WHAT ARE THE PARAMETERS TO BE REGARDED WHEN SELECTING THE MATERIAL OF CONDUCTORS?

Based on recent and past experiences that have resulted many times in the loss of lives, the loss of significant investments, and in indirect losses that have severely affected household chores and production, and in order to avoid similar situations in the future, we are starting now a series of technical communications to provide useful information regarding the Design of Electricity Distribution Networks

SUMMARY
The natural aggressiveness of the environment should be regarded when selecting the material of the conductors used in electricity distribution networks, so as to safeguard the mechanical and electrical performance of the design, as well as the useful life, reliability and safety of the installations.

1. INTRODUCTION
Electrical conductors are made of Copper or Aluminum, because both metals have the adequate basic characteristics for such purpose; nevertheless, said metals have physical differences as well as their own performance aptitudes when facing the aggressiveness of the environment. Generally, minor cross-sections are used in low voltage distribution systems and networks; and the aforesaid can make them more vulnerable when confronted with environmental demands.

2. WHAT CONDUCTING MATERIALS ARE USED AND HOW?

2.1 Metallic Materials for Electrical Conductors
The cost of the base metal as well as its conductivity, limit the choice of metals to Copper and Aluminum, which are also alloyed and thermally treated to improve their final characteristics. In the case of pure Aluminum, a steel reinforcement may be included, if necessary.

2.2. Use of Electrical Conductors
Initially, low voltage electricity distribution systems used Copper conductors only. As of 1945, the presence of Aluminum conductors was noticeable, mainly due to the their lighter weight and lower initial investment cost. Nevertheless, the good performance of Copper over the years, especially in dense atmospheres, industrial zones, and corrosive seashore areas, has enabled it to be regarded as the most convenient material.

2.3 Consequences of an Economic Conditioning
The minimum-cost-investment criterion, disregarding a prior analysis of the compatibility and performance of the conductors once in contact with the environment, may mislead to a wrong choice based only on lower initial costs. The aforesaid could result in: rapid deterioration and subsequent collapses, high maintenance costs, losses by premature replacements, as well as profit losses; apart from the latent danger, the loss of reliability on the system, and the reduction of the useful life of the installations.

2.4 Experiences with Electrical Networks that Collapsed
Out of the different experiences in Peru, two of them are worth noting for the seriousness of their consequences. They took place between 1978 and the early years of the decade of the 1980’s, and involved the new and refurbished Aluminum electricity distribution networks of the cities of Trujillo and Chimbote, which only two years after their installation began suffering serious deterioration with countless conductor fractures and collapses. The aforesaid resulted in chaotic services, the loss of human lives, and enormous economic losses.

3. WHICH IS THE CONTEXT OF ELECTRICAL INSTALLATIONS?

3.1 Copper Conductor Manufacturing
Usually, Copper conductors used in aerial electricity distribution networks are manufactured with 99.87% grade metal; however, light alloys are occasionally applied to improve its mechanical characteristics. This is done without having to sacrifice its conductivity or thermodynamic stability because it is a noble metal. Finishing thermal treatments permit the production of the three most frequently used conductors: Annealed, Semi-Hard Temper and Hard Temper Conductors.

3.2 Aluminum Conductors
Aluminum conductors are manufactured with 99.5% grade metal; nevertheless, light alloys are included in most production intended for aerial electricity distribution networks, so as to significantly increase the mechanical characteristics. However, the aforesaid causes certain deterioration of the metal’s conductivity and thermodynamic stability. Furthermore, the delicate finishing treatment required at the end of the conductors manufacturing process is decisive for their future performance when confronted with mechanical and environmental demands.  

3.3 Comparison between Conductors’ Properties
The analysis of performance requirements for Copper as well as Aluminum throughout their useful life, must take into consideration that the main properties of both base metals (TABLE 1) when used in conductors of identical size, can have different effects on the electrical and mechanical design of the electricity distribution networks. These may be translated into long lasting conductors, free from relaxation or permanent deformation, that require minimum maintenance and assure reliable and safe operations.

3.4 Influence of the Environment on Installations
Geographical location of installations defines the demands that the environmental places on conductors; these depend mainly on atmospheric and meteorological parameters, and also on the natural and artificial air pollution of the area. Generally, the selection of the conducting material will have to be made based on these conditions.

a. Outdoor Weather
Effects Weather effects are manifested mainly through the erosive action of ultraviolet radiation and the loss of mechanical resistance due to solar thermal gradients applied cyclically to the exposed conductors, which must also dissipate the heat created by the electrical current - Joule Effect. Major thermal gradients are observed in the Coast and in the Jungle regions with less incidence of ultraviolet radiation, while the opposite occurs in the Highlands. To counteract weather effects, conductors are covered with a rigid Protective Patina that is quite weather resistant and capable of withstanding different temperature levels without permanent deformation.

b. Wind
Wind is an effective mean of transportation and deposition of solids, gases and fumes that form the artificial and natural pollution, as well as the humidity of the environment. In the Peruvian Coast, winds enter from the Pacific Ocean, through the South and South-West, with speeds ranging between 3 m/s and 17 m/s, blowing marine and desert sand salts that produce abrasion of protective oxide layers in electrical conductors. In the Highlands and in the Jungle regions, except specific localized pollution cases, winds only blow masses of air with minimum amounts of dust and humidity.

c. Humidity
Environment humidity, except pluvial precipitation in its various forms, comprises natural water steam, condensed steam in the form of fog, and sometimes in the maritime littoral, fog and atomized sea water from sea waves. Intense humidification of the surface of conductors occurs in the Coastal region, due to the overlapping of thermal gradients starting from or towards lower temperatures. Daily natural humidity in the Highlands is normally low, oscillating between 40% and 75%, while in the Coast and in the Jungle it is high reaching 100%.

d. Active Pollution
Comprises chemical elements deposits (TABLE 2), accumulated on the surfaces of the conductors, which in the presence of humidity and condensation form electrolytic solutions - somehow conducting electrolytes - cause corrosion of different cells. The coastal region is affected mainly by marine and soil desert salts pollution; in addition, urban areas, are also polluted by contamination agents from vehicles, industries and industrial farming. The atmosphere in the Highlands and in the Jungle is clear; nevertheless, there are specific high pollution areas created by extractives centers, concentrators, cement factories, vast farming, highways, urban centers, etc. Rain, in some cases, counteracts these effectively, washing away the deposited pollution.

