The L.C/Gecamines main office is a production entity of Gecamines Company in the Democratic Republic of Congo, engaged in the processing of non-ferrous metals (Figure 1 & Figure 2).

The insulated wire produced by L.C/Gecamines main office, was mainly composed of 99.73% copper and some other impurities at acceptable levels (Table 1). The wire-bars received by the headquarters from Shituru mining plant, had a deformation of ±2%. These wire bars were recycled in a FRITZ furnace at 400˚C - 450˚C for 3 hours of heating time, half an hour of holding and 18 hours of cooling. This operation resulted in new characteristic as shown in Table 2. The plastic materials in the form of pellets (PVC) had a density of 1.52 g/cm³; a low tensile strength of 13.6 MPa (15 MPa - 18 MPa for the supplier); an elongation at 200% breakage (280% for the supplier) and a continuous operating temperature of 70˚C to −5˚C for the L.C/Gecamines main office (Table 3). Noted that all these values (for plastic) from the supplier were define according to IEC 60,250 and SABS Methods 503/4/7.

BALTEAU apparatus were use in this study; it is a generator of medium voltage from zero to 10 kV, used for tests of dielectric rigidity of the insulator (Figure 3). The objective within was to determine the lifetime of cables based on the resistance of the insulation. A good insulation was defined as the insulation resistance that has a great value or equal to 1/100 of the applied voltage [7].

This lifetime is a function of the insulator’s resistance. The insulator must resist to the current, and maintain the latest in its trajectory along with the conductor.

In order to determine the resistance of the insulation, five samples (for each temperature) of 60 cm length and 9.075 mm² cross-section were prepared and stripped at both ends; the central part of the sample was attach in the water bath (Figure 3(b)). The two ends were not in contact with the water (setup shown in Figure 4). The phase of the device was connect to the first stripped end of the

cable and the neutral was immerse in the water bath. A voltage of 8 kV (8000 V) was sent through the cable (the time depends on the test to be performed).

During the study, the applied method was to determine the leakage current of the insulating tape than based on Ohm’s law, deduced the resistance of the insulating tape:

U = R ⋅ I l e a k i n g (1)

In order to evaluate the chemical attacks resistance, the used of a round bottom flask of 11, a principal solution, distilled water for dilution, a volumetric pipette, sulfuric acid, two beakers and a stopwatch. After the samples and the sulfuric acid solution’s preparation, these samples were immerse in an acid solution for 48 hours and then dried after 24 hours in an oven at 150˚C until a constant mass was obtain.

The table shows 3940.88 Mega Ohms characterizes the resistance of PVC insulation. However, it should be noted that there is no such thing as perfect insulation (i.e., no insulation has infinite resistance); from that, a certain quantity of electricity always flows along or through the insulation to the outside environment. A good insulation essentially means a relatively high resistance to current. In order to describe an insulating material, a good insulation also means “the ability to maintain a high resistance” [8].

Having a look at the results obtained in Table 4, a confirmation come out that the isolation performed at L.C/Gecamines main office is good quality. Furthermore, if measured at regular time intervals, it is possible to monitor it degradation’s trends.

Table 5 shows the decrease in insulation resistance over time at 40˚C (near ambient temperature), 70˚C, 100˚C and 160˚C.

Table 4 below presented a highlighted lifetime of the insulation according to the different test temperature. Started from the graphical representation of each case, and then conducted through Excel, the trend line resulted to an equation of decrease’s evolution of the resistance according to time (Graphic 1).

Starting from the trend of the insulation resistance loss (plasticizer loss), R e s = − 0.0091 t + 3940 (with R 2 = 0.9992 : Determination graphical coefficient) where R e s = 8 Mega Ohms;

The lifetime would then be equal to:

t = 3940 − 8 0.0091 → t = 432 , 087.9121 Life hours corresponded to ± 50 years.

Graphic 1. Progress of PVC strength as a test time’s function at 40˚C.

