If you look closely you can see a wire running from the mains straight to the sensor board next to the arduino, clearance is on the order of a few mm. All it ta

If you look closely you can see a wire running from the mains straight to the sensor board next to the arduino, clearance is on the order of a few mm. All it takes is a little bit of condensation for a nasty accident, the voltage between those two conductors is 110V AC on one side and +5 V with some unknown level of insulation from mains on the other. Making assumptions about which line carries phase and which is neutral can cause very nasty surprises and should never be a factor. You're one broken circuit away from being the shortest path to ground, even in the neutral line (you'll still have the load as a drop resistor but it's not good).