4. IRREVERSIBLE DETERIORATION OF CONDUCTORS

A comprehensive electrical and mechanical sizing of conductors, without an environment compatibility analysis does not guarantee a 30-year useful life under normal operating conditions, as contemplated in the investment. Aggressive environmental agents will be present from the moment installation works are started, mainly through corrosion; causing conductors to collapse in the short and medium term.

4.1 Environmental Aggressiveness
Aggressiveness occurs in specific locations in the Highlands and in the Jungle regions. Studies of deterioration of aerial conductors in the Peruvian Coast, caused by fractured strands and collapses in medium spans and in points of attachment to insulators, have permitted to identify seashore different degrees of atmospheric aggressiveness up to + 500 m.a.s.l. If we correlate the aforesaid to similar locations overseas (TABLE 3) corrosivity is high and falls into the "medium" classification.

*Coastal area up to 500 m.a.s.l. 
TABLE N° 3 : Coastal Environment corrosiveness levels in specific locations overseas, with aggressiveness comparable to that of Peru’s Coast.

4.2 Deterioration experiences in the Peruvian Coast
Urban seashore networks, located up to 2.0 km from the sea, with lined Copper conductors and small cross-sections, suffered serious insulating damage, sporadic fractured strands and collapses, 15- 20 years after they were installed. Further away from the sea; Copper conductors without lining, seldom register similar incidents, even after more than 30 years of use. Aluminum conductors performance in the principal cities of the Peruvian Coast is not encouraging, due to their vulnerability to coastal abrasive and corrosive agents, which in some cases, caused the fracturing of strands and the loosening of connectors, as well as collapses similar to those mentioned in item 2.4 hereof. Analogous contingencies allowed us to prepare TABLE 4, which presents referential intervals of average duration of electrical networks with conductors without lining.

 4.3 Causes of Deterioration
Deterioration results from the overlapping of various environment related agents, mechanical specifications, the execution of the installations, and the characteristics of the materials used in the network. Despite these emerging demands, the deterioration process of conductors is supported by or identified with different forms of corrosion attacks that act simultaneously and cyclically, day by day, until fractures occur.

4.4 Possibility of Counteracting Deterioration
The deterioration process starts once electrical networks are installed, and the progress of such process depends mainly on the conductors material. There is no way of stopping or counteracting it, except by replacing faulty conductors with other of better performance. Maintenance initiatives including coatings or greases will be just short term palliatives.

5. WHICH ARE THE SUPPLEMENTARY DESIGN GUIDELINES?

Selection of conductors and connection accessories for distribution networks should be subordinated to the location of the project, and to the characteristics of environment related agents that will have a direct impact on the infrastructure. A detailed analysis of the aforesaid will permit the right choice of conductors material, specially when pollution sources and micro-climates are involved.

5.1 Examination of the Geographical Location
When the location is in a littoral area, an environment analysis is fundamental. Possible environment aggressiveness may be inferred a priori from TABLE 3, based on the distance of installations from the sea. Then, said indicators must be verified "in situ" with respect to the corrosion. If a location corresponds to Highlands or Jungle areas, a further examination is required as regards presence of pollution sources.

5.2 Examination of Pollution Sources
Examination should be carried out at the project site, disregarding whether it may be located in the coast or in the interior of the country; it is necessary to find out how proximate sources of pollution are. To infer or confirm environment aggressive agents and the area of land they have an impact on, firstly, samples of contamination deposited on the exposed non-metallic surfaces should be taken; and after that the saline equivalent Mg Cl Na/cm3 should be characterized, as well as the percentage of soluble salts and the pH. All free Chlorine content or Chlorides close to or above 0.7% or pH on the acid or alkaline side, will denote a corrosive atmosphere.

5.3 Examination of Environmental Parameters
Atmospheric and meteorological parameters should deserve a retrospective examination of at least 5 years of recorded maximum and minimum absolute magnitudes. Results from such analysis will permit to infer the incidence of relative humidity, temperature gradients, pluvial precipitation and predominant wind speed and course. This will help determine doubtful condition areas or identify the degree of "corrosivity" of the place.

6. CONCLUSIONS

a. Peruvian experiences as regards conductors performance in Aerial Electricity Distribution Networks show, that the Coastal atmosphere tends to behave in a hostile fashion against materials, according to their location with regard to the sea, up to 500 m.a.s.l.
b. The necessary analysis for the right choice of material for conductors of Aerial Low Tension Electricity Distribution Networks, is facilitated as of the correlation of the variation of environmental parameters and deterioration mechanisms associated with the materials.
c. A selection of material for conductors based on a minimal initial investment criterion, without examining the aggressiveness of the environment, may lead to vulnerable, unreliable, unsafe and short life installations.

This publication has been prepared by Engineer Justo Yanque M., M.E. Eng -UNI, M.Sc. App - FPMS - Belgium, who is a Specialized Consultant with ample technical experience in electromechanical projects. Procobre Peru will be glad to help all interested parties who wish to obtain further information on this subject, and/or know the base bibliographical references used by the author of this article.