Is this safe enough way to probe 240VAC mains voltage?

[BravoV]

One mistake .. is enough to destroy the scope .. or your limb ...... or your life.

Question is, how often you made mistakes on probing say at low voltage ?

[AnasMalas]

One mistake .. is enough to destroy the scope .. or your limb ...... or your life.

Question is, how often you made mistakes on probing say at low voltage ?

I know, it is a risk you shouldnt take, I did it once, and wont do it again. Ill be honest and say that I did want to do this for the longest time, which played into it.

Ive made one major mistake probing low voltage with my DMM, I was trying to troubleshoot something for someone, went for a toilet break, and when I returned they had were playing with the DMM... They placed the probe in the 10A socket, and I didnt check. But I wasnt trying to be careful, I was working "normally".

Now Ian.M's Tek story was spooky honestly, I kinda wish we knew what happened exactly, but I only now saw that you shouldnt touch the controls, well, uh, why?

If I had known that the rubber buttons and dials are even slightly risky I wouldn't have done this without connecting the scope to my PC wirelessly and leaving it alone

[Ian.M]

Except in the rare case of scopes designed and intended for fully floating operation (e.g. Fluke ScopeMeters), your scope was designed under the assumption that no hazardous voltages would be present on its chassis (or front panel PCB ground plane). You don't know how good the insulation is between the scope chassis or ground plane and your finger at the supposedly insulated control you are touching.    e.g. conductive dirt may have built up on the sides of a button from fingerprint oils and ambient dust.  If your scope isn't properly grounded, all it takes is one bad mains spike at the wrong moment i.e. while you are pushing a button, and there's a significant risk of one dead engineer.

There was absolutely no need to float your scope.  If you were probing within its (and its probes) ratings, with the ground clips removed and the scope set to display the difference between two channels (with equal Y attenuator settings), your scope would have been safer properly grounded.

The matter of not having a proper ground at your workbench is another issue:

I knew the risks, and I knew that I should never float my scope....
Except that the scope is floating whether I like it or not. By doing this, I just isolated the ground from my power strip and nothing else. Houses here don't have a ground. We thought ours had ground in the kitchen only, turns out the ground wire is just connected to neutral at one of the plugs, and not behind the circuit breakers. We do have codes, but no inspections, and so wiring tends to be a complete disaster. Against all advice, my father (I am a college student, this is our summer house where I live alone for 9 months of the year) wont do anything about it, and I was 6 when we this house was built.

If you advise me to wire the ground to neutral locally at this socket only, sure, ill do that, but something tells me this is worse than just leaving the scope floating, as it has been for a year now

Personally if I was in your situation,   I'd get a ground to my bench no matter what.  As long as its a single occupancy dwelling, driving a ground rod, and running a 4mm² ground wire to your bench should be possible.  A TT earthing system is recognized as safe and  is commonly used in rural areas in many countries.  You can set up just the socket strips on your bench that way without having to change any wall outlet or breaker panel wiring.  A RCD (GFCI) is *ESSENTIAL* especially in hot dry climates as you are likely to have difficulty getting a low enough ground resistance to reliably trip ordinary circuit breakers or fuses.   *DO* *NOT* use water (or any other) pipes for the TT ground connection - if their grounding isn't as good as you thought it was, one unnoticed fault on your bench could make every water tap in the whole building lethal.  All socket strips on your bench should have their grounds tied together, and you shouldn't be physically able to come into contact with any metal pipes or building metalwork while at your bench.  If the floor is dirt or concrete, you'll also need a good quality rubber or plastic insulating mat for your entire work area.  Don't get a black one as it may be somewhat conductive due to being carbon filled.   

If its actually a multi-occupancy building (e.g. apartments etc.) and/or you cant drive a ground rod and run a ground wire, ask about reasonably safe options that don't involve having to find an honest, competent electrician to come in to rewire it properly.

[BlackICE]

Different countries have different standard of "safety." Japan for example has both 50 and 60Hz system depending on where in the country. I don't know about other areas, but around Tokyo they use 100VAC at 50Hz non-grounded, non-polarized outlets in homes. My understanding is that the power isn't grounded at the service entrance box at is left floating. Are plug in devices all class 0?

Decades ago I remember my home having a plug in light fixture that gave a buzz if you ran you moist fingers over the metal shade. I've been to a friends home that was built in the 1950s when you touched the refrigerator's metal handle you also get a buzz. The refrig was really dangerous as you could touch it and a water facet at the same time! Both the devices had two prong non-polarized plugs connected in reverse or a reversed outlet wiring! If you ever looked at old homes with knob and tube wiring it you may be shocked to see some wires with crumbling cloth insulation. So you have bare copper running inside the walls. I will never buy a home with groundless knob and tube wiring unless I was prepared to rewire the whole home.

