By Antonio Ruffini, Editor, ESI Africa
The head of the working group dealing with the draft NRS 102:2013 specification covering theft deterrent earthing materials, Gavin Strelec, proposes to eradicate the theft of earthing conductors rather than simply mitigating this problem.
The theft of earthing conductor is economic sabotage,” Strelec says, and it is at the top of a hierarchy of risks to the earthing of electrical systems. “Even insufficient earthing will initiate protection in most cases, but if the earthing conductor is not there… It is a problem that we can no longer afford to tolerate.”
Poor connections due to lack of contact pressure or corrosion is the second highest risk to earthing systems. Steel has been proposed as an alternative to copper but below ground corrosion of steel earthing conductor is also high on the list of risks to be considered. A number of solutions have been considered to protect earth conductors of electrical systems from theft. One of these has been to encase the earthing in concrete, but Strelec points out that some perpetrators will break through concrete and suitable mechanical protection would become cost prohibitive.
“Many of the perpetrators have time and energy and will penetrate through practically any mechanical barrier,” he says. In addition, copper earthing in concrete is not cost effective, as temperatures can only go up to some 90OC, whereas the fusing temperature for copper is 1,083OC, resulting in poor conductor utilisation. “In fact the value of copper makes theft of conductor so lucrative, that it is equivalent to leaving your wallet lying on your desk in an open plan office,” Strelec says.
Another solution attempted has been to conceal the earthing, but with transformers there is no convenient way to do this, and such earthing conductor is regularly harvested by thieves, which means costly large power transformers may be lost due to lack of protection operation. In other instances it is possible to have internal earthing hidden in equipment support foundations. But the lack of visible earthing can cause problems since when some earthing in a facility is sealed and some is exposed it becomes more difficult to assess what is earthed and what is not.
“I believe the solution is something that essentially has no scrap value – and hence we are aiming for eradication of earthing conductor theft,” Strelec says. Based on information from the metal recyclers association in South Africa, recycled copper trades at some R50 to R60/kg, brass at R39/kg, aluminium at R11/kg, stainless steel at R11/kg and steel at R2.0 to R2.5/kg. The replacement of copper with aluminium is not feasible for buried earthing applications due to corrosion, but even in above ground applications it is not advised, since the latter still has relatively high scrap value. In some electrical applications aluminium has been used to replace copper but aluminium is easy to cut and bend and, while an improvement over copper, remains sufficiently lucrative for thieves.“It is simply moving the goalposts.”
There have been projects done where copper was replaced with aluminium, which has about 60% conductivity of copper, but due to the overdesign of the old copper busbars these projects were sometimes able to get away with similar cross sectional dimensions. However, this aluminium is also being stolen. The use of steel on its own is technically viable when applied with sacrificial anodes, but the heavy sections of steel for equivalent rating to copper become costly to install. In addition, conductors have to be connected by welding which requires a high skill level. Both of these render steel a less than ideal solution. Copper clad steel has been used for earthing in USA for almost a 100 years and copper plated earth rods have been used for about 15 years in South Africa for earthing systems.
Both materials possess very limited latent value as the copper is considered to be an impurity in the steel that can be blended into steel but is not desirable. The recovery value of these materials should be equal to or slightly less than steel, i.e. R2/kg. Thus the proposal to eradicate earthing conductor theft is to use copper clad or electroplated steel, and this is the recommended approach by the NRS 102 working group. Strelec says the cross sectional areas of such conductors are not prohibitive. The solution has been used elsewhere and has proven that using copper plated steel rods does effectively eradicate theft. Local experience in South Africa has shown that, despite some initial tampering, essentially there has been no theft.
The cutting of these conductors is quite distinctive from copper conductor. Cut marks have been observed, but the conductor is not taken as thieves realise this is not the copper they are seeking. Stranded alternatives are discouraged since they are difficult to differentiate from solid copper and some local experience in Cape Town in the 1960s indicated a large risk in this option. The ideal would be to have several hundred metre drums of the conductor, to minimise the number of joints. Once in place, tampering could be inhibited by putting up signs indicating that the earthing conductor is not copper, and thus worthless to scrap dealers.
