The asymmetry monitor MU 631 is available in several design versions for differing nominal system voltages. When the set voltage unbalance is exceeded, the built-in output relay releases once the set delay time has elapsed. In addition to voltage unbalance, the device also signals phase failure, phase sequencing reversal and balanced voltage reductions. Thanks to a low-pass filter shielding the measuring circuit, the device is also suited to applications in systems with harmonic wave content.