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 ?ows through the circuit breaker, Current ?owing 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 ?owing through the current path itself as well as the alternating current ?owing through adjacent current paths. So, it is required to study and understand all the factors which are a?ecting the thermal performance of ACB.
Factor affecting the thermal performance
• Skin & proximity effect
• Contact resistance
• Proper/improper termination
• Surrounding equipment heat characteristics
• Conduction, Convection, Radiation
When an alternating current ?ow through the conductor it will produce an alternating magnetic ?eld which will circulate around the conductor axis. This magnetic ?eld will link with the conductor itself and will induce an electrical ?eld 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 ?eld inside the conductor is known as an eddy current and the overall phenomenon is known as a skin e?ect.
When conductor carrying an alternating current placed parallel the current in a conductor will further redistribute due to magnetic ?eld of other conductor this is known as proximity e?ect.
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 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 ?at and roughened surfaces of the conductor. Many times in
real practice contact gel is being used before assembling.
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.
The resistance of a joint is mainly dependent on following two factors:
• The streamline e?ect or spreading resistance Rs, due to the distortion of the current ?ow 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 speci?cally for direct current (DC) applications. Where for alternating currents (AC) the changes in resistance due to skin e?ect and proximity e?ect must be taken into account for the joint zone.
When current ?ows through the joint formed by the conductors overlapping the lines of the current ?ow are distorted and the e?ective resistance of the joint is increased due to ?ow of current through a portion of the conductor. This is known as the streamline e?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 ?g 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
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 ?uid in the low voltage switchgear panel and the ?ow 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 ?uid (air) and the electrical equipment. When current ?ows 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 is taking place in gases, liquid and solids, solids o?er 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.