Off-cuts could be scattered around and distributed to show potential thieves this is the case. In some international jurisdictions it is illegal to deal in recycled copper clad steel, and this could be considered for South Africa. One would hope to get buy-in from the metal recyclers association. Another helpful intervention would be to pressurise government to introduce legislation to cap the amount of copper that can be exported, though that in itself is insufficient since there is little knowledge of how much copper is illegally taken out of the country. The consensus on the cost of the copper clad steel conductor is that it should be cheaper than copper on initial cost and this was ostensibly the reason for its initial application in the USA.
New York Stock Exchange listed General Cable says that stranded dead soft annealed copper clad steel is used for buried ground grid systems where a more economical alternative to copper conductors is desired. In contrast, some local suppliers in South Africa purport that the cost of copper clad steel may be comparable to that of copper due to the additional fabrication costs.
However, the life cycle costing of copper clad steel when combined with exothermically welded connection is a fraction of conventional copper installations (20% or better) utilising bolted connections that require frequent continuity tests and maintenance. The only disadvantage of copper clad steel, which is related to why it is not attractive to thieves, is that the disposal costs of this material are higher than that of copper, as the recovery of the metal requires a foundry to separate the copper from the ferrous component.
The field of earthing, as simple as it seems, is often one where people apply a great deal of creativity, with a myriad of systems and conductors used. In general, a search for creative solutions should be encouraged, as long as the end result achieves the objective in a technically sound manner, and this does not always happen. In terms of the earthing conductors themselves, Strelec would prefer to see a degree of standardisation and the type of conductors used limited to only a few.
“We need to apply ruthless standardisation, even if this is not always 100% optimal, but with reduction in engineering input in view of the lack of skills and the economic benefits of standardisation, it is recommended.” A goal is to achieve local manufacture of the copper clad steel conductor. Two South African companies exist that manufacture exothermic welding kits locally, and while there is no commercial production of copper clad steel conductor in the country, ultimately pressure from customers will force it to happen.
“We have seen local companies that can supply copper plated steel rods which can be used in some cases but are challenging due to difficulty in bending the high tensile steel of relatively large sections, and the rods are only available in typically up to 3.0 metre lengths.” Strelec says the transition to composite copper and steel ideally requires a flat strip alternative to rod and stranded cable.
Flat strips have mounting and installation benefits in certain applications whilst providing lower surge impedance. It raises the possibility of the usage of substantial galvanised steel conductors for earthing, though corrosion will be a problem that has to be taken into account. If the industry does require copper coated flat bar, electroplating appears to be the only practical means to manufacture this type of conductor.
Strelec’s personal view is that thin wide copper plated conductor could be stacked or laid side to side, though the heavy steel sections are difficult to work with. Of course the skin effect means that copper encased steel conductor performs disproportionally well compared to bare steel or galvanised alternatives. In general, Strelec believes too much is being invested in conductor in South and southern Africa, and proposes that new systems should have much lower life cycle costs.
“We are doing a lot of continuity tests of systems and there is a lot of risk associated with that, and a lot of this could be done away with.” A problem with this field is that a lot of solutions have been prevented from moving forward due to concerns raised about quality and safety. It appears that many local users are overdesigning earthing systems. The criteria used for the sizing of conductors are the peak amplitude of the neutral current and the duration of the fault.
The recommended duration is the clearing time of the backup protection scheme, which for high voltage systems could be around 500 milliseconds (ms). In South Africa short time ratings of 1.0 seconds, 3.0 s and even 5.0 s have been used, whereas in the USA 0.5 s is frequently used. Europe is generally more conservative, designing for 1.0 s. If the statistical distribution of fault amplitudes and durations are considered, the traditional deterministic approach for conductor sizing is overly conservative, which costs a lot of money and caters for rather miniscule probabilities.
Far more tangible risks lie in the appeal of the conductor to thieves, and unreliable connections. This aside, one of the advantages of the proposed solution using exothermic welded joints is that the conductor itself will fuse before joints lose integrity. Equivalent copper clad steel conductors are rated on the fusing temperature of copper, which buys some extra time due to the melting point of steel being some 1,500OC. Thus there will be more time before the conductor is lost in such an event.
