Electronic – Are touch lamps safe

Warning

• FYI - some early AC/DC radio sets had live to mains chassis when operated from mains.

Equipment chassis is usually grounded to wiring ground via earth lead on power plug where provided.

In any sane regulatory environment, appliances without an earth lead are required by law to be "double insulated" so that internal metal parts are not touch accessible during operation. (Note: Double insulation nowadays seldom involves two layers of insulation - it is more a state of mind in manufacture and testing that leads to touch safe equipment. Usually :-). )

While a chassis should be "touch safe", never assume it is, because:

• There are non-sane regulatory environments,

• or ones where the authorities enforce the rules so little that they may be ignored and

• It is excessively common for people to do silly and dangerous things with mains wiring, despite being aware that people are reported to have died due to such things.

Wikipedia - appliance classes

They say:

Class I

• These appliances must have their chassis connected to electrical earth (US: ground) by an earth conductor (coloured green/yellow in most countries, green in the U.S., Canada and Japan). A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. This current should trip either an overcurrent device (fuse or circuit breaker (CB)) or a residual-current device (RCD) also named as residual current circuit breaker (RCCB), or (ground fault circuit interrupter (GFCI)) or also, residual current operated circuit-breaker with integral overcurrent protection (RCBO). which will cut off the supply of electricity to the appliance.

Class II - See also: double switching (and double insulated)

• A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (US: ground).

  The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation.

  In Europe, a double insulated appliance must be labelled Class II, double insulated, or bear the double insulation symbol (a square inside another square)..

Class III

• A Class III appliance is designed to be supplied from a separated/safety extra-low voltage (SELV) power source. The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock.

  The extra safety features built into Class I and Class II appliances are therefore not required. For medical devices compliance with Class III is not considered sufficient protection.

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)