LV_And_HV_Electrical

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Figure 2 - In high-voltage cables the insulating material makes up a greater f raction of the tot alcross-sectional area than the conductor material

http://electrical-engineering-portal.com/discussing-about- lv-and-hv-electrical-cables May 13, 2013

Discussing About LV And HV Electrical Cables

Edvard

Space is really a crit ical criterion when discussing electrical cables and wires. In a low-voltage(LV) plastic-sheathed cable with conductor cross-sect ions of up to 10 mm2 per conductor or in

high-voltage (HV) cables (Figure 2), the lions share of the cross-sectional area is occupied bythe insulating material.

Ifaluminium rather than copper is used as the conductor material, the addit ional cross-sectional area required is more or less negligible in comparison.
http://electrical-engineering-portal.com/aluminum-vs-copper-conductors-in-low-voltage-dry-type-transformershttp://electrical-engineering-portal.com/general-considerations-when-choosing-power-cablehttp://electrical-engineering-portal.com/discussing-about-lv-and-hv-electrical-cables


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Lef t: Figure 3 - Mineral-insulated cables; Right: Figure 4 - The structure of f ireproo f plastic-coated cable and mineral-insulated cable

At

least that is the situation for conventional plastic-coated cables. Mineral-insulated cables and

wires (Figure 3) are not only absolutely fireproof, they also take up much less space (Figure 4)than conventional plast ic-sheathed cables.

For a t ime, these mineral-insulated cables were even equipped with an aluminium sheath, butthis never became established and copper sheathing remains the norm.

And in most European count ries, copper is still used predominantly, if not exclusively, forelectrical installation work in buildings.

So why is it that most European standards do not permit the use of aluminium conductors with

cross-sections up to 16 mm2 (or in some cases) up to 10 mm2?

There are three main reasons:

1 Although aluminium is quite ductile, it is not as ductile as copper. The ends of st if wires laid inwalls e.g. as connections to f lush-mounted sockets or wall outlets tend to break after beingrepeatedly bent back and forth. This can be problematic if the imminent f racture point islocated inside the insulating sheath and if t he wire cont inues to be used. In such cases the faultcan remain undetected unt il the wire has to carry a sizeable current (that is one close to itsrated maximum current) and although it could be years before this situat ion arises, when itdoes, the conductor material will melt at the fracture point and sustained arcing can occur.

Aluminium also tends to form these local constrict ions more readily than copper and as it has alower melting point and a lower coef icient of thermal conduct ivity than copper, this sort o f localmelt ing will occur more readily in wires and cables with aluminium conductors.

In the worst case, this can cause the aluminium to catch f ire and burn like a fuse wire.

2 When exposed to air, the surface of aluminium rapidly becomes covered by a hard, durableoxide layer that does not conduct electricity, thus making it harder to ensure electrical cont act.The build up of oxide at points where aluminium wires are terminated or connected, canincrease the local electrical resistance of the conductor. The increased resistance can causeelevated temperatures with the risk of heat damage to the insulat ing materials and possibly f ire.

Copper also undergoes oxidation when exposed to air, but perhaps surprisingly, the oxide layerdoes not inhibit electrical contact, even though the copper oxides (CuO and Cu2O) have

conduct ivities some 13 orders of magnitude less than elementary copper and can thereforehardly be described as electrical conductors.
http://electrical-engineering-portal.com/download-center/books-and-guides/electrical-engineering/abb-electrical-installation-handbook-part-i


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Figure 6 - Only in low-voltage high-current cables does the conducto rmaterial make up mos t o f the cable's to tal cross -sectional area

3 Aluminium has a propensity to undergo slow material creep. When subjected to highpressures, the material will yield over t ime. One result of this is that originally tight connect ionsmay gradually become loose.

Connection t echnology is available that can deal with t his problem and it is worth investing theextra cost and ef ort involved for installat ions involving relatively few connect ion points (e.g. HVoverhead transmission lines), but not for more complex branched networks such as thosefound inside buildings.

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Concluding

Because of the second of the three problems listed above, connect ions involving the ends ofaluminium conductors should always be made as tight screw-fastened contacts.

Unfortunately, the third problem discussed above means that t hese joints are of ten notpermanent. Spring con tacts can be helpful, but they tend to suf er from the problems

associated with the insulat ing aluminium oxide layers. In both cases, the result is a slow rise inthe contact resistance at the connect ion point and thus to an increased risk of f ire.

Grandfathering regulat ions cont inue to protect older aluminium installations in EasternGermany and in most count ries in Eastern Europe, but the only real prot ection being providedby this sort o f regulation is protection from the threat o f improvement!

Fortunately, methods are now available for ensuring proper electrical contact between theseolder protected installat ions and newer electrical systems. These connectors combine spring-loaded contacts with a special cont act paste made from grease and sharp metal particles.

When the connect ion is made the part icles penetrate the existing aluminium oxide layer whilethe grease protects the contact area from renewed corrosion.

Copper is also thepreferred conductormaterial in high-voltagecables. Although the useof aluminium would resultin only a slight increase in

the overall conductorcross section, theinsulat ing materials andthe exterior shieldingrequired for HV cablesare expensive and thegreater tot al cross-sect ional area of thecable would cancel outthe savings made byusing the cheaper

conductor material incontrast to the situat ionwith low-voltage powercables (Figure 6).
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It is also worth remembering that the cable shielding is always made from copper, because it isthe only material suitable for the job.

If aluminium is chosen as the conductor material, then processing the scrap cable at the end ofits (admit tedly long) service life will involve the addit ional step of separat ing the two materials.

As a material, pure copper has a pract ically inifnite lifetime. It can be reprocessed an indef initenumber of t imes without sufering any loss of quality.

About 45 % of the copper required today is generated from scrap, and the products for which itis used (cables, transformers, water pipes or rooi ng) will remain in use for a long t ime, onaverage around forty years. However, forty years ago, the demand for copper was only abouthalf of what it is today.

It follows t hat about 90 % of the copper used at t hat t ime is st ill in use today. This appliesequally to aluminium and other metals. Metals are not consumed, they are used.

SOURCE: Practical applications of electrical conductors Stefan Fassbinder

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