Oscilloscope to view mains power

You can safely view mains power lines with a bit of common sense. Never connect the ground (shield) return of the scope to the hot side of the mains. Make sure that the neutral and ground wires are correctly connected. If they are properly wired, then it's safe to connect the scope probe's shield connector to either neutral (preferred) or to the ground. Then, connect your probe's tip to the hot wire and safely view the mains waveform. For the 3-phase power, make sure that you have identified the neutral and/or ground connections. Scope shield/ground to power neutral/ground, scope probe tip to the phases. Use the same strategy on your inverters. Connect the scope ground to the ground or neutral out of the inverter, and the probe tip to the hot line. In all inverters that I have seen, the inverter's neutral and ground are connected together inside the inverter.

NEVER EVER use a 2-blade adapter to isolate the scope. If you get the scope's case (ground/shield) connected to the mains hot line, or high voltage inside a piece of equipment, you could easily cause your wife to become a widow. I'm sure you want to avoid that if at all possible.

Cheers!!!!

First, be very careful if you connect anything to the AC mains. The voltage it operates at and the current it can deliver can make your mistakes lethal. One simple way of reducing this risk would be to use step-down transformers with the primary connected to the mains and doing your measurements on the secondary side at a reduced voltage. Those cheap, plug-in AC power supplies would be good for this.

Make sure that the front end of your scope can handle the peak voltage of any AC voltage you will be measuring, divided by the probe factor if appropriate. For example a 120 VAC RMS line will have a peak voltage of 170 V to neutral/ground (1.414 * RMS Voltage). AC voltmeters read RMS voltage. In this case, your scope would need to have a peak rating greater than 170 VDC if you use 1X probes and greater than 17 VDC if you use 10X probes.

Also, remember that the shield on your scope is connected to ground so be careful not to connect the ground lead of your probe to the line side instead of the neutral. If you do, you will likely ruin your scope probe. Using a step-down transformer will provide isolation and eliminate this problem.

Be safe......

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Thanks for that.

instead of

I read elsewhere that if I don't care too much about accurate voltage measurements, I can just connect the probe tip to neutral or active and not bother about connecting the probe shield.

The scope will then show the PD between the probe tip and the internal scope ground (which should be the same as the power points ground since they'll be on the same house power circuit - except when I'm measuring the inverter).

No circuit between the probe tip and its shield would seem to be safer, on the face of it.

Does that make sense?

tia, RR

I second the low voltage transformer suggestion.

Assuming your probes are rated for the voltage to be applied, put the scope in difference mode (subtract channels). Don't hook the ground to anything. Probe around using both probes. If you're interested in high frequency events, hook the two probe ground leads together...won't make any difference at 60 Hz.

NEVER, EVER hook your probe ground to ANY PIN on the wall socket. Yeah, yeah, call me conservative, but I'm an ALIVE conservative.

It's IMPORTANT that you not make any assumptions about the socket in the wall. I've seen 'em with open grounds, reversed hot/neutral. Measure with your voltmeter and be sure.

Another problem with measuring line voltages is hooking it up. I've seen people stick screwdrivers in the socket so they have something to hook the probe to. You're messing around, got one more probe to hook up. What the hell, just stick this screwdriver in there temporarily...wanna see my collection of screwdrivers with big black divots in the shaft? I've also seen people put alligator clips on a line cord. "Sounded like a good idea at the time." Try not to do anything stupid. mike

If I wanted to scope mains, I'd not just poke the probes into a handy receptacle.

If you do everything correctly, you'll just see some rather boring, noisy sine waves; if you do something wrong then there's the potential (no pun intended) that, if you survived, you would be wearing the remains of the o'scope.

Build a test rig with a properly polarized mains plug, in-line fuses, current limiting resistors, and with shielded and polarized test points. If it's for a semi-permanent test rig (e.g., an inverter) then build the fuses, resistors, etc. into it.

BTW, your premise wiring is most likely anti-phase rather than 3-phase and comes from a single, center-tapped transformer that itself is across one of the three phases coming out of your substation.

The best way to do this safely is to put your scope in differential mode. What you'll be doing is looking at the difference between the two channels (A-B). In this mode you will NOT connect the shield of your probes to the mains at all. Simply connect one probe to one line of the mains, and the other probe to the other.

Make sure that both input attenuators are set the same.

Steve

Thanks for smart suggestions.

The probe specifies Maximum Working Input Voltage at X10 (10MOhms setting) as:

Yes, it's the noise (or lack of it) that I'm particularly interested in.

BTW, I try the simple low voltage transformer and saw a 12VAC sine wave. Interestingly, there was a little ripple in the curve just after each postive peak.

I assumed this was some noise added by the transformer. That's one reason I'd like to see the "raw" output from the socket.

Great idea. I'll do that.

I'm waiting on the arrival of my "Fundamentals of Power Electronics" book, so I look forward to properly understanding that last statement.

thanks, RR

Yes, it's the noise (or lack of it) that I'm particularly interested in.

BTW, I try the simple low voltage transformer and saw a 12VAC sine wave. Interestingly, there was a little ripple in the curve just after each postive peak.

I assumed this was some noise added by the transformer. That's one reason I'd like to see the "raw" output from the socket.

Great idea. I'll do that.

I'm waiting on the arrival of my "Fundamentals of Power Electronics" book, so I look forward to properly understanding that last statement.

thanks, RR

Yes, that's correct. "400 volts DC and 400 volts peak AC". As opposed to, say, 400V rms AC.

Peak = 1.4 x RMS, for a perfect sine wave. So they could have said "400VDC

• 284Vrms AC". But if already know it's a sine wave, there's no point looking at it on a scope... and anything other than a sine wave will have a different RMS to peak ratio.

I would interpret that to be 400 volts DC or 400 volts peak AC. For example it would exceed the scopes spec if you wanted to measure a 50 V p-p AC signal riding on top of a 375 VDC bias voltage.

Yes, we're saying the same thing. The bottom line is: the scope doesn't know the difference between AC or DC or a combination thereof, it just doesn't want instantaneous voltage to exceed 400V.

At 60 Hz., you're right. But the AC capabilities of a probe derate quite significantly as the frequency goes up. Hence the addition of the DC number plus the derated AC number. Usually a chart in the probe manual. mike