In most simple terms, Connectors are designed to provide continuous electrical path to two or more conductors in a manner that will provide continuous electrical path without loss, fatigue failure or relaxation of mechanical grip. Electrical connections for electrical conductors require the right Mechanical, Chemical and Electrical properties so that the connectors have adequate Current carrying capacity, Mechanical strength and do not corrode on account of Oxidation or Galvanic corrosion. Also the connection should give due consideration to differential thermal expansion, Inter metallic compound, Fretting, Stress relaxation and Creep.
Even though numerous connector forms have been developed through years, the connectors have three basic applications namely,
NEMA Classifies connectors in the following ways:
• 8-CC-I refers to all connectors bolted, welded or expansion fittings of the type generally used in the construction of indoor and outdoor substations.
• 8-CC-II refers to Pressure connectors of the kind generally used in installations conforming to the National Electric Code, for terminating or joining No. 8 AWG and larger electrical conductors.
♦ Pressure connectors are of the following types:
- Those in which the pressure to fix the connector to the electrical conductor is applied by integral screw, cone, or other mechanical means.
- Those in which the pressure to fix the connector to the electrical conductor is applied externally, changing the size and/or shape of the connector.
♦ Insulation Piercing Copper and Aluminum
- Mechanical tool applied.
- Other installation mean
• 8-CC-III refers to Connectors for outdoor overhead lines, as described in the following categories
♦ Split bolt and vise type connectors, with and without washers or spacers. Includes Copper split bolts and vise type connectors, Aluminum split bolts and vise type connectors, and plated split bolt and vise type connectors, unfilled or filled with compound. Service entrance connectors,bolted and mechanical type, Copper or Aluminum, with or without washers or spacers.
♦ Bolted clamps and U-bolt parallel connectors and/or clamps, clamps with one or more bolts, Copper or Aluminum, plated or un-plated, with or without spacers , including center bolt type, with or without liners, unfilled or filled with inhibitor or contact paste.
♦ Compression splices, dead ends and repair sleeves, tool installed, full or partial tension for connecting overhead line conductors. Full tension sleeves for ACSR may be of the double or single-sleeve type.
♦ Compression parallel tap connectors, tool installed, Copper or Aluminum, plated or un-plated, filled or unfilled with inhibitor contact paste, with or without tabs, one piece or assemblies.
♦ Hot line or hot tap clamps, either “V” type main and eye bolt tap or parallel groove main and tap types, designed to be manually installed on overhead conductors with conventional “shotgun” hot sticks.
♦ Wedge connectors, Copper and Aluminum
- Fired-on using a pyrotechnic charge
- Mechanical tool applied
- Other installation means
♦ Insulation Piercing Copper and Aluminum
- Mechanical tool applied
- Other installation means
• 8-CC-IV refers to Insulated or Un insulated separable and non-separable, cable connectors, splices, joints and accessories involving the connecting of cable to cable, or cable to equipment at voltages from 125 volts up through and including 69 KV. Applications include non-utility as well as utility electrical distribution systems, and cover all conductor sizes commonly employed in commercial, industrial, urban, residential and street lighting systems.
♦ Low voltage connectors, insulated or un insulated mechanical or compression, for application in residential and medium density underground distribution circuits operating at 600 volts or below. Typical product descriptions are:
- Junction bars; associated terminals or plug connectors.
- Multi- conductor terminals for joining cable to transformer spades or studs.
- Insulated splices or splice kits, consisting of both bare connector and insulating materials.
♦ Voltage insulated connectors, separable and non-separable, functioning as connectors, cable joints, terminations, or circuit element housings, in shielded cable systems operating voltages above 600 volts. Typicalproduct descriptions are:
- Elbow connectors or terminators, bushing well inserts, junction modules, and operation accessories, load break and non-load break, fused and un fused.
- Splices, taps, cable joints, plugs, receptacles and accessories.
- Insulated splices or splice kits, consisting of both bare connector and insulating materials.
