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Conversation between Bob Pease and Paul Rako taken from this video, where Rako talks about modifying an oscilloscope such that it is not connected to earth:

Pease: "Have you ever tried hanging a scope from silk threads?"

Rako: "No, but I have put it [the scope] on cardboard and cut the ground pin off so it floated - you can't touch the metal the old delay-line or you'll get killed ..."

What exactly is Rako talking about?

What I'm looking for:

An explanation of what the dangers are of having an oscilloscope that isn't connected with earth (i.e. a floating scope). Ideally, with an example of a dangerous situation with a schematic of the circuit.

Carl
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3 Answers3

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I believe the situation in the video is such that a scope is used for debugging the unisolated mains side of a fly-back type switch mode power supply.

As no point in the unisolated side is at ground potential, you can't just poke a grounded scope there. I will explain both.

First, the reason why the mains side of the power supply is not grounded. Sure, in electrical wiring, ground and neutral wires are connected together, but in the power supply, neutral and live are full-wave rectified into a capacitor. So if you live in a 230V country, there will be 325V over the capacitor. But neither end of the capacitor is no longer at 0V ground level. During positive mains cycle, the negative leg of the capacitor is near 0V ground level and the positive leg is +325V, but during the negative mains cycle, the negative leg is at -325V and positive leg is near 0V.

The scope however is using the mains ground as 0V reference, and for safety reasons the metal case and probe ground lead are connected to mains ground.

So, with the scope grounded, you can (in theory) poke the probe tip to measure either leg of the capacitor, but positive leg will show a waveform between 0V and +325V while the negative leg will show a waveform between 0V and -325V.

But to show what is the voltage waveform over the capacitor positive leg in reference to capacitor negative leg, you would have to do one of a few things. Either get a suitable differential probe, or use two scope channels to make differential measurements, or the dangerous option, lift the scope ground and connect the probe ground lead to negative side of the capacitor.

The scope mains ground must be cut or otherwise there is a short circuit and tens of amps of mains current will flow through the scope ground clip during the negative mains cycle and your mains circuit breaker or fuse trips, likely with a loud bang, sparks and smoke, it may even damage the probe leads or the scope.

So when scope mains ground is cut, the scope is said to float, and then you can connect the probe tip and ground to the capacitor.

But as the probe ground is also connected to the scope metal case, it means, every exposed metal part on the scope and connected to the scope will now be live with rectified mains, wiggling between 0V and -325V, not safe to touch.

So if scope has other probes or anything else connected that are now floating at between -325V and 0V they will be deadly too. Or trip the breaker if you make a connection to grounded device.

But if you float the scope ground, the scope itself can be dangerous now, because it is missing a protection from the case becoming live due to an internal fault, not because you deliberately made it float to make it live.

Also the scope mains inlet likely has a filter which connects both live and neutral to ground through capacitors. It means that if you disconnect the mains ground, the scope ground, probe ground, and metal chassis will be floating at 115VAC because of the filter capacitors forming a capacitive divider. That may not be an issue, but again poking the scope tip or ground to a sensitive circuit might blow it up by discharging a capacitor with instantaneous voltage max +/- 162V through your circuit.

Justme
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  • This cleared up many things, thank you. Are "metal case", "case" and "chassis" synonyms for the same thing? – Carl Oct 22 '23 at 18:19
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    @Carl Good question - yes in this "case" they are (sorry pun not intended). I can edit to use a single word. – Justme Oct 22 '23 at 18:21
  • Thank you. One last thing: All scope channels have common ground, meaning if I float the scope and connect the ground clip of channel 1 to -325 V, all the outer conductors of the BNC-input channels become -325 V. But even so, I could still touch the outer metal conductor of channel 2 with one hand without being in danger, as I don't form a closed loop for the current to flow, right? I ask, because I read on a forum that simply plugging in a BNC-cable into channel 2 when scope ground was not earth would send you flying into a wall, which doesn't seem reasonable... – Carl Oct 22 '23 at 18:33
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    The ground would not be -325V DC, it would be just half-wave rectified mains waveform, AC. It would be just as safe or unsafe as just touching the live wire directly with one hand only. So not very safe. It depends on how well insulated you are as a whole, i.e. what shoes if any you are wearing, are you standing on metal, concrete, wooden or plastic floor, how moist has the humidity made the floor. And that's only for the leakage resistance. Even if us humans are just pouches of conductive saltwater, we have stray capacitance to our surroundings. And AC can pass through stray capacitance. – Justme Oct 22 '23 at 19:40
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    "your 10A breaker" -- why explicitly a 10 Amp one? (As a counter-example, all my socket circuits are on 13s or 16s, only lights have 10s). – Dan Mašek Oct 23 '23 at 16:54
  • @DanMašek It depends on your electrical system and wiring. For out of no reason I put 10A there. Your breakers may vary. – Justme Oct 23 '23 at 16:59
  • "But neither end ... is no longer at 0V ground level." did you mean "neither end is at 0V ground level"? Or "both ends are at 0V ground level"? – jcm Oct 24 '23 at 23:21
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No need for a schematic, in fact I'm not going to show a schematic to avoid noobs trying this. Consider a current shunt resistor, maybe 1 Ω, in series with the hot terminal of a mains supply. You disconnect the scope earth, and connect your probe to one side of the current shunt resistor, and the probe ground to the other side.

You don't need much imagination to see how that's dangerous, to the point of deadly.

Neil_UK
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  • Neil, Although tempted to give an example, I'll defer - as you have. Suffice to say, I still have my thumb that was squeezing the 'scope probe alligator clip. The clip (sadly) did not survive, saving the 'scope. – glen_geek Oct 22 '23 at 18:53
  • Lonnnnnng ago I braced a screwdriver on a chassis and probed a tag with mains on it with the tip !!! :-) :-(. The screwdriver welded to the tag and the arc burned a nice clearance half circle in the chassis edge. :-) – Russell McMahon Oct 27 '23 at 23:47
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What I'm looking for:

An explanation of what the dangers are of having an oscilloscope that isn't connected with earth (i.e. a floating scope). Ideally, with an example of a dangerous situation with a schematic of the circuit.

This is supplementary to the above query. There is "more than one way to float your scope".
Paul and Bob discuss cutting off the earth pin. Jim noted another way -

Jim asked in a deleted comment:

Am I missing something here or would not a 1:1 AC line matching transformer have done the trick?

Using an isolation transformer floats the oscilloscope. BUT if you then eg test between point at say 350 VDC above true ground and 300 VDC above true ground the scope sees 50 VDC BUT the scope ground is at 300 VDC above true ground.

The operator will experience the 300 VDC if they are grounded and touch the oscilloscope now-floating "ground".

Russell McMahon
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