In order to reduce the mechanical (due to electro-dynamic forces) and thermal stresses on the object to be protected, the current must be interrupted right during the initiation of the short-circuit, before the full prospective value can be attained (as for example to avoid the welding of the contactor contacts).
This is achieved by:
• Quick opening of the main contacts.
• Rapid build-up of a high arc-voltage (move the arc quickly away from the contact tips and guide it to the arc chamber).
The effects of the reduced let-through values are:
• Reduction of the electro-dynamic forces on the bus-bars (as for example increased spacing between supports).
• Reduction of thermal stresses. The welding of the contacts of contactors can be prevented. Over-dimensioning of the contactors can be avoided or at least kept within reasons. The result is reflected in the short-circuit co-ordination tables - compact starter combinations with components selected mostly on the basis of their rated currents.
The current limiting circuit breakers are used in a wide field of applications. It is no longer necessary to carry out complex calculations of the short-circuit current at each point of the network where a circuit breaker is installed. The subject of short circuit co-ordination takes about as much planning effort as in the case of fuses.
The circuit breaker should be constructed in such a way that it can interrupt the short-circuit current under all possible situations without any problem whatsoever.
The features, which make the planning with circuit breakers as simple as that with fuses, are :
• High breaking capacity makes calculation of short-circuit current superfluous: in actual applications, the fault level (prospective short-circuit current) at the point where circuit breakers for motor branch circuits are installed lie mostly in the range of 1….…20kA. If the breaking capacity of the circuit breaker is higher than this, no further calculation is necessary. The circuit breakers can be utilised in any point of the installation without calculations for its dimensioning, similar to a high rupturing capacity fuse.
• Low let-through values: the contactors connected downstream are less stressed as the short circuit current is appreciably limited by the circuit breakers. Short-circuit co-ordination is simplified and it is not necessary to consult the short-circuit co-ordination tables (the manufacturers perform tests for the short-circuit co-ordination and supply tables in accordance with the IEC 947-4-1 for, as for example, types "1" or "2"). The combination of a circuit breaker and a contactor, both selected on the basis of their rated currents, can in most of the cases fulfil the requirements of the type of co-ordination "2", without any other considerations.
This is achieved by:
• Quick opening of the main contacts.
• Rapid build-up of a high arc-voltage (move the arc quickly away from the contact tips and guide it to the arc chamber).
The effects of the reduced let-through values are:
• Reduction of the electro-dynamic forces on the bus-bars (as for example increased spacing between supports).
• Reduction of thermal stresses. The welding of the contacts of contactors can be prevented. Over-dimensioning of the contactors can be avoided or at least kept within reasons. The result is reflected in the short-circuit co-ordination tables - compact starter combinations with components selected mostly on the basis of their rated currents.
The current limiting circuit breakers are used in a wide field of applications. It is no longer necessary to carry out complex calculations of the short-circuit current at each point of the network where a circuit breaker is installed. The subject of short circuit co-ordination takes about as much planning effort as in the case of fuses.
The circuit breaker should be constructed in such a way that it can interrupt the short-circuit current under all possible situations without any problem whatsoever.
The features, which make the planning with circuit breakers as simple as that with fuses, are :
• High breaking capacity makes calculation of short-circuit current superfluous: in actual applications, the fault level (prospective short-circuit current) at the point where circuit breakers for motor branch circuits are installed lie mostly in the range of 1….…20kA. If the breaking capacity of the circuit breaker is higher than this, no further calculation is necessary. The circuit breakers can be utilised in any point of the installation without calculations for its dimensioning, similar to a high rupturing capacity fuse.
• Low let-through values: the contactors connected downstream are less stressed as the short circuit current is appreciably limited by the circuit breakers. Short-circuit co-ordination is simplified and it is not necessary to consult the short-circuit co-ordination tables (the manufacturers perform tests for the short-circuit co-ordination and supply tables in accordance with the IEC 947-4-1 for, as for example, types "1" or "2"). The combination of a circuit breaker and a contactor, both selected on the basis of their rated currents, can in most of the cases fulfil the requirements of the type of co-ordination "2", without any other considerations.
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