Several other phase sequence discriminator circuits c

 For places where there is no neutral line at the power supply , the phase sequence discriminator shown in Figure 3-91 (a), (b), (c), and (d) can be used.
In Figure 3-91 (a), the neon bulb in Figure 3-90 is replaced by a light-emitting diode , which solves the problem that the neon bulb is easy to burn out after long-term operation.