How to attach (safely?) an o-scope to various sources

Question

I've heard of course that probing mains can ruin your oscilloscope, so that is out of the picture. There are a few situations I do not quite understand however:

Is probing an i.e. 10:1 transformer safe if I were wanting to see mains signal? Even if it is not an isolation transformer?

If I cannot do the above, how does a wallwart make the situation different? Is ground being part of the linear regulator the device of which makes it safe to probe directly? Just anything that is not "earth" referenced?

Can I probe something that is on battery (silly question) with no earth referenced ground?

I've heard people mention something about the ground clip, and if it is not properly placed somewhere it could damage the alligator clip with large currents, however I have no clue what that is referring to, or how massive current could flow through the o-scope to the ground clip or something.

I am sure I could answer the before with some various manuals, however the ones before that I am just completely unsure of.

Answer 1

Russell's answer is excellent as usual, I just want to add a bit extra.

Most oscilloscopes have a "standard" at least as far as impedance goes (1 Megaohm - note some have a 50 ohm input but that is less common and not relevant here)

The amount of protection and rating of front end components can vary pretty widely from what I've seen. For instance I have seen schematics for scopes rated for something like <50V input with no protection other than a 10k resistor in series with the opamp input.
In comparison you can get old (and probably new although I haven't seen inside one) tektronix scopes with >600V rating and heavy duty protection.

The only safe way to know what the limit for your scope is to read the manual carefully. If they sat you can probe mains voltage with the probe set to 1x then it should be fine - if it's under warranty and it breaks then you are covered anyway. However, heed Russell's advice about transients - if you have to probe mains voltages, whatever the input is rated for I would use a probe with 10x or 100x setting only, so you can't accidentally set it to 1x (see below)

Personally, I rarely probe anything high voltage on my DSO (OWON SD8202) - I use my big old tank of a scope (Tektronix 7633) for stuff >100VAC with a 10x probe and the DUT run from an isolation transfromer. I must admit a long time ago I accidentally used 1x probe setting for 230V (UK) mains on the Tek a few times and it never complained, although I certainly wouldn't recommend this to anyone - I mention it just to give an idea of how well these things were built (guess they assumed some idiot was gonna come along and do silly things like this :-P )

As far as the ground clip goes, on most plug in (to wall plug) scopes this is directly connected to earth ground.
In floating (i.e. no connection to mains ground through anything - USB, charging leads, etc) battery powered scopes with plastic cases then floating measurements may be taken, but as always, follow the manufacturers advice. What this means is that if you attach the ground clip to anything that is ground referenced (like the mains live wire) and at a potential higher than earth ground, it will create a low impedance path for current to flow (i.e. a short)
Ground referenced means, that one side of the potential is connected to ground - with mains voltage, when the utility wires come into your house, they are split into live, and neutral/earth (which are both connected to each other)
The earth wire is at the same potential as the neutral, but is not meant to carry current under normal circumstances - if there is current flowing in it (for instance if a live wire has fallen against a metal chassis connected to earth) then there is a fault.

If you isolate the ground referenced mains voltage using a transformer, then (as long as the secondary has not been connected to earth) you can connect your ground clip to either side of the secondary and be safe, as the current does not "want" to flow through it (aside from a small amount of capacitive leakage current)

If in any doubt, a good idea is to measure to see if there is any common reference between your ground clip and whatever it is you want to connect it to.
For example, say you have an unknown power supply with two leads and you want to figure if they are ground referenced - one way is to connect one multimeter probe to ground clip and the other to either wire to see if you get any voltage.
Another way is simply to unplug the unknown supply and measure continuity from it's earth plug pin to the output connections - if there is no continuity (or extremely high, say > 1 Megaohm) then there is no reference to ground.
Just in case it's a transformerless supply (or just a badly designed one) you should check that there is no continuity from the live and neutral pins too.

If still in any doubt, don't connect anything up until you completely understand everything.

There are also differential probes (example) you can buy for any scope that can be used to measure the difference between two floating voltages.

Here are a couple of references on grounds/probes:

Tek reference on probe grounds

All About Circuits worksheet on scopes read all of this and the answers to the questions (press reveal)

All About Circuits - Electrical Safety - not about scopes, but very useful information about electrical safety. The section on "Safe Circuit Design" is particularly relevant. Note this does not deal with isolation transformers though (although there is plenty on transformers on another part of the site)

Answer 2

Standard oscilloscope front ends are not rated for use with direct AC mains input. While some do have voltage ratings which are higher than the nominal mains voltages. mains also can contain transients and "spikes" of very high amplitude which can break down circuitry which is not designed to withstand such spikes.

High voltage probes are readily available (all you need is $) which allows high voltage or even EHT to be measured, depending on the probe used.

An oscilloscope 10:1 probe, which reduces signal voltages by a factor of 10:1, WILL reduce mains voltages to safe levels for input to essentially all oscilloscopes BUT a probe used for this purpose MUST also be mains rated for the same reasons mentioned above.

A wall wart has a transformer which provides isolation. This is either an iron cored transformer that works at mains frequency or a high frequency transformer that transfers energy at typically 10's to 100's of Khz and which uses ferrite (or in some higher power designs may use powdered iron). Irin cored designs are "older tech", usually MUCH heavier and require no electronics for the actual transformer action. an active regulator is often used to stabilise the output voltage. HF versions use an oscillator to drive a high frequency "switch" to convert input rectified AC to HF AC output which is then converted to regulated output DC. This is more complex than an iron core design but more compact. much lower mass and may be cheaper overall. Either way the output is transformer isolated from the input.

While one "leg" of AC mains may be nominally and notionally at earth potential (the Neutral leg), mains signals MUST be treated as if either or both legs are at full mains voltage to ground as on occasion this is the case.

If you use a 10:1 reduction transformer then the voltages will be safe for all normal oscilloscopes*. ie 110 VAC becomes 11 VAC and 240 VAC becomes 24 VAC. 24 VAC will produce about +/- 35 VDC peak to ground. This is within the input range of essentially any scope with an input scaler and using BNC connected probes. A transformer used to provide 10:1 AC step down from mains MUST be mains rated.

• Some specialist or super cheap and cheerful oscilloscopes may have a + 5VDC (or similar) maximum input voltage rating.Try not to use such devices for general purpose use.

A Wall wart is just a variant of the transformer discussed above.
The isolated output is (if properly designed) from from dangers of mains voltages but in this context have the advantage that the power supply output and so anything connected to it is "mains isolated". There is now no electrical path from AC mains or from mains ground to any point on the connected circuit so the oscilloscope is never mains exposed.