Patent Application
20090257156
The present invention pertains to the art of electric circuit trip units and, more particularly, to a method of setting a ground fault trip function for a trip unit.
Circuit breakers are used to protect electrical circuits from damage due to an overload condition or a relatively high level short circuit condition. Upon sensing an overload or short circuit condition, the circuit breaker interrupts power to the electric circuit to prevent, or at least minimize, damage to circuit components. Many industrial circuit breakers employ a trip unit that may be programmed for a particular trip characteristic. In many cases, the trip characteristic is based on an inverse time function that accelerates interruption time as current increases.
Generally, the trip characteristic is based on a slope that establishes a reaction time for the circuit breaker. Trip characteristic are based on a number of different slope forms that are selected to match system protection requirements/parameters. The slope forms, or inverse time response curves, generate a trip response time that is inversely proportional to current. That is, the greater the current the faster the response time. Slope forms can be selected from constant slope forms, or inverse time functions such as Î2t (ampere-square-seconds) and Î4t (ampere-to-the-fourth-power-seconds) or other shapes described by more complex functions. Many trip units also employ ground fault protection circuitry that interrupts an associated electrical circuit upon sensing a current flowing to ground via an undesired path. In the case of a ground fault, leakage current must exist for a period of time before the circuit is interrupted. Towards that end, ground fault protection circuits employ a fixed time delayed, or definite time response, trip or are limited to Î2t inverse time functions similar to that employed in circuit breakers short time characteristics for normal overload protection and also demonstrated by thermal magnetic circuit breakers.
In accordance with one exemplary embodiment of the invention, a method of setting a ground fault trip function for a trip unit includes enabling a ground fault protection mode for the trip unit, and setting one of a plurality of a trip functions for the trip unit. At least one of the plurality of trip functions being an Î4t inverse time function that establishes a trip characteristic to interrupt current upon sensing a ground fault indication. The method further includes monitoring an electrical circuit for a ground fault.
In accordance with another exemplary embodiment of the invention, a trip unit for a circuit interrupter includes a ground fault sensing unit operatively coupled to an electrical circuit. The ground fault sensing unit provides a ground fault signal corresponding to current flow in the electrical circuit. The trip unit also includes a trip device operatively coupled to the electrical circuit. The trip device interrupts current flow in the electrical circuit based upon the ground fault signal provided by the sensing unit. In addition, the trip unit includes a processor operatively coupled to the trip device. The processor establishes one of a plurality of trip functions for the trip device. At least one of the plurality of trip functions being an Î4t inverse time function. The Î4t inverse time function establishes a trip characteristic for the trip device to interrupt current flow based on the ground fault signal.
In accordance with yet another exemplary embodiment of the invention, a system for monitoring an electric circuit for a ground fault indication includes a trip unit including a central processor. The central processor is interconnected functionally via a system bus to a ground fault sensor, a user interface adapter, and at least one memory device thereupon stored a set of instructions. The set of instructions which, when executed by said processor causes said system to monitor an electrical circuit for a ground fault indication, and interrupt the electrical circuit upon sensing a ground fault condition based on a trip function. The trip function includes at least one Î4t inverse time function option that is set in the processor though the user interface.
With initial reference to
Reference will now be made to
Once the ground fault band is selected in block 46, a ground fault trip function or slope is selected as indicated in block 48. Trip unit 4 includes a plurality of ground fault trip function options. The particular option chosen depends or circuit component and/or breaker parameters for electrical circuit 10. As best shown in
Pickup threshold=PGF*IGN̂4
Where PGF is ground fault pickup and IGN is ground fault current.
By providing trip unit 4 with Î4t trip functions, GF monitoring system 2 provides personnel with a greater level of flexibility in designing circuit protection schemes. Once the particular trip function is selected in block 48, the monitoring algorithm is executed and indicated in block 104 and GF monitoring system 2 is enabled to provide ground fault protection for electrical circuit 10. At this point, ground fault protection is executed every half cycle, e.g., every 10 ms for 50 Hz and every 8.33 ms for 60 Hz. During ground fault protection accumulator heating will occur per the selected ground fault trip function. When an Î4t function is selected, Accumulator threshold is determined by the formula:
Accumulator threshold=2KGF*f*Pickup threshold.
Where KGF=the trip function Î4t; and f is the frequency.
Once the accumulator exceeds the accumulator threshold, a trip or circuit interruption occurs.
At this point it should be understood that the present invention provides a ground fault circuit monitoring system having greater flexibility than previously possible with existing circuit protection schemes. By enabling personnel to select between multiple ground fault trip functions, that includes Î4t inverse time functions, the present invention provides a system that provides a level of circuit protection that match overload and short circuit protection speeds when desired. That is, as best shown in
In general, this written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of exemplary embodiments of the present invention if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
