A magnetic field measuring device for detecting a localization current (Ip) in a branched AC power supply system, wherein a load current having a mains frequency flows through a line section to be tested of the AC power supply system and the localization current (Ip) has a fundamental frequency lower than the mains frequency, the magnetic field measuring device having a current sensor arrangement mountable on the line section to be tested, characterized in that the current sensor arrangement consists of a first current sensor, a second current sensor and a compensating coil (Ak), which generates a compensating magnetic field (Bk) acting in a detection range of the first current sensor and serving to partially compensate a mains-frequency alternating magnetic field which is caused by the load current, and in that the magnetic field measuring device has a first signal processing block, a second signal processing block and a compensating block (Vk), the first current sensor and the second current sensor being disposed such that, in addition to the compensating magnetic field (Bk), the first current sensor detects the alternating magnetic field and a common-mode magnetic field (Bp) caused by the localization current (Ip) as a partially compensated magnetic field and the impact of the compensating magnetic field (Bk), which is generated by the compensating coil (Ak), on the second current sensor is negligible, the second current sensor thus detecting only the alternating magnetic field and the common-mode magnetic field (Bp) caused by the localization current (Ip) as a resulting magnetic field, the first signal processing block generating a localization signal (xp) indicating the localization current (Ip) from a first sensor output signal generated by the first current sensor, the second signal processing block generating an alternating voltage signal (xs) corresponding to the resulting magnetic field from a second sensor output signal generated by the second current sensor and the compensating block (Vk) connected downstream of the second signal processing block generating a compensating current (Ik), which feeds the compensating coil (Ak), from the alternating voltage signal (xs) and the second signal processing block having a signal input for receiving a synchronization signal (xsync) in order to synchronize a specified measuring period (Tm), during which the localization current (Ip) is detected, with the alternating voltage signal (xs), and the current sensors (S1, S2) being designed such that, compared to the first current sensor, the second current sensor has a lower magnetic field measuring sensitivity and a greater magnetic field measuring range, for which reason the partial compensation occurs only to the extent that the partially compensated magnetic field does not exceed the magnetic field measuring range of the first current sensor. a load current having a mains frequency flows through a line section to be tested of the AC power supply system and the localization current (Ip) has a fundamental frequency lower than the mains frequency, the magnetic field measuring device having a current sensor arrangement mountable on the line section to be tested, characterized in that the current sensor arrangement consists of a first current sensor, a second current sensor and a compensating coil (Ak), which generates a compensating magnetic field (Bk) acting in a detection range of the first current sensor and serving to partially compensate a mains-frequency alternating magnetic field which is caused by the load current, and in that the magnetic field measuring device has a first signal processing block, a second signal processing block and a compensating block (Vk), the first current sensor and the second current sensor being disposed such that, in addition to the compensating magnetic field (Bk), the first current sensor detects the alternating magnetic field and a common-mode magnetic field (Bp) caused by the localization current (Ip) as a partially compensated magnetic field and the impact of the compensating magnetic field (Bk), which is generated by the compensating coil (Ak), on the second current sensor is negligible, the second current sensor thus detecting only the alternating magnetic field and the common-mode magnetic field (Bp) caused by the localization current (Ip) as a resulting magnetic field, the first signal processing block generating a localization signal (xp) indicating the localization current (Ip) from a first sensor output signal generated by the first current sensor, the second signal processing block generating an alternating voltage signal (xs) corresponding to the resulting magnetic field from a second sensor output signal generated by the second current sensor and the compensating block (Vk) connected downstream of the second signal processing block generating a compensating current (Ik), which feeds the compensating coil (Ak), from the alternating voltage signal (xs) and the second signal processing block having a signal input for receiving a synchronization signal (xsync) in order to synchronize a specified measuring period (Tm), during which the localization current (Ip) is detected, with the alternating voltage signal (xs), and the current sensors (S1, S2) being designed such that, compared to the first current sensor, the second current sensor has a lower magnetic field measuring sensitivity and a greater magnetic field measuring range, for which reason the partial compensation occurs only to the extent that the partially compensated magnetic field does not exceed the magnetic field measuring range of the first current sensor.