Author Topic: A way to measure a earth ground circuits with your scope safely  (Read 2906 times)

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Offline aether22

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A way to measure a earth ground circuits with your scope safely
« on: February 09, 2016, 11:56:07 pm »
See:

Alas this didn't work with my Rigol DS1054Z, but maybe I don't know how to drive it right yet?

Can someone with a Rigol try this method and see if it works as in the video?

Does anyone have any other suggestions for ways to measure live circuits with an earth ground safely?

Other than with expensive differential voltage probes?

Isolation transformer on the oscilloscope?  (essentially turn it into a hand held)

Capacitor in between the scope ground and point being measured?

What about 2x 30uf caps (one on live, one on neutral) to isolate the scope as I can't seem to find a suitable isolation transformer?

What ideas are the least crazy/dangerous to me and the scope?

Thanks
« Last Edit: February 10, 2016, 12:03:51 am by aether22 »
 

Offline Muxr

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Re: A way to measure a earth ground circuits with your scope safely
« Reply #1 on: February 10, 2016, 12:34:29 am »
Your Rigol should be able to do it.

I know of 4 ways to make this type of measurement:

- Differential probe for the scope. (these can be expensive)

- isolating transformer for the unit under test (it is not advisable to isolate the oscilloscope due to safety reasons, but you can isolate the unit under test)

- hand held oscilloscopes are often used as well since they are floating by design

- and the a minus b method (which is what that guy described in the video)

Also Tektronix has a PDF about it you might take a look at: http://jp.tek.com/dl/3AW_19134_2_MR_Letter.pdf
« Last Edit: February 10, 2016, 12:37:12 am by Muxr »
 

Offline vk6zgo

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Re: A way to measure a earth ground circuits with your scope safely
« Reply #2 on: February 10, 2016, 02:23:12 am »
Actually,"earth ground" circuits are the ones you normally look at with Oscilloscopes,where one leg of a power supply is connected to chassis & hence ac ground.("Earth",or "PE")
In that case,both the ""ground" lead on the 'scope & the device ground are at the same potential.

I think you are referring to situations where neither side of the voltage you are probing is at ground potential.
This could be looking at the voltage drop across a resistor in the middle of a network.

There are two ways commonly used to do this:
(1) Look at the voltage w.r.t ground on each end & work out the difference in your head----this is easiest for dc
measurements.
(2) Use the method shown in this video.
(3) Use a differential probe-----expensive but good!

Let's bite the bullet of the big "nasty"---probing the Mains supply.


Note: Only use a X10 or X100 probe for this,& check the specs of both probe & 'scope before proceeding.
Mains supplies usually have three connections---Active,Neutral,& Earth.
In Oz,& various other places,Neutral is connected to Earth at the entry to your house.("meter box")

You can't just call Neutral "near as dammit" Earth,because it carries return current for all the Electrical stuff you have running,& will have a voltage drop  due to circuit resistance ,elevating the potential at your GPO above that of the Neutral/Earth junction at the "meter box".

You may just get away with clipping your 'scope probe "earth clip" onto the Neutral line & probing the Active, without any "zaps"BUT
(a) The Electrical Authorities won't like it at all--some of the return current from the appliances on that circuit will flow in the Earth line,which contravenes Regulations.
(b) The RCD will operate,if you have one.

As you are unlikely to be messing around with the fixed wiring,the most likely place you will be looking at Mains is inside a device you have on the bench.

In a perfect world,all devices would be wired & colour coded correctly,so the worst that could happen is (a) or (b) above.
If the device is wired with Active & Neutral reversed,or you misidentify them,you are shorting out the mains supply via your 'scope  & its Earth circuit.
Large BANG!--goodbye probe,maybe 'scope,maybe you!

There is a sneaky trick if you just want to check for the presence of Mains in your device----use one  X10 or better probe with the earthclip completely removed.
The 'scope is now only grounded via the Earth lead in its power cord & the house wiring.
Some current then flows in the Earth circuit,but it is miniscule.

Obviously this means that any high frequency artifacts won't be accurately displayed,but if all you want to do is see if the Mains are there,it works well.

If you absolutely have to "hang across" Active & Neutral,use the method shown in the video,or preferably,a differential probe,or a portable 'scope of suitable ratings,with insulated connectors.
 

Offline alsetalokin4017

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Re: A way to measure a earth ground circuits with your scope safely
« Reply #3 on: February 10, 2016, 04:54:02 am »


Alas this didn't work with my Rigol DS1054Z, but maybe I don't know how to drive it right yet?

Can someone with a Rigol try this method and see if it works as in the video?

(snip)

Well, you certainly should be able to do this simple differential measurement on your Rigol. Let's see if we can figure out why you weren't able to do it.

You need two channels turned on, obviously. You will also need the Math trace turned on, since the Rigol is a DSO that does not have a simple "trace add" setting like the Analog scopes do.

The Math menu will give you the opportunity to select "Operator", and you want "A-B". (Or you could select "A+B" but then you'd have to invert one of the channels in its Channel menu.) Select "Operation" ON, and the math trace will appear on the screen. Set the vertical position ("Offset") where you want it to appear,  and Scale appropriately for the voltage you are measuring. Set the "Source A" and "Source B" to the channels you are using.

Now you should be able to get the differential measurement appearing on the Math trace when you use the two probes, without ground leads connected, just as is shown in the video.


Try it first with the 9v battery so that you understand what's happening, before you go trying to scope your mains!

« Last Edit: February 10, 2016, 04:58:08 am by alsetalokin4017 »
The easiest person to fool is yourself. -- Richard Feynman
 

Offline aether22

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Re: A way to measure a earth ground circuits with your scope safely
« Reply #4 on: February 10, 2016, 07:37:09 am »
I figured out what is wrong.

2 of my Rigol probes are way way off.  Hence they weren't cancelling right.

They read voltages notable higher than the other when on x10.

With the 2 good probes I tried it, and it worked, but the problem is my signal generator is digital and that means discrete steps in the sinewave, this works but it makes the math trace very hairy looking.

So it kinda works, but it is only kinda  plus unlike the video demo the 2 raw results from the 2 probes also shows onscreen which is a mess, and that is what the trigger is based off of.

So it works, and it can be used in a pinch, but it isn't pretty.

John
 

Offline kripton2035

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Re: A way to measure a earth ground circuits with your scope safely
« Reply #5 on: February 10, 2016, 07:39:50 am »
you can often get used differential probes on ebay for less than $100
only put alert emails for them and wait for the right one
 


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