Using oscilloscope safely with AC mains

Question

I know there is quite few discussions already on this topic, but since English isn't my first language, I though it better to get some more information about this.

I have used oscilloscopes to troubleshoot electronics problems and measure AC engine current spikes at startup, but I have no experience using them to measure high voltage levels and my experience is with old analog scopes.

So now I have this new DSO and the input rating is 400V peak to peak AC, so it can handle about 140V AC RMS? Not that I'm going to input that much there.

Since I live in Europe the mains here are 230 V @ 50 Hz I can't measure them directly.

If I set the attenuation from the scope and probe to 10X, the voltage is 23 RMS and it is safe to measure? And yes the probes are rated for 1kV.

If that is true, it's also safe to measure voltage between two main lines (400V RMS), since the peak-to-peak would be about 112 volts?

SAFETY: When I am measuring AC, I will be using the scope from its internal battery or with a isolating transformer (1:1) so that it is floating, meaning that there is no connection to earth ground. Are there any other safety issues I have not realized?

Answer 1

You must be very careful when measuring voltages on the mains, especially in your country where the voltages are absolutely deadly.

The best way to approach this is to make a resistive divider box. This is a simple resistor divider housed in a safe non-conductive project box. Connect the top and bottom of the resistor divider to a line cord with a correctly polarized plug. Then bring the bottom of the divider and the center tap of the divider out to 5-way binding posts or banana jacks. Also route the Earth Ground lead of the line cord to another banana jack, or 5-way, on the enclosure.

Select a resistor divider ratio so you get an output voltage which is both safe to touch and suitable for your scope's input range. Also, select the resistor values so they have a low enough impedance to not affect your scope's accuracy, but are high enough that you are not burning up too much power in the upper resistor and creating a lot of unnecessary heat.

As you will be multiplying all of your scope readings by the inverse of this ratio, choose a ratio which is easy to manipulate mentally - e.g. 10:1, 15:1, 20:1 - but still provides a safe-to-touch voltage level on the output jacks. ( Not that you will be purpously & routinely touching the output terminals, but accidents and slip-ups do happen. )

Make sure you construct this box in such a way and seal it up so there is NO CHANCE of accidentally touching the Hot Wire. You might also include a pilot light to indicate the box is plugged into the mains. You can't be too careful when messing with the power mains!

Mark the resistor divider's ratio on the outside of the box. Multiply all of your scope readings by this factor to get the actual line voltage.

Answer 2

I just tried this. To be safe, I verified that the outlet was properly wired, e.g. hot and neutral are not reversed, no open ground, no open neutral. I also used the same outlet as the scope is connected. That way, the grounding is already happening inside the scope and reduces to possibility of a ground loop. I connected the grabber to the hot (small) side of the plug as it enters the outlet. The little gap creates a low risk of accidentally touching it.

Bottom line, it worked, and I lived to tell about it. And got pics to prove it. Signal looked clean, very little harmonics and noise.

Answer 3

I would tend to use a transformer and be safe! It may loose some noise at the high end, but...

Note that in Europe most mains systems are protected via leakage current trips; 30mA is common, some are 100mA especially where "leaky" systems regularly trip or where inductive loads are an issue.

If you must use a resistor divider to earth, you must ensure that the current passed does not exceed this trip value (there will be leakage from other devices, so a divider must draw well under the trip value). I would also tend to use multiple, high wattage wire wound ceramic resisters that will not break down. Using multiple resistors in series protects against an unlikely short circuit within the resistor. Thus if you need a 1M resistor use 2x 500K in series, or, 2x [2x 1M in series] in parallel (for clarity, that's four resistors to create a single resistor value - often easier than trying to match the correct E values).

(note that the use of a parallel/series combination has the advantage of "smoothing out" values, especially where tolerance is high and of course the values vary within the sample - you will of course need to know an accurate value for your divider)

Note that wire wound resistors may introduce unwanted inductance

Fit your divider in an insulated case with approved connectors, wiring and wall plug.

Be safe!

Answer 4

Many others have made the point that you should use a proper differential probe when measuring high voltages, however something to watch out for if you are measuring line to line is that the common mode voltage rating of the differential probe is high enough. You might find differential probes which are cheap and measure up to ~500V, but the cheaper ones often can't tolerate high common mode voltages that you can see when you put the probes line to line.

Answer 5

I don't have much time to answer this question, but I have to say:

PLEASE PLEASE PLEASE

AC mains are potentially deadly: be VERY careful when experimenting with the grid.

I would strongly advise using a differential voltage probe like this one: Voltage probe (many other models exist)

This kind of gear is designed by professionals, and you won't have to experiment with deadly voltages when trying to debug your own circuits.

Some warnings about self-made solutions:

• as other contributors have pointed out, floating the scope is simply NOT an option
• using a voltage divider could be an option, but only if you really understand the odds of such a design. If used incorrectly (e.g. when connecting directly a phase to the scope GND), you create a short circuit which can, at best, destroy your scope, and at worst harm yourself. Please see this about arc flash. And if you try to watch voltage across two phases, you are guaranteed to create a short circuit.
• you could try to build your own differential voltage probe. Basically, it is a differential amplifier with R1 = R2, R3 = R4 and R1>>R3, R2>>R4. But, by buying one, you'll get certainly a better-designed probe (more precise attenuation, far better common mode rejection, larger bandwidth), and you will avoid hazardous experiments

Answer 6

user36607 you have asked a great question. One that will keep you from damaging or destroying your brand new DSO. Some scope input basics are in order. Keep in mind that the input attenuators are the same for both the analog scopes and todays DSOs. The input amplifier/A/D & attenuator in the scope has a maximum input capability no matter what probe you are telling the scope you have. All that changes is how the scope reports the input voltages. Think of the probe as an additional attenuator ahead of the scopes internal attenuator. The probe multiplier reduces the input connector voltage by the factor on the probe. 10V on a 10X probe will generate 1V at the connector. Likewise with a 100X probe the connector voltage will be 0.1V. By informing the scope what the probes attenuation factor is allows the scope to accurately report that voltage to you the operator.

Now on to your second question. You are correct about the voltage. The 230V is across the lines not from the lines to ground. The phase is split(L1=+180deg, L2=-180deg). Half the voltage from each line relative to ground. Assuming your scope is properly grounded which also means the input connectors outer ring is also grounded(verify with a DMM). So what the input to the scope sees is the line voltage relative to ground or 115VRMS (162Vp-p). Then you use the MATH function to show the difference between the lines and viola you have the full voltage measurement with full scope bandwidth. Now where you ground your scope probe determines if you have created a ground loop. Sorry ney sayers the only way around creating a ground loop is to isolate the scopes ground. Ground loop is another topic for another question.

user36607 hope this theory helps so that in the future you can apply this thought process to determine if you are safely taking a measurement.