# Measuring the waveform of mains voltage using an oscilloscope

I understand this is a recurring discussion, but I still can't find a concrete answer to this question.

I would like to know how can I inspect the waveform of my UPS output (230V) using an oscilloscope, to find out whether its "sine wave" is actually an acceptable sine wave or one of the high-marketing, low-quality versions.

My idea is to purchase this handheld DMM/oscilloscope and simply stick the probes on the C13 output contacts of the UPS.

There are videos on YouTube of people doing this on mains. I am knowledgeable enough to understand just how dangerous mains voltage is, and I'm aware that I'm not knowledgeable enough to understand how to measure it safely.

The only difference I see between mains and an UPS is that the UPS should not be as dangerous regarding transients, but other than that I believe they're the same thing for practical purposes. The UPS will obviously be unplugged from mains, so it will be working on battery power and it will not be connected to earth ground.

So, is there a safe way to measure the waveform of a UPS using an oscilloscope like that one?

Assuming that the oscilloscope's CAT II rating is honest, that I do not touch the tip of the probes, and that I ensure they do not touch each other, what hazards are present?

• That's the big assumption with these types of instruments - is the CAT II rating honest. I'm always wary of using test equipment that doesn't have any regulatory approvals - without a safety mark, not only is the design not proven to be compliant but the manufacturing may not be well controlled. An approved product demonstrates design compliance through testing and manufacturing compliance through factory auditing. – Adam Lawrence Sep 18 '20 at 15:21

The alternative is to buy a cheap differential probe. I had bought one on Ali- Micsig DP10013 and it works surprisingly well. However a handheld scope should have a triax connector and differential input. Many cheap handheld DSO don't have this option, there might be some lurky datasheet that you won't understand at first glance, but then it's too late. Go visit eevblog site for opinions about the DSO you want to buy.

So if the handheld scope is not accessible, then go for HV differential probe.

• I don't understand your answer. It is better to purchase a differential probe for this purpose instead of an oscilloscope? The Micsig DP10013 seems to cost twice the price of the scope I was considering. I also don't know what you mean with "differential input". – guest Jun 18 '20 at 1:42
• @guest Due to forum restrictions about shopping recommendations, I gave you a more general answer about using a DSO and live potential. Now bad and cheap handheld DSO starts at $800, while a decent starts at$2400. For a good multimeter you also pay more than \$200. So, I can't give you an opinion about the item you are looking, personally I have a DSO and I need to turn many knobs to get the correct signal view, I would never buy that item, because in my opinion is useless. – Marko Buršič Jun 19 '20 at 10:28

Two reassuring checks in the on-line manual:

maximum waveform 700V

Seems it is designed for "ease-of-use". Put in the hands of an experienced user, it (and the user) likely survive just fine. I always wonder if such an instrument is tested by putting it in the hands of an inexperienced user, and then doing a review after a failure.
This instrument appears to be engineered for use by less-experienced users. Attempts have been made to eliminate some failure modes.

What hazards are present?...
This instrument has many measurement modes...it is possible to make measurement connections to a device-under-test that result in smoke.
For example - current can be measured. If you arrange for a current measurement in error believing that it is set to measure voltage - bad things can happen: perhaps a blown fuse saves you.
This is a very common user-failure mode that this particular instrument has attempted to make less likely - a determined person can bypass its efforts.

• I know how to use a multimeter, and I understand the difference between connecting in parallel or series. I would first turn the knob to the "OSC" probe, which I believe is a voltage reading over time, and I would connect each probe - which is supposed to be insulated - to each pin in the C13 socket. I also understand that it is a VERY bad idea to turn the knob with the contacts already connected (so if I pass through an ammeter, I will short the circuit). – guest Jun 18 '20 at 1:48
• In YouTube videos, people freely touch their multimeter contacts to the mains socket. To me, it makes sense that they are not electrocuted: the probes are insulated, and they are not touching each other, so the mains voltage should only be running inside the circuit, and never through the user. The only hazards I can imagine are a faulty circuit (with insufficient clearance inside the PCB) or an accidental touching of fingers or probes. My question is what other hazards are present that I may not be aware of :-) – guest Jun 18 '20 at 1:49

Personally, I would build a 50:1 voltage divider out of some old fashioned through hole resistors before I would trust one of those budget meters that may or may not actually be rated for the voltage. Heat shrink the live ends to an old power cable, make sure the total resistance is a few hundred kOhm and only expose the low voltage side. Gives you something you can verify is safe before you plug it in.

Downside is that voltage won't be exactly accurate unless you use precision resistors and account for the input impedance of your scope, but it sounds like you don't care about the exact voltage.