The situation occurred because the system ground is faulty or the neutral and ground has been improperly connected together in a sub-panel, and/or the ground screw and neutral are wired together
In the power industry, wiring the neutral to the ground screw is called a bootleg ground. There is only one tester that will tell you that this situation is present in an outlet, without removing the outlet.
The $2 three prong tester will not detect this situation. The Ideal SureTest tester ($200 plus) is the only tool that will identify this situation, without removing the cover plate and inspecting the connections.
If you are installing a satellite system and have a coax that has been grounded to a secure ground, your best way af testing for a "bootleg ground" or a floating ground is a DMM (digital multi-meter) set on the AC voltage setting. A n inexpensive one costs about $20, but make sure that it is marked "double insulated." I prefer one that is auto-ranging.
Set the DMM on AC voltage. If it is not autoranging, set it at 0-250 volts. Do not touch the tips of the leads. Put one lead into the wider "neutral slot" on the receptacle. Place the other lead on the outside "shield" connection of the "f" connector.
If you see a voltage above 1-5 or 75-150 volts, you have a "bootleg ground" with another situation called a "floating ground."
Basically, the electrical system's main ground is disconnected or faulty, and the neutral has been connected to the ground screw.
Placing the leads on the neutral and hot cavities of the receptacle will result in a voltage reading of between 75-150 vac.
This problem often occurs, when the neutral and ground wires are wired together in a sub-panel. These wires "can" be wired together in a main panel, but NEVER in a sub-panel.
Contrary to conventional wisdom, NEUTRAL is NOT GROUND! It is a LIVE RETURN WIRE. A GFCI will NOT protect you from a LIVE neutral.
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