♦ Connector components installed in or on equipment that ultimately will be joined to, and function with, connectors described above. Typical product describtion are:
- Bushings, load break and non-load break, high or low voltage.
- Bushing wells.
- Parking stands and operating accessories
The fastening of two metal conductors is a Mechanical process and can be accomplished by pressure with the help of tightening devices or Compression tool.
A well-designed connector should have sufficient mechanical strength to maintain the joint mechanical integrity under normal as well as overload conductor operating conditions. It should also provide adequate contact pressure to maintain sufficient contact area. A contact area is the metal-to-metal contact called as “a-spot” which provides the conducting paths for the transfer of electrical current. If the contact is porous, where Oxygen and other corrosive gases can enter, they react with the exposed metal and reduce the metallic contact area. This will lead to reduction in the electrical contact even though mechanical contact may still be preserved. The real contact is therefore only a fraction of the apparent contact area. Hence electrical current paths will be fewer and the current will be distributed more uniformly if there are more contact spots.
Fastening can also be done by Wedge type, Helically formed wire type and Automatic splices which require neither of tightening or compression. Supreme’s Wedge type connectors are under testing. It can also be accomplished by thermal means such as Welding and Soldering too which do not fall under the scope of Supreme’s supplies at present.
Use of Copper as an electrical conductor assure longer life because of the metal’s properties, which allow problem free connections. Aluminum has been widely used because of its lower cost. Making connections with Aluminum, unless accompanied by adequate mitigating measures, have certain drawbacks such as corrosion by Oxidation of Aluminum surface due to its high solution potential, Creep and Stress relaxation.
Firstly, When the bare Aluminum surface is exposed to the Oxygen containing environment Aluminum Oxide forms a duplex layer. The Aluminum Oxides are harder compared to Copper Oxides and not that easy to disrupt with less contact force. The current flow is restricted to flow through the areas where the Oxide film is ruptures. Thus an adequate contact force should be applied to establish the connection.
Secondly, Aluminum conductors have the tendency to creep when subjected to a constant external force over a period of time. The rate of creep depends on stress and temperature and is higher for Aluminum than for Copper.
Thirdly, Stress relaxation, which depends on time, temperature and stress, occurs at high stress levels and is evidenced by a reduction in the contact pressure due to changes in Metallurgical structure. Excessive conductor deformation and high stresses produced by certain connector system having no means of providing residual mechanical loading to the contact interface cause accelerated stress relaxation and failure of joint.
The use of two dissimilar metals in an electrical connection adds a new dimension to the electrical connections due to the difference in their Physical, Mechanical and Metallurgical properties. Galvanic corrosion and degradation due to differential thermal expansion are significant factors to be considered in the connector design.
Galvanic corrosion is an Electrochemical reaction between two dissimilar metals in the presence of an electrolyte in which the anodic metal is continuously corroded and deposited on the cathodic metal. In case of Aluminum – Copper connection, Aluminum being the anodic metal dissolves and gets deposited on the Copper surface in the presence of humidity. This corrosion can cause an Electrical failure by drastically reducing the contact area, a Mechanical failure by severe corrosion of connector or a combination of both.
The difference in the coefficient of thermal expansion of Aluminum and Copper is a serious matter to be factored in by the design engineer during connector design. Aluminum expands at a greater rate than Copper when exposed to an increase in temperature. This differential expansion leads to large lateral movements occurring in contact area shearing the metal contact or occurrence of plastic deformation adjacent to contact surface.
Bi-metallic welds made by friction welding, pressure welding, diffusion and roll bonding, flash welding and explosion welding have relatively stable joint interface but, when in service, frequent current surges can cause conditions favorable for the formation of electron compounds which is a kind of intermetallic compound. This is an intermediate compound formed due to the difference in the crystal structure, valency and one of the metals being electro positive and the other being electro negative. As the intermetallics have high electrical resistance and very low mechanical strength, the interface between the two metals in contact becomes brittle, making the interface more susceptible to environmental effects and generation of fissures. Also, formation of intermetallics increases the contact resistance dramatically resulting in overheating of the contact.