IEEE 80 suggests the use of the structure supporting the power equipment as part of the earth path. Copper conductor in parallel with steel structure is superfluous. South Africa is the anomaly among BRICS countries in that it is the only one that uses copper for its earthing conductor. In Russia, as well as India and China, steel is used in the earthing systems, primarily due to cost. The Russian experience using steel has shown a long service life (sacrificial anodes are used). In Sweden it was found that theft was reduced through the use of copper clad steel. Some utilities in Germany only use copper in high fault current applications close to source busbars, but downstream distribution networks utilise steel earthing systems.
In India steel flat bar or rebar is used, which results in large cross sections to cater for transmission system fault levels. The downside is that it cannot be bent, is difficult to work with, requiring substantial manpower, as well as high skill levels to do welding. The higher resistivity of steel is slightly offset by a higher fusing temperature.
“The question is why is South Africa still using so much copper? We have to move forward for economic reasons as well,” Strelec says. He believes that any new installation should not have pure copper in any form. The long term solution to theft is to eradicate copper, which can be removed from installations in only a few minutes.
Power cables are the next thing to look at, but dealing with this problem will be more challenging. Feedback obtained by the NRS 102 working group is that, like South Africa, many countries in Africa use pure exposed copper bar. The relative ductility of copper clad conductors of the usual sections make this the preferred option in African countries.
However, in many of their cases they experience no theft. Among these are Mozambique, Zambia and Ghana, and it is not because the countries are utopias where the people refrain from theft, since many other items such as bread and water get stolen. It is the absence of scrap dealers, as stolen copper cannot be traded. In contrast Eskom reports that high levels of copper and pylon theft persist. During the year that ended on 31 March 2013 Eskom’s losses due to conductor theft, including the theft of copper, cable, transformers and tower related structures totalled R50.5 million and involved 5,187 incidents. A consideration when it comes to earthing conductor is that of corrosion in the ground, and that together with wide cross sectional areas used with galvanised steel explains why it is not used as much. Typically within a few years of burial, galvanised steel conductors of usual sections are compromised due to corrosion. Galvanised steel conductors can be protected, but this becomes a difficult system to manage. In contrast copper clad steel with a copper thickness of 250 microns has service life of 40 years in most soils based on an extensive US National Bureau of Standards survey.
“One needs to determine the nature of the soil to assess whether additional corrosion protection is required as would be the case with solid copper. Work on the corrosion effects has been done and proven in numerous applications overseas.” Copper clad steel is available in different standard copper equivalent conductivities: 20%, 30% or 40%.
Conductivities of 30% or 40% are preferred in high current applications to minimise conductor sections. The fusing of the copper and the steel during manufacture occurs at a molecular level for copper clad steel, which means there is no connection interface for corrosion. This leads to the second highest priority, after eradicating theft: that of ensuring the connections involving earthing conductors are sound. The connection is often the weak point in an earthing system. Strelec says that bolted earthing connections might be a poor solution, unless a lot of maintenance is done to check these, since the connections can come loose and are prone to corrosion.
“These typically have to be checked every two years, and one needs to consider the torque applied on the bolt, etc. Even if the bolt is correctly applied this type of connection can limit the earthing capacity of the system due to a maximum temperature of 250OC. Bolting can also introduce bi-metallic connections, which is not a technically acceptable option. Crimp connections are a better alternative to bolting.”
The ideal connection system is one that can be forgotten about for 50 years. In general, the idea is to progress from complex connections that require a lot of maintenance to exothermic welds which result in 100% conductor utilisation during fault conditions and have no maintenance requirement. Since the components of the systems are fused together there are no connection interfaces that need to be maintained. An earthing system also has to take into account surge impedance caused by lighting or similar high frequency effects.
One way to deal with surge impedance is the use of multiple connections. At high frequencies, such as lighting frequencies, the skin depth is extremely shallow, which means the copper portion covering the outer part of the conductor will be relevant. While today, in South Africa, copper clad steel conductor would have to be imported and suppliers might resist the change, as will scrap dealers, ultimately the change is coming. Once demand for these new alternatives increases, local manufacture will begin and costs will come down. “Copper clad steel is coming for earthing applications. In future we would like to see similar technology implemented in power cables,” Strelec says.