One of the three-phase AC power neutral line open circuit alarm circuits

As we all know, if the neutral line of the power transformer is open and the three-phase load is unbalanced, it is easy to cause
damage to electrical equipment (such as household appliances, lights, etc.) due to overvoltage (caused by neutral point potential deviation). The neutral line open alarm circuit is
designed based on the principle that the neutral point potential shifts when the transformer neutral line is relayed.
    The circuit is shown in Figure 13+80. When the neutral line of the power supply is open , the light-emitting diode VL lights up. In the figure, T is the power transformer
, Ru, Rv, and Rw are the loads.