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ACB Mounting Requirements

Updated August 24, 2022
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ACB Mounting Requirements essay

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Generally ACB is used as an upstream device to feed electrical supply to connected load. General practice ACB is mount inside the panel which is connected to other equipments with the help of bus bars. During the normal operation rated amount of load current always shows through the circuit breaker, Current growing through the current carrying path of ACB get distorted due to electromagnetic forces acting on it.

This electromagnetic forces are produced due to an alternating current growing through the current path itself as well as the alternating current growing through adjacent current paths. So, it is required to study and understand all the factors which are aecting the thermal performance of ACB. Factor affecting the thermal performance

  • Skin & proximity effect
  • Contact resistance
  • Ventilation
  • Proper/improper termination
  • Surrounding equipment heat characteristics
  • Conduction, Convection, Radiation Skin effect

When an alternating current how through the conductor it will produce an alternating magnetic field which will circulate around the conductor axis. This magnetic field will link with the conductor itself and will induce an electrical field which will drive current in a conductor in such a way that current density in the centre of the conductor will get reduce and increase on the outer periphery. The current induced due to its own magnetic field inside the conductor is known as an eddy current and the overall phenomenon is known as a skin ect. Proximatiy effect When conductor carrying an alternating current placed parallel the current in a conductor will further redistribute due to magnetic field of other conductor this is known as proximity effect.

If the conductors carry the current in the same direction, then the magnetic field of the halves of the conductors which are close to each other is cancelling each other and hence no current flow through that halves portion of the conductor. The current is crowded in the remote half portion of the conductor. When the conductors carry the current in the opposite direction, then the close part of the conductor carries, the more current and the magnetic field of the far off half of the conductor cancel each other. Thus, the current is zero in the remote half of the conductor and crowded at the nearer part of the conductor. Contact resistance Contact resistance is mainly dependent on two factors

  • Surface condition
  • Contact pressure

In real practice an electrical contact between two surfaces are formed at some discrete areas which are known as a-spot which are the only current conducting path. The a-spot occupy only 1% of the overlap area, obviously the larger number of a-spot the more uniform will be the current distribution throughout the joint area. This can be maintain by having the that and roughened surfaces of the conductor. Many times in real practice contact gel is being used before assembling. Contact pressure Generally the joint resistance decreases with increase in the pressure, size and the number of the bolts.

As this increases the number of a-spot. Contact resistance falls down rapidly with increase in the contact pressure which is as shown in the gure, but above 30N/mm2 there is little decreases in the contact resistance so it is generally preferred to have pressure up to 30N/mm2. Joint resistance The resistance of a joint is mainly dependent on following two factors:

The streamline effect or spreading resistance Rs, due to the distortion of the current how through the joint

The interface resistance or contact resistance of the joint Ri. The total joint resistance Rj, is given by: Rj = Rs + Ri This formula is valid specically for direct current (DC) applications. Where for alternating currents (AC) the changes in resistance due to skin effect and proximity effect must be taken into account for the joint zone. Streamline effect When current shows through the joint formed by the conductors overlapping the lines of the current how are distorted and the effective resistance of the joint is increased due to how of current through a portion of the conductor.

This is known as the streamline ect. Current density in the perpendicular direction of the busbar e.g. as current transfers from one bar to another bar is highly non uniform and is concentrated to the edges. Here in the resistance ratio e is the ratio of resistance of a joint due to streamline to the resistance of the equal length single bar.

e =(Rs/ Rb)=(ab /?l)Rs Ventilation Generally the ACBs are mounted in the panel and surrounded by many other equipment and the thermal performance of all this equipment are critical, their performance will a?ect the downstream load. So, it is mandatory to maintain the temperature in side the panel and the temperature of the equipment. This will need the careful attention to the ventilation of the panel. Air is working as a fluid in the low voltage switchgear panel and the show of air is dependent on the ventilation scheme as well as the supply connection inside the panel e.g.top feed or bottom feed. A good ventilation can improve the thermal performance of the ACB. Conduction, Convection, Radiation Any electrical switchboard or a panel system is made up of a fluid (air) and the electrical equipment. When current shows through this equipment they generate heat as an energy loss and to achieve the thermal equilibrium they dissipate the heat to the surrounding by following three phenomena.

  • Conduction
  • Convection
  • Radiation

Conduction is taking place in gases, liquid and solids, solids other the least amount of resistance to heat transfer by conduction. The physical contact between the materials is required for conduction of heat through it. Molecule available at higher temperature side moves at high velocity and transfer the kinetic energy to the adjacent molecules which are at the lower temperature this is known as the conduction process.

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