"Is it attaching a forward biased diode from your circuit of interest to ground?" NO, this removes any isolation that may be present.
Basically, an isolated circuit has no connection at all to earth ground. Sometimes this is as easy as using a transformer in the power supply.
Most circuits use ground or earth as a zero volt reference point.
In domestic power supplies one pole requires to be earthed for safety, hence a neutral which is earthed at the star point of the supply transformer and a separate protective earthing conductor, which is either totally separate from the neutral or combined with it.
The neutral is the return path for single phase on a 3 phase distribution system. The earth conductor is the safety conductor should a fault develop between live and metal parts and conveys the fault current back to the supply to disconnect the supply.
To have an isolated circuit means no pole of the supply is connected to earth, therefore only short circuit protection is provided.
Some equipment do not require an earth, such as double insulated equipment. These are deemed not dangerous to life as the equipment cannot convey voltage outwith itself.
Some distribution systems such as used on hazardous cargo carrying ships are not allowed to have an earthed neutral system, as the earth is the hull of the ship and fault currents flowing through the hull would be extremely dangerous. This is a subject of it's own.
In electronics say, to supply a +5 -5 supply you may have a 10v supply using a mid point as your zero but not connected to earth, it may be grounded to the chassis to supply a return path, but not to earth.
Means u'll have to use an Isolation Transformer.
The primary side is grounded. So your circuit on the secondary doesn't have 'ground' as the reference voltage.
So, if an operator touches the circuit (which is on the secondary side) he won't get shocked.
But he will if he touches the negative as well! obviously.
In normal public supply systems, one pole of the supply is connected to earth at the supply transformer. In the UK and Europe, this is known as a 'TN' system. Not connecting the neutral to earth at the supply creates an 'IT' system or as I remember it 'insulated terra '(ground).
This is not used for public supplies as it can become unpredictable and dangerous.
You can create your own ground isolated supply simply by using a double wound isolating transformer with a 1:1 ratio, ie same voltage out as in, take your supply from the secondary without grounding one side.
The voltages in the circuit operate entirely independent of ground or there is a potential difference between voltage sources, be they negative or positive and ground.
"Isolate from" in this respect means "disconnect from".
The circuit in question should have no electrical connection to ground.
Is it attaching a forward biased diode from your circuit of interest to ground? What does isolation look like physically?
