How to maintain the zero potential at the connector end?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A latching current relay.

How it works:


With no load connected RLY1 turns off.
When load is connected the ON button is pressed. This causes current to flow through RLY1 and its contact closes. ON can now be released and power will stay on.
If the load is disconnected RLY1 will open again. The OFF button can be used to switch off power before disconnection.


RLY1 should be chosen with a coil current rating to match the load under test. The switch contacts also need to be rated for the load.