Ghost? voltage on lamp outlet when turned off (72V/50V)
When you have a load that is controlled by two switches, there is a cable that runs directly between those two switches that contains two conductors. From your description, that cable is at least 10 meters long.

When the power to the load is off, one of those conductors is live and the other conductor is connected to the load. There is a significant amount of capacitive coupling between the two conductors &mdash; about 70-100 pF per meter &mdash; which will allow a tiny current to flow.

I have seen a situation in which a much shorter run between the switches (about 3 meters) passed enough current to cause several LED bulbs to glow dimly on a 120 VAC circuit even when they were supposed to be off. In this case, I calculated that the capacitance would allow about 13 &micro;A to flow. In your case, with a longer cable and twice the voltage, you could be gettng something on the order of 80 &micro;A.

The voltage you measure will be a function of the load impedance. With no other load than your multimeter, it is not surprising that you measured 72 VAC, and that you felt a significant shock.

The cure is to either disconnect the power from the circuit altogether (at the main distribution box), or to provide a low-impdeance path for the current to flow through. In other words, temporarily shorting the line to neutral would be perfectly fine. Just be sure to remove the short before you try to switch the power back on.