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INTRODUCTION
Sales of fuses in the U.K. market have stabilised whilst circuit breaker sales have continued to grow. The U.K. has always been a fuse dominated environment unlike Europe where the many benefits of circuit breakers have been more readily accepted. Many reasons exist for the continued improvement in circuit breaker sales such as the requirement for constant information to be made available by system devices. The adoption of new international standards has also ensured a better understanding of the performance of devices by system designers and specifiers. A host of pre conceived ideas about the limitations of circuit breakers are normally held by ardent users of fuses, such as the fact that circuit breakers are not current limiting - this is certainly not the case and indeed the current limiting effect is something which should be fully exploited when selecting devices. Detailed below is an overview of the characteristics and options with both solutions.
COMPARISON
FUSES
ADVANTAGES
Excellent current limiting properties.
Methods of achieving discrimination accepted and easily understood.
Requirement to calculate fault level normally eliminated due to high rupturing capacity of fuse link.
DISADVANTAGES
Requirement to have spare fuses available.
No adjustment or test facility
Trained personnel should always be available to replace fuses
Scope for auxiliary equipment is limited
Larger volume switchboard normally associated with fuse style distribution
May deteriorate over a period of time due to thermal stresses and the process of ageing.
Possibility of replacement fuses being of incorrect rating.
Three phase isolation not normally achieved in the event of a fault due to fuses blowing on only one or two phases.
CIRCUIT BREAKERS
ADVANTAGES
Complete range of load currents available therefore total packages can be offered.
Current limiting capability allows the cascading principle to be used - this ensures the economic selection of devices based on actual prospective short circuit current.
Cascading with full discrimination possible with newer electronic trip units.
Wide choice of trip styles and units available allow protection settings to be made based on actual system load conditions.
Extension of installation possible due to wide adjustment range of units and possibility of changing trip unit.
Close overload protection with high value short circuit protection.
Testing possible on electronic trip units under no load conditions due to ability to secondary inject.
Primary injection possible on all units enabling tripping characteristics to be verified.
Simultaneous opening of all poles in the event of a fault and for isolation purposes.
Rapid identification of the fault circuit via the toggle operator allows for rapid reclosure of the device once caused of trip has been identified.
Ability to combine overcurrent/earth fault protection with communication of system /device parameters all remotely controlled in one unit.
Standard options available include:-
Earth leakage/earth fault option
Remote control via motor mechanism
Tripping by voltage release
Door interlocking Plus in/withdrawable
Mechanical interlocking
Load shedding
Communications facility
Automatic changeover facility -
To guarantee continuity of service
To shed non essential loads
To remove the need for human intervention in the event of mains failure.
CONCLUSION
In summing up it should be noted that the increasing sales of circuit breakers is proof enough that wider acceptance is emerging. Publications such as the Application Guides are valuable tools and should be used when considering the arguments between both options. Detailed recommendations exist for the selection of devices based on system conditions, these include the protection of motors, generators and transformers. Discrimination tables also confirm actual discrimination limits, often these figures reflect the fault levels typically experienced under short circuit conditions. Cascading tables too should be fully utilised as an alternative to fuse back up protection.
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