I have had a few experiences of dead short across as 120VAC 20amp wall outlet, not fun. A large loud spark is emitted before the breaker trips. I wouldn't ever want to find out what a 240VAC with a 50amp breaker would do!



https://en.wikipedia.org/wiki/Appliance_classes

[nightfire]

At my last job we had a Fluke 435 available for such things. The current clamps would provide Volts back to the Scopemeter and they were fast enough to cope with inrush current from capacitors of server PSUs- I measured those to get some feeling how a 19" Rack of servers would behave after a complete blackout and coldstart, where you will have a severe capacitor inrush current after providing voltage again...

Seriously, this sounds as a CAT III  zone, and even some errors in wiring those leads can short something out. Here you should only use power clamps rated for that environment and a metering device that is capable of dealing with that kind of transients usually present in the supply infrastructure.

As we were back then replacing a distribution cabinet for the HVAC systems in our main data room I asked for implementation of a 4mm panel to get some test points for that scopemeter, so that I could measure not only currents, but power whilst running those systems- the power leads from that cabinet have been bent at different distances so I could fit those current clamps without hassle...

[BlackICE]

Thanks to all that contributed with warnings and suggestions. I made a small PCB with 50ma fuses and 240V MOVs from HV to GND to short out the transients. It worked well for less than 10 test cycles. Then I wasn't getting any voltage readings. I checked the fuses and all 3 were blown! If I didn't make that PCB maybe my scope's front end would have been fried! I had a curiosity about the voltage reading but it wasn't really needed. So I didn't want to buy a real diff probe.

I knew that the transients spikes and voltage drops from the large motor starting and stopping would likely cause problems with the scope's operation. So I ran it off a battery powered inverter, grounding the scope to earth. However my inverter soon failed, so I tried using a wall outlet, no dice! I kept getting premature false triggers when only the small fan motor started. I had to grab a UPS from my PC to power the scope to get back to testing.

Anyway I got some great information and think because of the improvements I can run my 5 ton AC compressor using my generator. Testing that Monday.

Summary unmodified the startup lasted 0.22 seconds with a RMS current of 163A.
With soft start the startup lasted 0.45 seconds with a RMS current of 44A.
The 1st run using the soft start the startup lasted 0.2 seconds with a RMS current of 65A.
After 5 learning cycles, the current draw was further reduced by extending the startup time and reducing the duty cycle of the power given to the motor.

Net result a 73% reduction in startup current while extending the startup time by 100%.

Without any mods


With soft start

Current waveform using soft soft.

[wizard69]

I want to monitor at 240V US mains voltage during a HVAC compressor startup. I have a Siglent SDS1104X-E and the scope probes are rated for 300V at 1x and 600V at 10x. At full scale at 50V per division should give me a 400V range, enough for at nominal 240VAC*1.41 I plan to use an inductive current probe on one channel and use 2 other channels in subtract mode to monitor the AC mains voltage, leaving the grounds on the probes disconnected. The ground is connected via the power cord, not idea but good enough for my purposes. I will power off the equipment to attach the probes and then never touching the probes or probe wires while the power is connected. The 1x to 10x switch is pretty easy to move accidentally so best to set and never get near the switch.

If you are worried it probably isn't good enough.   You didn't mention if this is single phase or three phase, and what exactly you are looking for.   However the first thing that come to mind is do  you really need to monitor the voltage with a scope?   Generally a decent DVM or even a good old analog meter will be good enough to monitor line conditions at start up.    Voltage generally isn't as interesting as current draw where a good current probe on a scope can be useful.    If you think you are suffering from excessive voltage drop for an extended time, you should be able to see that on a meter.

To put it another way the best way to avoid the hazards of a dangerous measurement is not to make the measurement in the first place.   Instead consider if you really need to know the values in the first place.    If you do need to know, consider equipment that makes getting the data safer.    Being able to hook us a meter before start up is also a huge advantage as you don't need to touch anything to get your data.

As others have indicated short circuit currents can be excessive before a breaker can activate.   In a home it might be more limited but even 10,000 amps for a cycle or two can generate a huge bang.   This also assumes a breaker will actually actuate fast enough or at all.   While currents in industry can be significantly higher I've seen what can happen when motor controls can't handle the currents they need to interrupt - they literally explode.   So safety should always be a concern when working with mains power.