Hi Gabo, Good morning. A photocell is basically a photoresistor connected in series with the coil of an electromagnet, components more components less. With the light your photoresistor becomes zero or almost zero, letting the voltage pass through the coil and when you measure the neutral it measures the 127 volts of the phase, When the night comes the photoresistor becomes very large that the voltage tends to zero , the electromagnet coil is de-energized and the power contact is closed so that the passage of current turns on the lamps at night. When you connect the neutral, the voltage returns to zero because the circuit closes and each pole does its part, the phase feeds the coil and the neutral closes the circuit, the voltage in the neutral is zero if everything is OK. You can also have a return through the lamps but that will depend on whether this photocell is normally open or normally closed. I hope I have placed some light on your doubt. Greetings. Comment