Safety issue: What appliances will constitute a mains ground reference?

[Minor Tom]

If you measure higher voltage equipment (tube amplifiers, for example), it is sometimes important to know which of your appliances used will establish a reference to mains ground.

What about the common wallwart? The older ones will use conventional transformers, while the current ones will be switching power supplies. I assume the output of both of these will have no fixed reference (floating). I assume this, because their mains connectors establish no connection to safety ground (at least the german versions), so having any reference to mains at the output will be potentially dangerous. So any equipment powered by wall warts should establish no mains reference. Correct?

Traditional oscilloscopes with metal casing will be safety grounded - and so will be the ground connection of the probes, regardless whether the scope is analog or digital, I suppose.

What about the modern plastic casing digital oscilloscopes?

And a final question - is there an easy, reliable way to check for a mains reference with hobbyist measurement equipment?
I assume a multimeter ohm measurement will tell if there is a connection from output to mains connector, but a simple capacitor could defeat this.

I hope this bunch of questions is not to overwhelming - and I will be thankful even for partial answers.

[CatalinaWOW]

While this may not meet everyone's standards, I find it sufficient to measure connections to the power cord with an ohm-meter. 

[Minor Tom]

Thanks for your answer!

I just modified my post and recognized your answer only afterwards, so this was not in response to it 

[madires]

SMPSU based wall warts usually have an EMI suppression cap between primary and secondary which passes a low current. So a SMPSU isn't really floating. In modern scopes/DSOs the probe's ground is still connected to PE/earth. But you can get scopes/DSOs with isolated inputs or simply buy differential probes. A DMM is fine. You can detect a cap by measuring the voltage between the output (or signal input) and PE/earth when the device is switched on. Check signal ground and the signal line. In most cases the cap is connected to ground, but you can't assume anything until checked ,)

[Shock]

Obviously if you have a low resistance that is the first warning sign.

Some older equipment service manuals (Sony CRT TVs comes to mind) tell you how to test for leakage current. They either recommend a proper leakage tester, perform a current measurement, or use a resistor and capacitor in parallel to earth and measure AC across them.

The reason to measure current (in one way or another) is voltage alone may show as ghost voltage. An alternative is comparing a DVM to a multimeter that has a LoZ function, or try a voltmeter with a lower impedance than a DVM. These will slightly load the measurement and if you see decent drop in voltage you can get a rough current calculation (just don't do this on sensitive circuits).

When deliberately shunting an unknown current to earth or loading down a mains measurement with your test equipment keep in mind you can blow fuses and trip RCD type devices. Always treat damaged or unknown equipment like it has exposed mains and the potential for high current.

Read up about appliance classes and PAT testing as well.

[CatalinaWOW]

A neon bulb could be used to detect a capacitive connection.

[Minor Tom]

Many thanks for all of your answers and valuable tips so far!

SMPSU based wall warts usually have an EMI suppression cap between primary and secondary which passes a low current.

Well, this reminds me of (very) old tube amps. In addition to having two pronged mains chords (Safety earth simply wasn't provided in these times) there was a capacitor wired between one of the mains leads and chassis ground to provide "noise suppression". Nowadays this "feature" is known as the "death cap", as this would be very dangerous indeed if it failed short.
Well I HOPE the EMI suppression cap in a SMPSU lets pass a few "user safe" volts at most if it failed short.
If it should be of a certain type for safety reasons, I would count on our friends from the "Won Hung Lo" factory to cut some corners.

It seems, to be really insured against unexpected earth references, I should only use battery operated equipment!

I didn't really understand your measurement tips concerning "Detection of mains/earth reference in mains operated appliances".
As I have expected, there are some problems .... ghost voltages, references via capacitor ....
Could you give me a more noob friendly description and tips how to measure? 

[madires]

Yep, the EMI suppression cap is a class Y2, typically with a low value in the nF range like 2.2nF. A possible path for DC current is simply checked by a resistance measurement as you've already figured out yourself. For checking for an AC current path through a EMI suppression cap you'd power the device/SMPSU and measure the AC voltage between PE/earth of the mains socket and the device/SMPSU's output. If there's no cap you shouldn't see any significant voltage. But if there's a cap you will.

Edit:
A ghost voltage is caused by coupling from a wire installed in parallel usually. For example, if you have two mains cables and one is switched. Even when the second one is switched off you'll measure a high voltage and might think it's switched on. The coupling creates a quite low current, but with the high impedance of a DMM of 10M Ohms it's sufficient to see mains voltage. To cope with this problem some DMMs have a low-Z mode, i.e. the impedance is lowered to 10k Ohms or so. The load of the low impedance is too large for the low coupled current and the voltage measurement will show a few mV maybe.

[Minor Tom]

Thanks for the additional explanation!