# How can I measure a ground voltage of a circuit?

#### Dong-gyu Jang

Joined Jun 26, 2015
115
Hello.

Our lab has an RF generator which output power is 600 W in maximum. I want to measure its ground voltage (a voltage at a metal case of the generator) with respect to a ground voltage on a wall socket in our lab during a full operation. I consulted this issue with an electrical engineer and he said the voltage difference between these points is simply measured by typical fluke multimeter; a red line is touching the metal case of the generator and a black line (com) is touching the wall socket ground. He has an opinion that the ground voltages are always considered DC so this approach is reasonable.

Could you please give me some comments on this? Is it true? If not, why and how can I measure the ground voltage of a circuit with respect to other ground?

#### Dodgydave

Joined Jun 22, 2012
11,152
Simply put your voltmeter across the mains ground and generator casing, select AC or DC.

Ideally it should be Zero, because the casing should be Earthed at the plug.

#### Papabravo

Joined Feb 24, 2006
20,607
With a multimeter you should be able to make both AC and DC measurements. Interpreting the results can be challenging. If you know the geometry of the connection between the two grounds it is somewhat easier.

Example 1: Suppose you have two grounds with a total resistance of 1 Ω between them. Then a 1 mV difference means there is 1 mA flowing between the grounds. Now consider what happens when there is 1 MΩ between the two grounds. Now a 1 mV difference means there is 1 nA flowing between them. Using RMS values with the meter on the AC setting you come to the same conclusions. Here is the problem, If the meter measures to 3 significant figures you will be unable to measure very small voltages and currents. If you don't know the resistance between the grounds you will have some difficulty interpreting small non zero values.

Example 2: On long network cables that distribute power and ground to satellite nodes it is not uncommon for there to be several volts of difference between the ends of the cable. This is true for both the power pair and the data pair. Robust network physical layer components account for this.

Last edited:

Joined Jul 18, 2013
28,050
Hello.

Our lab has an RF generator which output power is 600 W in maximum. I want to measure its ground voltage (a voltage at a metal case of the generator) with respect to a ground voltage on a wall socket
Where does the RF generator get its case ground source from?
Or do you mean measure a possible difference from case to its own power source socket?
Max.

Joined Jan 15, 2015
7,256
A Google of the term "ground loops" should bring up some interesting reading for you. When we have multiple paths to ground and small currents flowing through those paths we get small voltage drops which show up as voltage differences at ground. These are generally very small voltages, often in the mV range but in some cases with sensitive equipment these tiny ground loop voltages can be problematic. Ground loops and potential differences in ground can make for interesting conversation.

Ron

#### BobTPH

Joined Jun 5, 2013
8,108
[not really recommending this, just a little humor]

Connect the case to the inner contact of an RCA jack and the socket ground to the outer. Then plug it into an audio amp plugged into that same socket. A loud hum would indicate the grounds were different!

Don't ask me how I know.

Bob

#### crutschow

Joined Mar 14, 2008
33,358
Why are you concerned about ground voltage?

#### Dong-gyu Jang

Joined Jun 26, 2015
115
Where does the RF generator get its case ground source from?
Or do you mean measure a possible difference from case to its own power source socket?
Max.
The RF generator metal case is grounded by connecting it to the wall ground (so it is wall socket ground) via a ground wire. The ground wire length is about 2.5 m and its copper core diameter is about several mm.

And yes, I would like to measure any possible voltage difference between the metal case and the wall ground. The every conductor has impedance consisted of R, L and C components, so if there is some current flowing through the ground wire between the metal case of the generator and the wall ground, there should be some voltage developed along the wire according to Ohm's law. This voltage is what I would like to measure. Can a simple voltmeter work enough for this?

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#### Dong-gyu Jang

Joined Jun 26, 2015
115
Why are you concerned about ground voltage?
Hello.

I would like to measure the live voltage of the RF generator output by using a single-ended voltage probe, which is connected to an oscilloscope. The oscilloscope is powered and grounded by the wall socket. So, I'm worrying the possibility that a measured voltage is not accurate if there is some voltage difference between the RF generator metal case (local ground of the RF generator circuit) and the wall socket. Yes, I assumed that the wall socket ground and the oscilloscope ground are on the same level.

#### crutschow

Joined Mar 14, 2008
33,358
I would like to measure the live voltage of the RF generator output by using a single-ended voltage probe, which is connected to an oscilloscope.
You should get a good measurement as long as you connect a short probe ground lead directly to the generator output ground.

#### Dong-gyu Jang

Joined Jun 26, 2015
115
You should get a good measurement as long as you connect a short probe ground lead directly to the generator output ground.
I guess "measurement" you mentioned is the measurement of the voltage between the live line and the ground of the RF generator output port. Can I have some question? Imagine that the ground lead of the single-ended voltage probe is touching the local ground of the circuit and this probe is connected to the oscilloscope which is grounded differently. If there is some voltage difference between the circuit ground and the oscilloscope ground, could you tell me what happens on this whole measurement circuit?

#### Dong-gyu Jang

Joined Jun 26, 2015
115
With a multimeter you should be able to make both AC and DC measurements. Interpreting the results can be challenging. If you know the geometry of the connection between the two grounds it is somewhat easier.
Hello. Thanks for commenting me

So..you said that a ground voltage difference can be basically measured by using a typical multimeter, right? I have a difficulty to accept this as our frequency is at 13.56 MHz. If our frequency were DC or 60 Hz, I would fully accept this. However, our frequency is 13.56 MHz. The ground potential rise is made by the current flows on a ground wire between two ground points. In my case, the RF generator metal case is connected to the wall ground via the ground wire (green thick wire). If there were any current at 13.56 MHz flows on the wire, I guess there must be a ground potential rise rippled at 13.56 MHz, due to Ohm's law.

Am I right? If I'm right, can the multimeter follow such a high frequency?

#### crutschow

Joined Mar 14, 2008
33,358
If there is some voltage difference between the circuit ground and the oscilloscope ground,
But if you connect the two grounds together through the probe ground, then there won't be any significant difference in ground potential between them.
To check, just connect the probe to the ground.

#### nsaspook

Joined Aug 27, 2009
12,309
Hello. Thanks for commenting me

So..you said that a ground voltage difference can be basically measured by using a typical multimeter, right? I have a difficulty to accept this as our frequency is at 13.56 MHz. If our frequency were DC or 60 Hz, I would fully accept this. However, our frequency is 13.56 MHz. The ground potential rise is made by the current flows on a ground wire between two ground points. In my case, the RF generator metal case is connected to the wall ground via the ground wire (green thick wire). If there were any current at 13.56 MHz flows on the wire, I guess there must be a ground potential rise rippled at 13.56 MHz, due to Ohm's law.

Am I right? If I'm right, can the multimeter follow such a high frequency?
The wire from the electrical wall socket to the generator case is a electrical protection ground not a signal ground so you should see almost no RF flowing in that wire unless it becomes incorrectly becomes part of the RF transmission line from RF source to RF load. The 13.56 MHz generator output current path should be only via the transmission line loop path. If the load is an antenna that needs a earth ground connection then a completely separate grounding system for RF is used but it's normally directly attached at the matching network end of the transmission line, not at the generator to eliminate current loops.