Self-Calibrating Circuit Interrupter Trip-Time Control
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Solution Overview
Problem
Circuit interrupters, such as GFCIs, face calibration shifts due to environmental and operational factors like temperature fluctuations, input voltage, and load changes, leading to inconsistent trip-time responses, potentially resulting in delayed or premature tripping during ground faults.
Innovation Solution
A self-calibrating circuit interrupter with a controller that adjusts the resistance of a variable resistor based on sensed fault current values, maintaining the trip-time response within a predetermined range by using differential and ground/neutral transformers, and a microcontroller to periodically recalibrate or adjust the trip-time response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the circuit interrupter is calibrated to trip within a specific time for a given fault current, then the trip-time response meets UL 943 requirements, but environmental and operational factors cause calibration shifts leading to inconsistent trip-time responses
Solution Approach 1:
The patent employs a feedback mechanism where the microcontroller periodically measures the actual trip-time response by injecting a test signal through the differential transformer and measuring the resulting fault current. Based on this measured value, the system automatically adjusts the variable resistor to compensate for calibration shifts caused by temperature, voltage, and load variations, thereby maintaining consistent trip-time response.
Solution Approach 2:
The circuit interrupter performs self-calibration without external intervention. The microcontroller automatically executes calibration routines, measures trip-time response, and adjusts the variable resistor based on predetermined target values, enabling the device to maintain its own performance characteristics despite environmental changes.
2Reliability
If the trip-time response is adjusted to account for environmental factors, then trip-time consistency is improved, but the device complexity increases due to additional calibration components and control circuitry
Solution Approach 1:
The microcontroller serves multiple functions: it controls the variable resistor for calibration adjustment, measures trip-time response through the electrical sensing line, manages the overall circuit interrupter operation, and executes calibration algorithms. This multi-functionality reduces the need for separate dedicated calibration hardware, thereby limiting the increase in device complexity.
Solution Approach 2:
The variable resistor acts as an intermediary element that the microcontroller adjusts to compensate for calibration shifts. By using this single adjustable component in the sensing circuit, the system achieves calibration adjustment without requiring complex reconfiguration of multiple components, thus limiting complexity increase.
3Measurement precision
If periodic self-calibration is performed to maintain accurate trip-time response, then measurement precision is improved, but loss of time occurs during the calibration process
Solution Approach 1:
The microcontroller performs self-calibration at predetermined time intervals rather than continuously or after every trip event. This periodic calibration approach maintains measurement precision by regularly updating the trip-time response while minimizing time loss by concentrating calibration activities at specific intervals rather than continuously interrupting operation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The self-calibrating mechanism ensures consistent and accurate trip-time responses, maintaining safety and compliance with standards like UL 943, even under varying conditions, reducing false tripping and enhancing performance across different voltage and load ranges.
Implementation Method 1
The sensing circuit can incorporate a high turn ratio differential transformer connected to detect the difference of current in the neutral and phase wires of an electrical distribution system
Implementation Method 2
A ground/neutral transformer may be coupled to the phase conductive path and the neutral conductive path
Data Source
AI summary
A circuit interrupter includes phase and neutral conductive paths between a line side and a load side of the circuit interrupter. Differential and ground/neutral transformers are coupled to the phase and neutral paths. A circuit interrupter chip is coupled to the phase and neutral conductive paths, and to respective fault inputs from the transformers. A calibration circuit including a calibration resistor is coupled to a circuit interrupter controller. An interrupter circuit is configured to open the phase and neutral paths when a predetermined ground fault current condition is sensed. The circuit interrupter controller is configured to perform a self-calibration function to adjust a ground fault current trip-time response to maintain the ground fault current trip-time response within a predetermined range. Methods for using the self-calibrating circuit interrupter are also disclosed.