Q: Why are just the center LEDs of the Voltage Indicator On?

A: For DC applications, just the center 4 LEDs will be lit and the outer LEDs will not. The reason is that the two LEDs on each phase are wired in opposing directions and will only light up the "+" LED. In an AC application, both will flash as the positive LED lights up on the positive aspect of the sine wave and the negative LED lights up on the negative aspect.