Separation of Grounds

[Zman]

What is meant by seperation of grounds when we say seperate the grounds of control signal and power electronic circuit?  But we still use the same ground even after using optocouplers. So, how are the grounds seperated.  Would that be correct to say there is only one ground in a circuit?

[Jerome Trent]

Ground is a term that is often misused.  Most circuits have a ground in them somewhere but some circuits do not have a ground and instead have what is referred to as common.  Take a battery operated flashlight for example.  How could it possibly be connected to ground on one side when there is no connection external to the flashlight?  Your car would be the same.  Your car battery doesn’t connect to ground because it is isolated from ground with rubber tires.  In fact, pretty much anything that is battery operated does not have a ground.  It has a common instead.

Now to your question about isolating grounds; this sometimes comes into play when you are connecting to pieces of equipment together.  If the two pieces of equipment are a long distance apart like hundred yards apart, then they probably each have their own ground connection.  These two ground connections may be at a different potential causing there to be a voltage between the two grounds.  That’s when you need isolation.  There are various ways of isolating circuits such as optical isolators and transformers.