According to this theory, the lifetime seems to be long because the main way of PVC insulation deterioration at this temperature consists in the evaporation of the plasticizer, the deshychloration is so low and even negligible. The Brebu’s work [9], who studied waste medium voltage cables (6 - 10 kV) after 18 years of operation, concludes that the PVC’s deshycloration is very low under service conditions. Indeed, according Hishler [8], it would take at least 2 billion years to have a release of 1% by mass of HCl at 40˚C. clearly, this estimate has no physical significance since the material itself will not survive that many years, but it does confirm that deshychloration of PVC is a negligible phenomenon at low, near ambient temperatures.

Graphic 2 shows the pattern of insulation resistance loss as a function of time, this pattern translates into the straight line R e s = − 0.0138 t + 3940 (with R 2 = 0.9996 : coefficient of graphical determination).

From this line, a lifetime of ±33 years is obtained. This loss of strength is largely due to the loss of plasticizer at 70˚C. However, in the case of PVC, the loss of plasticizer mass, as shown in the previous point, is mainly due to evaporation [5], Ekelund [10] studied the thermal ageing of a PVC electrical cable insulation. The results showed:

· The plasticizer (DEHP) loss process is controlled by evaporation below 100˚C;

· The evaporation rate of the plasticizer is constant in the studied time interval (150 days), looking at the strength loss curve, the trend of the line is almost linear, so its rate is constant also during 8 hours of research.

The trend of straight line of the decrease of the insulation resistance R e s = − 0.0205 t + 3940 ( R 2 = 0.9968 : coefficient of graphical determination), inducted to a lifetime of 22 years (Graphic 3).

If the temperature is at 70˚C, the only mode of plasticizer loss was evaporation;

Graphic 2. Progress of the strength of PVC as a test time’s function at a temperature of 70˚C.

however, at 100˚C, this loss is largely due to evaporation of the plasticizer but also to a slight deshychloration. The Hishler’s work [8] shows a very low release off HCl below 105˚C (close to 100˚C) (Graphic 4).

The PVC insulation decrease resistance is show by the straight line R e s = − 0.0394 t + 3940 (with R 2 = 0.999 : coefficient of graphical determination), this inducted a lifetime of ± 12 years.

This low lifetime obtained is justified by different aspects: at 160˚C, out from the loss of plasticizer (which is generally known), the PVC itself undergoes deterioration due to deshychloration as shown by the Ekelund’s work, between 120˚C and 180˚C, the deterioration of PVC in electrical cables consists mainly to a deshychloration. The latter is a bad phenomenon because it propagates by sequential removal of HCl molecules along the chain. Each removal creates a

Graphic 3. Progress of the strength of PVC as a test time’s function at a temperature of 100˚C.

Graphic 4. Progress of PVC insulations resistance as a test time’s function at a temperature of 160˚C.

structural irregularity (double bond) that destabilizes the next monomer unit. Several deshychloration acts created polyene sequences along the chain that ultimately caused material’s yellowing and its thermal, optical and electrical properties changes [11].

Each insulating tape’s presence constituents particularly affects the acid resistance value of the cables. Table 6 shows the change in mass of the samples after 48 hours test.

The above table shows that after 48 hours of immersion in sulfuric acid (pH 1.5 and density 1.84 g/cm³), a mass loss of 0.2470 g was observed, i.e. a rate of 2.027% per day. Thus, at a pH of 1.5; L.C cables have very poor resistance to acid attack. These results coincide with those proven by IPEX [12], performing a similar operation on PVC at 23˚C and 60˚C; the results showed that the PVC cable is non-resistant to 98% sulfuric acid attack but it states that it resists to sulfuric acid attack with a mass percentage of less than 80%. On the other hand, the commercial company Alfa Flex [13] proposes a good resistance of standard PVC from 10% to 50% mass of sulfuric acid, and a worst resistance for a higher massive percentage evaluated at 50% in acid. These operations were carried out at temperatures of 20˚C and 60˚C.