Apart from all these, there is another common problem called Fretting, which can cause significant damage to the equipments. Fretting is an accelerated damage occurring at the interface of contacting materials subjected to small Oscillatory movements which may be due to Mechanical vibrations, Differential thermal expansion, Load relaxation and Junction heating. The Connector design needs to address above issues with suitable mitigating measures.
In order to accomplish this, the connector design criteria primarily factors in a) Area, Shape and Dimension of the Conductor b) Conductor material and type c) Current carrying capacity/Ampacity. It may also include 1) Provision of Mechanical contact devices 2) Surface treatments with Tin, Silver or Nickel 3) Use of suitable Contact aid compound 4) Provision of Bi-metallic transition contact plates and transition washers Wherever required to keep the above discussed limitations under control.
Sometimes Voltage rating becomes an important criterion. In case of connectors with Insulated protection enclosure such as Insulation Piercing Connector, Connector with Insulated Cover, Insulated Sub-Station Connector or Underground Power cable accessories like joints and terminations, Dielectric strength of protection enclosure is a very important consideration. In case of connector for EHV applications, Voltage rating is also an important factor to be considered in design as the degree of smoothness, radius at corners, geometrical shape and surface continuity have an important role in satisfactory Corona/RIV performance at rated Voltages.
Supreme & Co has facilities and access for Routine testing and Type testing as per relevant International standards for verification of such important parameters as given above.
CLAMPS & BOLTED CONNECTORS
HOT LINE CLAMPS
AUTO MID SPAN JOINTS.
INSULATIONS PIERCING CONNECTORS
SUBSTATION /EHV CONNECTORS
STANDARDS AND TESTING
GLOSSARY OF TERMS.
Compression connectors ensure durable connections having high mechanical integrity and good electrical conductivity with the help of specifically designated tools and dies. The portion of the connector called as barrel or sleeve, is deformed in a predetermined shape and to a pre determined extent by the designated tools and dies. The deformation of the connector barrel is permanent so that the clamping force is maintained.
The level of electrical conductivity of the connection is related to the amount of deformation. In the initial stages of deformation the contact resistance is inversely proportional to the deformation. However once the conductor is firmly fixed in the sleeve, the decrease in the contact resistance levels off until no decrease is observed with further deformation.
There exists a definite relationship between the force required to accomplish a given deformation and the installation die configuration. In the same way there are relationships between the degree of compression and the resulting electrical and mechanical characteristics.
The degree of compression is determined by the following factors:
- The Compression force provided should be sufficient to meet the mechanical requirements of the connection.
- The contact resistance should be low enough to meet the electrical requirements of the connection.
- The deformation should ensure the permanent maintenance of the compression force.
- The force required for this operation should be within the capabilities of the installation tool, leaving a sufficient margin of safety.
Compression methods help in attaining all the three basic types of connector forms such as Taps, Terminals and Splices. These connections are available for aluminum, Copper,Steel and any combination of them.
- Compression system provides relatively low cost installation.
- Due to the relatively symmetrical geometry of the compression sleeve it is easier to insulate or tape when compared to mechanical connectors.
- The compression process ensures a good degree of enclosure to the conductor, which retains the oxide-inhibiting compound, which prevents the contact area from being exposed to atmosphere and thus eliminates the need for frequent maintenance.
- The uniform forces applied during installation provide a connection, which ensures better electrical and mechanical performance. High forces help in breaking down the oxides and provide contact points with lower contact resistance.
- Compression connectors are well suited for full tension applications and areas of wind, vibration and ice build-up.
- The use of designated tools and dies eliminates the error of human judgment in the choice of type and shape of the crimp.
- Obtaining suitable installation tools and dies involves high investments.
- The personnel must be trained in the installation techniques.
