The field of the disclosure relates generally to current measuring systems and methods of assembling the same, and more specifically, to a current measuring system that includes multiple magnetic field sensors.
Circuit breakers currently used to protect, for example, a residential or commercial environment, generally detect the presence of overcurrent conditions and release an operating mechanism to separate the circuit breaker contacts. Current flow may be monitored by positioning a shunt resistor in the current path and measuring a voltage drop across the shunt resistor. However, undesirable heat is generated by the shunt resistor when current is monitored in this manner. A sense transformer may be used to detect a level of alternating current (AC) within a conductor without positioning a shunt resistor in the current path. Solid state magnetic field sensors, for example, a Hall effect device or a giant magnetoresistance (GMR) device, may be used to measure AC or direct current (DC) flowing through a conductor without positioning a shunt resistor in the current path. Hall effect devices measure a magnetic flux and output a voltage that corresponds to a level of magnetic flux measured. However, such sensors are subject to error if they are exposed to stray magnetic fields from the surrounding environment. A Hall effect device cannot distinguish between the magnetic field produced by the current in the conductor and a stray magnetic field. The Hall effect device merely measures a level of magnetic flux.
Magnetic fields from adjacent devices create noise and prevent accurate measurement of current through the conductor. This can be a significant problem for circuit breakers when they are used in applications where they are in close proximity to multiple devices that can see high in-rush or short circuit currents. Although these high currents are typically transient, the high currents create large magnetic fields. For example, a multi-phase circuit may include multiple circuit breakers, each circuit breaker configured to disconnect a phase if the current in that phase exceeds an overcurrent level. In the multi-phase circuit, an overcurrent occurrence within one phase could generate enough magnetic flux to cause a circuit breaker in an immediately adjacent phase to provide a false indication of an overcurrent occurrence in the adjacent phase.
Typically, distance and magnetic shielding have been used to reduce effects of stray magnetic fields on a Hall effect device positioned to measure a current level within a conductor. If not shielded, current in an adjacent pole may cause the Hall effect device's output to correspond to a current level that is greater than the actual current within the conductor (i.e., the Hall effect device does not provide and accurate measurement of the actual current within the conductor). This false signal may be greater than the circuit breaker's nominal setting which would result in tripping of the circuit breaker. Tripping of the circuit breaker when the actual current within the conductor is below an overcurrent level is referred to herein as nuisance tripping. For example, a magnetic core may be used to concentrate the conductor's magnetic field and shield the sensor from stray magnetic fields. A magnetic field shielding material draws magnetic flux to keep it away from the sensor. Furthermore, increasing a distance between the sensor and sources of stray magnetic fields reduces the effect the stray magnetic fields will have on the sensor.
For systems with potential for large magnetic fields, it may not be possible to provide enough shielding or distance to ensure a good signal to noise ratio (i.e., low noise from adjacent devices). Size constraints in some circuit breaker applications, for example, a molded case circuit breaker, limit the effectiveness of shielding and distance due to the close spacing of the circuit breakers. Even when size is not a constraint, material for shielding adds cost to the circuit breaker.