Measuring AC phases on oscilloscope

[Collaro]

Out of pure academic curiousity I have been investigating how people can use oscilloscopes to visualise three phase ac.

I checked out this youtube video. https://www.youtube.com/watch?v=yY3VHfxVnJA

Everything is OK until about one minute into the video. Then, when he is describing how to measure voltage potential difference between A and B he describes how he would connect phase B to the ground.

Surely that would lead to a BANG? Phase B is shorted straight to ground leading to wire-melting oscilloscope destruction?

[MrBungle]

I haven't looked at the video but it's possible the phases aren't relative to real ground but relative to another phase which is described as ground here. Also the scope might be floating (don't do this!)

[peter_scott]

If he was talking about mains 3 phase and he hadn't made his scope floating then yes, he would blow something.

He is actually talking about a 3 phase supply that is floating (has not got its star point connected to ground) so he's just creating a measurement reference point using the three resistor star.

Peter

[cmjones01]

It looks like he's using a battery-powered motor controller intended, I suspect, for remote-control models. That means it will have no ground reference and so connecting the scope ground to one of the phases is fine.

If he was working with a 3-phase supply that was referenced to ground, he'd need to use a suitably-rated differential probe instead.

Chris

[Collaro]

Ah yes, thanks for the enlightenment everyone.

I was under the impression he was measuring three phase from the mains, but looking at it again, that probably isn't the case.

[MotorBikeLes]

Yes, he clearly said connect phase B to the ground connector, but he did NOT say 3 Phase mains.
When I started playing about with rotary converters 10 years or so ago, I wanted to compare phase voltages on a single phase 230v in, 3 phase 230v out unit. Obviously I was after optimum phase balance. I thought of doing exactly the same as he did, using three 100K resistors connected in star, and using the star point as my reference "artificial neutral". I was using DVMs, rather than a scope, but I suspect if I set up the scope's controls first, stood it on a rubber mat with ground lead disconnected, I would be fairly relaxed about using it to study phase shapes and angles. (Don't do this at home!)
It was instructive to note that as I improved balance with the correct capacitors, the current drawn from mains fell (measured with a clip on ammeter), the "noise" from the motor also fell. Optimum voltage balance across the resistive matrix as measured with DVMs led to "coolest, quietest and best" result all around.
Les.

[BlueMoon]

Standard practice at college in the 1980s was to have a floating scope (earth disconnected at plug) so that the chassis could be connected to one of the phases. Any adjustments would be made very carefully! Fingertips only to the control knobs. How Times have changed... can't imagine a tutor allowing that now, let alone setting it as standard class practice. Thank you Captain Three Phase.

[Collaro]

That does sound a bit scary!

[ex 2 Base]

We use to used a CRO and the Lissajous configuration when synchronising alternators using a home made high voltage box attached to the scope input. We also had three lamps mounted on a box, lights dark or bright you made your choice and a syncroscope, belt and braces. Never heard of any one getting a shock or any thing else, folk were very careful those days.Ted