed. Some of the existing technology on the market to mitigate this situation involves the use of barriers attached to structural members and rails. The two technologies in the market are Heyco’s SunScreener and Spiffy’s Solar screening
It can be noted that compliance with the relevant standards will certainly achieve the code compliant and safe cable management for PV cables. As a result of adhering to the design and installation requirements, the entire PV cabling network can be adequately protected.
The PV cable system will ensure reliability guaranteeing the much required rate of return on the capital investment for clients. Maintenance of the installed system must follow to ensure that the investment does not deteriorate and this can be achieved by periodic inspections and verification of the soundness of all PV cables and equipment. The maintenance can be achieved by drawing up a maintenance management plan which can be agreed with the clients. With the evolving of new technologies, installers must advise clients on these new innovations and be able to advise on ways to improving the old existing plants. Factors for consideration in selecting the right technology for protection of the PV cables should be based on code compliance, ease of installation and cost.
Morgan Havire is a Chartered Electrical Engineer with experience that includes site surveys, design, supervision, testing and commissioning of new buildings and refurbishment of existing ones. He has undertaken major projects as a principal and lead design engineer responsible for all electrical and electronic services on projects throughout Africa. Morgan has undertaken design assignments within teams working on projects in South Africa, Zambia, Mozambique,, Nigeria, Australia, United States, Nigeria and Equatorial Guinea.