# Will I see a sine wave if I connect one oscilloscope terminal to the phase and another to the earth?

I have a doubt regarding a thought experiment I am conducting since I lack access to proper grounding at both my workplace and my house.

The experiment involves the following setup: I intend to connect one terminal of a battery powered oscilloscope to the phase and the other terminal to the ground. My main query is whether I will observe a sine wave in this configuration. If not, I am curious to know what waveform I would encounter.

The schematic would look something like this:

simulate this circuit – Schematic created using CircuitLab

My reason for posting this question is that I assume the earth wire to be neutral but possesses some additional resistance due to the properties of the earth. Could anyone confirm if my assumption is accurate?

Any insights or explanations would be appreciated. Thank you for your time and assistance.

NOTE: The original question was using a mains powered oscilloscope, but now I updated the question using a Battery powered oscilloscope to avoid the accidents that may cause due to internal short of earth wire to the ground of probe.

• You should google for "how not to blow up your oscilloscope" first, e.g., EEVblog #279 - How NOT To Blow Up Your Oscilloscope! (YouTube video). Jul 24 at 15:20
• please check first the rating of your oscilloscope if it can withstand the phase of the mains. for safety considerations, it would be nice to have an electrical diagram for that. Jul 24 at 15:23
• Use a 10x or 100x probe or you'll quite possibly blow up your scope. I haven't seen a scope rated for more than 300 V on its inputs (ONLY in high-impedance mode!), and if your mains is 240 V AC, the peak voltage will exceed 300 V. Jul 24 at 15:42
• @SKGadi How about using a transformer between the mains and the oscilloscope? A small transformer with a 12 V secondary would be suitable, and you will still see a lot of the noise which is present on the mains in addition to the sine wave. Jul 24 at 15:42
• @SKGadi There's a great big transformer at the other end of that mains cable - adding another one won't make a difference to seeing the wave form. Jul 24 at 15:46

You specified a specific model of oscilloscope. Relying on that claim, I see no problem connecting the oscilloscope live to earth because that particular model claims to be rated well beyond foreseeable voltages (416V x 1.414).

## Neutral is not earth.

That's not a headline, that's me shouting :)

In the AC mains power system, there is no "common" or "Vss" -each live wire comes with a companion neutral that returns power only for that live. They can't be carelessly combined because they'd be subject to thermal overload, and neutrals are not circuit breaker protected. Even where neutral goes through the breaker, overcurrent on neutral is not monitored.

Anyway, back to ground/earth.

The general concept is that neutral is defined as "the one near earth". However, different countries do their practical bonding to earth in different ways, and that can cause neutral to not be reliably at earth voltage.

Also, depending on the method, earth may not be bonded to neutral with a very low impedance bond - consider the European "TT" system where neutral comes from the utility but earth is acquired locally without a North American style neutral-ground bond in the service equipment.

So it is a worthwhile measurement, both generally interesting and potentially informative of problems.

The safe way to do this is use a filament or similar transformer to drop the voltage to a save voltage maybe 15 plus or minus 10 Volts. Then connect your scope on the secondary of the transformer and observe the sinewave. You can use AC or DC coupling as there is no DC offset If there is distortion etc in the power from saye a phase controlled device it will show. Safe testing!

Then you need a non contact voltage probe for your scope. Look at the Hioki SP3000-01 Non-Contact Voltage Probe w/ Amplifier Box and Probe, that should do what you want. It is not inexpensive but it works.

• My objective is not to see the sine wave between line and neutral. My objective is to see the wave between earth and phase. Also, you may suggest to put a transformermer primary to phase and earth, But I don't like that idea because when some current flows, the noise will reduce. I want to see through high impedance terminals what is the waveform between earth and line. Since it is a thought experiment, I can imagine I have an oscilloscope that work on battery to avoid all accidents may be caused. Jul 24 at 15:57
• @SKGadi Not if the oscilloscope inputs can’t handle the peak voltage directly (some high end LeCroy can, but the vast majority can’t). Jul 24 at 17:50
• @winny Ok, if they could handle, will it show a sine wave? Jul 24 at 17:54
• It will show whatever your utility company is providing. Depending on local legislation (or lack thereof), anything from a perfect 50/60 Hz sine wave to crippled asymmetrical brownout with overtones. Jul 24 at 18:24