Why can't you use "any" ground test point when measuring voltage?

Question

For example, let's say you want to konw the voltage at node x. You MUST have your +ive probe at x and your -ive probe at GND within THAT circuit. But, why cant I just put the -ive probe at any GND.. not necessarily the GND of the circuit i'm looking at..

Answer 1

gnd for the total system and the single circuit is over all the same. That's why its call ground.
So yes you can use any ground in the system as ground point for your probe

Answer 2

Ground for DC is usually easy. If all the grounds are connected together, and they should be, they will be the same. If you have a high current situation, there may be voltage drop in the ground system. In this situation you will want to know which ground point to use for an accurate measurement.

However, when you are dealing with AC things can change. Depending on the circuit and the frequency involved there may be voltage nodes on the ground system. In this situation you may have to use a specific ground point to get accurate measurements.

Almost any circuit or wiring might have ground loops. This can cause problems, especially in low distortion applications like quality audio amplifiers. Plugging some equipment into one outlet and other equipment into a different outlet can cause a very bad hum in your sound regardless that all the power goes back to the same ground point.

Answer 3

Supposing you are using a scope on some electronic board:
True, there might be more than one ground, usually true if you have a board with both digital and analog functions.
Nevertheless, all grounds should be connected to a single point at the end.
Look for your power supply entry point, and use the GND at the supply point.
Differential grounds are rather rare, but do exist. In this case, you would need a differential lead for your scope.

Answer 4

Because not all grounds are the same. You can have a chassis ground and an earth ground at different potentials. This is especially true when working in circuits which have both positive and negative voltage!

A good example of that is in a circuit where you have a +15 volts and a -15 volts to ground.where the ground specified is chassis ground. and your -15 volts is at earth ground potential. If you measured your positive voltage from earth ground instead of chassis ground or chassis common your positive voltage would be 30 volts instead of 15.

chassis ground is generally show as
_\_
/ / /

and earth ground is generally show as an upside down triangle!

Answer 5

All ground points are interconnected.
There is no question about which ground point you are using to measure.

Answer 6

Because all grounds will not earth.