Electric Line Safety Circuit Using Dual Low-Pass Thermal Modeling
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Solution Overview
Problem
Existing circuit breakers and electrical lines in vehicles have different thermal responses to overcurrents, leading to inconsistent break-time characteristics and potential measurement inaccuracies that can cause unnecessary disconnection or failure.
Innovation Solution
A safety circuit using a current detection means and an evaluation and control unit with two first-order low-pass filters of differing cut-off frequencies to simulate the thermal responses of both the circuit breaker and the electrical line, summing their outputs and comparing the result to a reference value to control the circuit breaker's switch-off.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single threshold-based control is used for the circuit breaker, then the response is simple and fast, but it causes unnecessary disconnection or failure to protect due to not accounting for the electrical line's thermal response
Solution Approach 1:
The control circuit is segmented into two parallel control paths: a first control path with a first threshold value for fast response, and a second control path with a second threshold value that accounts for the electrical line's thermal response. This segmentation allows each path to handle different aspects of overcurrent protection independently, improving overall reliability without excessive complexity
Solution Approach 2:
The second control path performs preliminary thermal response assessment before triggering disconnection. By evaluating the duration and magnitude of overcurrent against the electrical line's thermal characteristics, the system determines whether the line can withstand the overcurrent condition, preventing unnecessary disconnections while maintaining protection
2Speed
If the circuit breaker is designed with high thermal sensitivity for fast protection, then it responds quickly to overcurrents, but it disconnects even when the electrical line can withstand the overcurrent due to its greater thermal mass
Solution Approach 1:
The control circuit dynamically adjusts the disconnection decision based on real-time overcurrent characteristics. By continuously monitoring current magnitude and duration, the system adapts the effective threshold: using the lower first threshold for immediate danger and the higher second threshold for sustained overcurrents that the electrical line can tolerate, optimizing both response speed and accuracy
Solution Approach 2:
The control circuit incorporates feedback from current detection means that monitors the actual overcurrent condition. This feedback allows the system to assess whether the overcurrent has persisted long enough to cause thermal damage to the electrical line, enabling accurate timing of disconnection decisions that balance fast response with reliability
3Measurement precision
If measurement inaccuracies occur in detecting the current magnitude, then the control decisions may be erroneous, but using more complex measurement systems increases device complexity
Solution Approach 1:
The control circuit uses two threshold values where the second threshold is higher than the first, creating a cushion zone. This cushioning effect compensates for measurement inaccuracies: if the measurement is slightly erroneous, the dual-threshold system ensures that disconnection only occurs when the overcurrent clearly exceeds both thresholds, preventing erroneous disconnections while maintaining simple measurement hardware
Data Source
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Figure 3
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AI summary
Described is a safety circuit comprising a current detection means (24) for providing a measurement signal (I_LOAD) that represents the magnitude of a current flowing through the electrical line (14) and thus also through the conduction path (18) of the circuit breaker (16), or from which the magnitude of this current can be derived, and an evaluation and control unit (22) for controlling the circuit breaker (16) to deactivate its conduction path (18). The evaluation and control unit (22) receives the measurement signal from the current detection means (24) and generates a deactivation signal (TRIP) for the circuit breaker (16).The evaluation and control unit (22) has a first first-order low-pass filter (32) with a first cut-off frequency (36) and a second first-order low-pass filter (34) with a second cut-off frequency (38) that is higher than the first cut-off frequency (36), a summer (40) for summing the output signals of the two low-pass filters (32, 34) and for outputting a sum signal, and a comparator (42) for comparing the magnitude of the sum signal with a reference value (44). The measurement signal (I_LOAD) or a signal derived therefrom is fed to the two low-pass filters (32, 34), wherein the evaluation and control unit (22) can be fed, and the evaluation and control unit (22) generates the switch-off signal (TRIP) for the circuit breaker (16) when the circuit breaker (16) is switched on and the sum signal is higher than the reference value (44).