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Tin-Coating Busbar helps Improve Weak Connections

A connection of conductor represents weak points in most electrical circuits including switchboard and switchgear. Most standards and practice codes typically recommend necessary safety measures to reduce potential failures from those weak points. Normally, two conductor surfaces in contact can never be perfectly matched. On a microscopic level, each surface resembles rough terrain with peaks and valleys. When the two surfaces are in contact, the peaks and valleys from one will randomly match up with those on the other surface. These direct contacting points are also known as “A-spots”.

To obtain a good electrical connection, it must create as many contacting points or A-spots as possible. Normally, this large number of contact points will increase when more pressure is applied and the peaks are crushed. The more contact points that can be established within a given area, the lower contact resistance. It is important to note that a connector’s long term performance is directly related to the contact points originally established. Therefore, not only must the connection maximize the contact points during fabrication or installation, it must maintain that contact over the projected life of the connection.

Although making good connection of electrical conductor seems straightforward, many factors influence ability of connection in establishing and maintaining a low contact resistance. Surface contaminations or corrosion will interfere with making initial contact, thermal fatigue can loosen the connection and reduce the number of contacting points, and mechanical stress and long term corrosion can diminish contact surface directly. Two main factors that primarily affect the initial contact resistance of the surfaces are (1) The condition of the surfaces and, (2) The total applied contact pressure.

Layers of copper oxide will reduce the number of contacting points in a connection, thus increasing the contact resistance. Given electrical resistance nature of conductor oxides, it is crucial that these oxides must be broken or removed before making up connections. Normally copper oxide can be broken down by reasonably low value of contact pressure. However if the copper is badly oxidized, a minimum cleaning is required to obtain good contact. To prevent re-oxidation after removing oxide layers, joint compounds are recommended in connection of copper conductors.

Corrosion and oxidation of conductor surface is highly dependent on nature of atmosphere. Due to electrical resistant nature of oxide films, the reliability and lifetime of electrical connections or jointing are also highly influenced by climatic conditions and surrounding atmosphere where they present. The corrosive nature of atmosphere of installation sites is one of the primary causes of jointing and contact degradation. Reliability of switchgears’ components directly affects reliability and useful life of switchboard and switchgear.

	Ri = C/Pn
Where	Ri = resistance of the contact
	p = total contact pressure
	n = exponent between 0.4 and 1
	C = a constant

The greater the applied total pressure the lower will be the joint resistance and therefore for high efficiency joints high pressure is usually necessary. This has the advantage that the high pressure helps to prevent deterioration of the joint. Figure 2 shows the effect of pressure on joint resistance.

Joint resistance falls rapidly with increasing pressure, but above a certain value of pressure, approximately 15 N/mm 2 for copper busbar, there is little further improvement. Certain safety measures must be observed to ensure that the contact pressure is not excessively high, since it is important that the proof stress of the conductor material or its bolts and clamps is not exceeded.

It is important to note that as a bar heat up under load the contact pressure in a joint made with steel bolts tends to increase because of the difference in expansion coefficients between copper and the steel. It is therefore essential that the initial contact pressure is kept to such a level that the contact pressure is not excessive when at operating temperature. If the elastic limit of the bar is exceeded the joint will have a reduced contact pressure when it returns to its cold state due to the joint materials having deformed or stretched. To avoid this, it is helpful to use disc-spring washers whose spring rating is chosen to maintain a substantially constant contact pressure under cold and hot working conditions.

Corrosion of Conductors

Corrosion is essentially a process of oxidation and is the result of an electrochemical reaction between the metal and its environment and especially with oxygen. For copper conductor, the result is the production of blue or green layers on copper. Moisture, oxygen and a salt (e.g. sodium chloride) all contribute to the corrosion process. In a humid climate, especially in coastal areas where moisture is saline or salty, corrosion can occur readily. Two general types of corrosion that are of concern in electrical connections are Oxidation and Galvanic Corrosion which affect both the initial contact and the long-term performance of an electrical connection.

Oxidation

Copper, like other electrical conductors, is oxidized immediately upon exposure to the atmosphere. Once oxidation is stabilized, this oxide prevents further oxidation of the conductor. When copper conductor is heated, for example by bad connections or loose joints, more oxides are formed. These oxides are of high-resistance and will continue to increase the heat until the conductor breaks. Copper oxide is easily broken down by applying contact pressure.

Aluminum oxide, on the other hand, is a hard, high-resistance film that forms immediately on the surface of aluminum exposed to air. This tough film gives aluminum its good corrosion resistance. However, after a few hours, the oxide film formed is too thick to permit a low-resistance contact with cleaning. The film is so transparent that the bright and clean appearance of an aluminum conductor is no assurance of a good contact. After cleaning the oxide film from aluminum, a compound must immediately be applied to prevent the oxide from reforming.

Galvanic Corrosion

Corrosion is the electrolytic action of moisture and other elements of the atmosphere in conjunction with the metals of the connection. When the surface of a metal or of two different metals connected together is exposed to a corrosive liquid, such as salt water or water with dissolved oxygen, a micro-battery is established. The surface of the single metal or of the less-resistant of the two metals will disintegrate.

