Ground is Ground?
I overheard another person discussing electrical ground the other day, and they said something along the lines of 'ground is ground'. What he was referring to is the electrical ground in reference to another electrical ground shouldn't be different. I shook my head and silently walked away.
Electrical ground is not the common bonding circuit that some people assume it to be. Ground can mean a number of things, from the body of your car (which is not earth grounded), to the common circuit in your cell phone. Often different systems will require grounds specifically referenced to that system. That is the key, referencing.
What you reference your ground to, determines what it is common to. If you reference all the ground outlets in your homes electrical circuit to the same ground rod beside your house, then yes, all those circuits have a ground reference to the same ground rod (in a TT system). That is also assuming that the neutral wire is bonded to the electrical panel, which is also attached to that same ground. In this case you are also referencing the neutral to the same ground, which is important as many devices do not utilize the 3rd ground pin in an outlet.
However when you have a structure with various sources of ground... lets say, 3 ground rods, and one electrical ground coming in off of the powerline feed, then those 4 ground sources will be different unless they are all well bonded. You can not expect the ground reference at one ground rod to be equal to the ground reference at another ground rod. There will be differences. In fact you can even make a battery by placing metal panels into the ground... This is called an Earth Battery.
So no, unless each ground is bonded, there is no way that every ground rod will reference exactly the same.
Like I said before there are also other systems which require their own grounds, outside of an electrical ground. For example, you generally want to separate RF Ground from Electrical Ground, as you do not want to either induce high frequency RF into your power circuits, or 60 cycle hum onto your antenna circuit. You also do not want to send RF power down an electrical ground which is connected to other devices.
Audio is generally the same way. When designing a studio, I used to have to run a separate ground, from a clean 'signal ground' source, to each studio. Sure the studios already have electrical grounds connecting to the equipment. But the last thing you want is the 60 cycle hum being inducted into your audio lines. Therefore all audio cable shields are connected to the signal ground, usually at the main mixer, and left disconnected at the device end. This prevents what are known as ground loops, which is caused by a difference in electrical ground from device to device.
So no, ground is not just ground... Ground can be very different between point A and point B.