CLAMPS & BOLTED CONNECTORS:
A Mechanical Fastening Device is a device, which keeps two bodies in a position relative to each other by applying a clamping moment, which results in a Mechanical force. Thus a Mechanical Connector makes an Electrical Connection between Conductors by the application of Clamping force using a mechanical fastening device to create contact points. Joint integrity thus is a function of clamping forces resulting from the optimum application of the torque.
Apart from good conductivity and Mechanical integrity, the Mechanical connectors have durability of connections. It is important that connecting materials are not susceptible to stress-corrosion- embrittlement or Hydrogen embrittlement failure.
High strength materials are often selected for Clamping elements but they may be susceptible to stress corrosion failure. Hence, the selection of material for the clamping element involves giving due consideration to
- Prevailing environment
- Thermal coefficient of expansion of the material in comparison with the body material.
- Presence of stresses.
Aluminum alloy clamps can be provided for aluminum connectors as it provides good mechanical strength and becomes less susceptible to stress corrosion failure. Also as the coefficient of expansion of Aluminum conductor and the bolts are the same, high contact pressure is maintained during both heating and cooling cycles, which effectively avoids the failure due to differential thermal expansion.
The Contact resistance of a bolted joint is determined by the resistivity of the contact members, the Contact area and distribution of the a-spots in the interface between the contact members. The Contact resistance is closely related to the clamping force applied to the joint, which determines the satisfactory performance of the joint assembly. But the use of contact force as an indicator of a good assembly is almost impractical. Practically, Tightening torque is used for this purpose. The empirical relation relating the tightening torque with the applied force generated by the bolt in a joint is given by,
T = K * D * F
T - Tightening torque (Nm)
K - Constant (friction factor- dimensionless)
D - Bolt diameter (mm)
F - Contact force (N)
- The conductor should be protected against damage or deformation caused, if any, during clamping as the bearing area of a hexagon head bolt or nut is very small. Thus the clamping element should be designed in such a way to avoid direct application of pressure on the conductor cores. But generally it is common practice to drill bolt holes over size which causes head and nut to sink gradually into the conductor under the normal pressure conditions. The use of pressure pads to distribute compression stress over a wider area is a step in this direction. Use of specially designed pressure pad is an improvement over use of washers. Washers distribute compressive stresses over a relatively small area. Use of wider washers can improve distribution but not very substantially. Washers are square or round in shape and stresses are distributed in all directions from the center. However, ideally compressive stresses should be distributed along the length of clamp for good connection. Secondly washer sitting on the clamp is based on flat-to-flat contact. When the tightening torque is applied, it causes uplifting moment at washer extremities and thus limiting the distribution of compressive stresses. Supreme has designed its pressure pad to overcome these limitations. Supreme’s pressure pad has got a concave shape, which fit exactly into the convex shaped top of the clamp and also made available to the whole length of the clamp.
- The size of the clamping element should be based on the amount of clamping force required to establish, considerably low resistance, which ensures good conductivity between the joining conductors. The under torque results in insufficient contact, increasing the resistance and excess torque will damage the conductor without further improvement in conductivity. Also, Tightening torques depend on the contact pressure required. The amount of pressure for a given torque varies over a wide range depending upon whether threads are dry, lubricated, and hot galvanized or otherwise treated. In general, the maximum torque equal to 95 % of the yield strength of bolt is recommended. Supreme is offering shear heads as optional accessory as a step in this direction.
- The contact area can be increased by brushing and proper lubrication as the use of suitable contact aid compound prevents the oxidation of clean metal surfaces. This results in low contact resistance and protection of the contact zone from adverse environmental effects. Apart from these benefits, Lubrication also influences the force generated in a bolt when tightened, which is commonly known as “ the influence of Friction factor”. It is experimentally proved that same tightening torque generates much higher force in the bolts and also joint when assembled with lubricated bolts than with dry bolts. In some cases depending upon the metallurgical state of the bolt material , a higher force generated by lubricated bolts can exceed the bolt yield strength and cause permanent deformation. Thus Metallurgical properties and the condition (lubricated or dry) shall also be considered. The values of Friction factors considered in a bolt tightened connectors are K = 0.3.
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