Different metals, when used together, have different tendencies to corrode. The corrosion series shown in Table 1 lists metals from the least resistant to the most resistant. The farther apart the two metals are in this series, the greater is the potential for corrosion. The material higher in the list becomes the anode of the micro-battery to the material below it and will corrode. For example, joining aluminum and brass would readily produce corrosion in the aluminum when moisture is present. When two dissimilar metals are joined or a metal fastener is used to join metal parts, care should be taken to use metals that will not unintentionally accelerate corrosion.

Least-resistant	.
Magnesium	.
Zinc	.
Aluminium	.
Untreated iron or steel	.
Solders (lead-tin)	.
Copper, brass, bronze	.
Stainless steel	.
Silver	Noble Metal
Gold	Noble Metal
Platinum	Noble Metal
Most-resistant	.

Improvement of the Weak Connections

Cleaning Conductor Surface

Surface contamination, specially surface oxide, must be expected on all conductors. These surface oxide films are insulators and must be remove or broken to achieve the metal-to-metal contact required for efficient electrical connections. In addition to cleaning, the surface should be covered with a good joint compound to (1) prevent formation of oxides on the cleaned metal surfaces and (2) to prevent moisture from entering the connection thus reducing the chances of corrosion. Various commercially used compounds and their specific applications are available from several suppliers.

Coating Conductor Surface

The primary reason for coating conductor surface is to increase corrosion resistance. However, some coatings are used to increase the hardness of the base metal for improved wear resistance. There are many metals that are used to coat electrical conductors. Tin and tin lead alloys are very common in applications where solderability is important. Gold is used for its exceptional corrosion resistance. Platinum, palladium, and rhodium were widely used in the past, although recent increases in price are making them less attractive. Silver is also used for high electrical conductivity. Of these coating metals, tin and tin alloy are frequently considered as low cost options on electrical conductors. With appropriate design considerations, they may often be successfully utilized as more cost effective and reliable alternative to expensive metal

Tin coatings are soft and easily abraded. In normal indoor exposure, thin gives protection to most metals, even if some corrosion of tin may be expected in certain outdoor exposure conditions. Tin, like the base metal, copper, will naturally form a hard, brittle oxide on its surface. This oxide is stable and helps to prevent further corrosion of the copper. Similar to copper oxide, tin oxide is also an electrical insulator, it therefore must be removed in order to achieve a good electrical contact. Fortunately, the tin layer is much softer and more ductile than the oxide surface, thus, with sufficient jointing force, good electrical contact can be made through the oxide film.

In addition, in case of very rough jointing surface, tin coatings may results in some improvement in jointing efficiency. Therefore, for the best results, the surfaces of conductor should be tin-coated prior to the final joint clamping, especially there the joints operate at unusually high temperatures or current densities or when subjected to corrosive atmospheres.

Several codes of practice and standards recommend that the jointing surfaces are tinned. Although jointing of copper busbar has mechanical stability and therefore less chance for fretting corrosion, jointing compound should still be used to prevent atmosphere oxidation. To ensure long-life contact, the bolts must be tightened to a specified torque. With properly prepared conductor surfaces and correct workmanship, joint integrity does not then deteriorate in normal service due to oxidation or corrosion.

Standards and Practices Related to Tin-Coating Copper

Several international electrical standards and guidelines have recognized superiority of Tin-Coating copper conductors such as:

IEC 60943 – Guidance concerning the permissible temperature rise for parts of electrical equipment, in particular for terminals – This IEC guideline discusses general considerations concerning the nature of electrical contacts and the calculation and measurement of contact resistance. Based on theoretical review in this guideline, tin-coating copper contacts show the lowest resistance compared with other kinds of contacts. However the layer of thin must be sufficiently thin to prevent its resistivity from being involved in the contacts but must be thick enough to generate sufficient hardness. Comparative values of various types of contact resistance for a contact force of 10 N and after 1,000 hours exposure to ambient air are shown as Table 1.

Table 1 : Comparative Values of Contact Resistance

Material	Resistance in m Ω
Bare copper	20
Nickel-plated copper	35
Tinned copper	6.8
Silver-plated copper	0.3

IEC 60694 - Common specifications for high-voltage switchgear and controlgear standards - This IEC standard specifies different temperature rises for different type of terminal materials. As shown in Table 1, Bolted contacts with coated copper conductors are allowed to withstand higher maximum temperature rise and working temperature compared with bare copper conductors. This is due to the advantage of coated copper conductors in protection against corrosion, hence resistant oxide film will be less at high temperature.

Table 2 : Maximum Permissible Temperature of bare copper versus tin-coated copper bolted contacts

Type of Contact	Maximum Permissible Temperature
bare copper bolted contacts (in air)	Maximum temperature rise: 50 ° C
	Maximum working temperature: 90 °C
tin-coated copper bolted contacts (in air)	Maximum temperature rise: 65 °C
	Maximum working temperature: 105 °C