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I have an old analog oscilloscope: http://www.ebay.co.uk/itm/Advance-OS-240-10MHz-Dual-Trace-Oscilloscope-with-Instruction-Manual-/261220770137 It doesn't have 3 pins power connector instead it has 2 pin connector(Line and Neutral). It has external earth symbol on it. I think it looks like it is not earth grounded. Are modern oscilloscopes earth grounded for the GND part of the signal or it is for safety? I mean are they grounded through Line or earth ground? I mean housing systems are such that at most of them earth and neutral are connected to each other at the end.

My question is : Is my oscilloscope measuring signals correctly?

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There are two 'ground' points in play here:

Earth ground is for safety. Connect to earth ground if you like.

Signal ground is for measurement. Connect to a ground on the circuit you are testing. Preferably something near the signals you want to probe.

The signal input of the scope has two front panel BNC connectors for probes. When you connect the probes, the center pin of the BNC carries the signal you are probing, while the outer ring of the BNC carries the signal ground.

If you are probing a digital circuit, for instance, you'd want to connect the signal ground to whatever ground all the digital logic uses.

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The earth ground pin of the power supply is for safety only, it does not have any impact on the signal measured by your scope probe.

The scope channel input measures the voltage between the two pins of the BNC, the inner pin and the outer pin (acting as a shield). You must always connect the two to get a proper voltage, because a voltage is the difference between two electrical potentials.

You do not always want to measure the voltage between one potential and the earth ground. For example, you could observe the voltage between the ground of a digital IC, and the earth ground.

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  • \$\begingroup\$ which ground do u mean? Im asking the GND of channel input. Channel input of an oscilloscope has 2 shields: plus and ground. If I only connect the plus of my BNC cable to plus of the Oscilloscope I dont see anything. I should also connect the GND of BNC to GND of oscilloscope channel. I mean the signal ground. Where is this signal ground connected? Is it connected normally to the earth ground. In my case there is no earth ground. \$\endgroup\$ – user16307 Nov 26 '13 at 11:03
  • \$\begingroup\$ The channel input is made of two pins, and it measures the voltage between these 2 pins. You call it plus and ground, but more generally it is plus and minus. A voltage is always defined by the difference of 2 electrical potentials (plus and minus). It's common to use earth ground for the minus potential, but now that you are speaking about electronics, please forget that. The BNC is made of 2 conductors, and you must connect the two. The minus potential (signal ground), is not connected to the earth ground of the scope, and except if you know what you are doing, it must not be connected. \$\endgroup\$ – RawBean Nov 26 '13 at 11:43
  • \$\begingroup\$ Note if you use a device powered with grid power (not battery) you WILL get readouts even without "ground"/"minus" line not connected - although it will be extremely noisy and inaccurate, as the "minus"-"Ground" connection of the circuit goes through your wall socket. That's a troubleshooting tip: if your waveform is somewhat right but ridiculously noisy, check if the "minus" connector of the oscilloscope didn't go loose. \$\endgroup\$ – SF. Nov 26 '13 at 11:59
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On a few oscilloscopes, each channel will independently measure the voltage difference between the inside and outside of a BNC jacks without allowing any current to flow between the different jacks. More commonly, the outer ring of all the BNC connectors will be connected together. Typically, for scopes with a three-prong mains plug, they will be connected to earth ground as well.

From a safety perspective, connecting the scope ground and earth ground has a number of effects, some bad and some good. On the bad side, it means that when working with voltages which are substantially above earth ground, it creates a potential path for dangerous currents to flow which would not otherwise exist. If one were constructing a scope with a single input, this reason would probably represent a good justification for not connecting the input ground to earth ground. While it is true that having the scope lead be earth grounded could help avoid the possibility that an equipment chassis was live without someone knowing it, it's also entirely possible (if no other exposed objects are earth-grounded) that the even though the chassis was live it would pose no real hazard but for the scope ground itself.

The real safety advantage to earth-grounding scope leads arises when there are multiple probes whose ground clips are connected to different objects. Were it not for the earth grounding of the scope, connecting one probe's ground clip to something which was live (relative to ground) would cause the other scope's ground clip to also become electrically live. If that other clip was connected to an exposed metal surface which was not earthed, the entire surface could be dangerously electrified. Even if the other surface was earthed, touching the clip to it could cause sufficiently dramatic sparks as to startle the operator, thus resulting in the live lead touching other things it shouldn't. Nasty stuff. Having the scope ground clips be earthed would mean that the attempt to connect the scope lead to something that was live would quickly get noticed.

Conceptually, it would seem like the ideal thing from a safety standpoint (aside from having all probes isolated) would probably be a scope which had a two-poles-per-input resettable interruptor, and a safety circuit which would disconnect everything if excessive ground currents were detected along any path. Such a device could combine the safety advantages of preventing dangerous current flows between probes' ground clips, while simultaneously avoiding the introduction of a dangerous current path of its own. I don't think I've ever seen such a scope design, but it probably shouldn't be too hard to construct such a thing.

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For safety only!

The circiut has a ground that you defined and the osciloscope's signal is biased with the two pins.

Remember that the power generator and the osciloscope cannot be earth grounded at the same time because you will have a short circuit when measuring a signal of a single component in a circuit.

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