Products > Test Equipment
Floating Scopes
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serggio:

--- Quote from: tronde on June 27, 2017, 05:55:06 pm ---If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket.

--- End quote ---
This is completely stupid. This is prohibited in Russian standards. If during any works at mains, phase's and PEN conductor will changed each other before branching to your socket, that is means that you'll have phase at your ground contact in socket.
PEN should be divided to PE and N at building central switchboard or floor central switchboard and PE should not be interrupted after that division.
Never make connecting ground contact to "neutral" contact directly in socket!

I see, that you very experienced at electrical safety and have knowledge about (I)T(N)-(C)-S and etc. protective grounding schematic, but still telling about evident's safety errors.  :(
exe:

--- Quote from: Vtile on June 26, 2017, 09:31:12 pm ---PPS. The water circulating radiator under your workbench where you rest your feets seeking a warmth is also propably a directly grounded.. What a nice electrocution chair there.  ;)

--- End quote ---

I got a shock every time I simultaneously touch the radiator and my laptop (that has metal enclosure). Why is it so?
tronde:

--- Quote from: serggio on June 27, 2017, 07:37:09 pm ---
--- Quote from: tronde on June 27, 2017, 05:55:06 pm ---If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket.

--- End quote ---
This is completely stupid. This is prohibited in Russian standards. If during any works at mains, phase's and PEN conductor will changed each other before branching to your socket, that is means that you'll have phase at your ground contact in socket.
PEN should be divided to PE and N at building central switchboard or floor central switchboard and PE should not be interrupted after that division.
Never make connecting ground contact to "neutral" contact directly in socket!





I see, that you very experienced at electrical safety and have knowledge about (I)T(N)-(C)-S and etc. protective grounding schematic, but still telling about evident's safety errors.  :(

--- End quote ---

You must read the post I answered to.


--- Quote from: serggio on June 26, 2017, 07:51:41 pm ---
TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

--- End quote ---

What I say, and this IS true, is that if you find TN-C in a wall socket, it will be wired with PEN connected to both the earth connector AND the neutral conector.

"serggio" gave the impression that TN-C in a wall socket would give you a floating scope. This is wrong, because the earth connector of the socket will be connected to the neutral (which will be the PEN-wire in that particular instance).

It is correct that this is bad practice (and I guess illegal in most countries by today), and that is also stated by "serggio" when he called the practice "obsoleted".

We must understand that you will find a lot of different wiring around the world, and it was in fact legal to wire TN-C in wall outlets in some European countries long time ago.

This is why I said in my first post that we must know what is present in our home or lab. You can not rely on what other people will have.
IanMacdonald:
"Just because a dangerous practice was acceptable in the 1980's did not make it safe back then, nor acceptable now - perception and acceptance of risk has changed in the intervening 30 years (for the better IMO)."

I would disagree. In those days a lot of equipment was live chassis, and bench engineers took their own safety very seriously.  More so than today, when people have gotten complacent thanks to much safer gear. In those days it was projectors with 35kV on the anodes, or Marshall guitar amps with a 1kV supply that could deliver 250mA. Either could kill you stone dead if you weren't careful enough.

The insistence of manufacturers on earthing 'scope inputs arises, I suspect, from a misunderstanding of the purpose of a protective  earth conductor, which is to form a Faraday cage around and enclosing live connections. However, if the live connections are not inside the Faraday cage, then no protection is given. In that case, an earthed object which is liable to be touched by the operator is actually a safety hazard in its own right, and the interests of safety would be best served by eliminating that object from the testbench. 

Building site practices have clearly identified that hand tools with insulating cases and NO earth have a much better safety record than earthed metal tools. If the case has to be metal then earthing is the lesser of two evils compared to a floating case, but the least hazard is achieved by eliminating the touchable metal. 

I mean, we have DMMs with fully isolated data ports. That is to prevent hazardous situations arising though the test prods being earthed via the data cable.

I daresay that isolated scope inputs might be a little harder to design since they handle much higher frequencies than meter prods, but it would be a really big safety advantage if they were isolated from earth, and from each other.  Although, with modern opto-isolators and PSU modules I really don't think it would be that hard to provide post-preamp earth isolation.

-How many lives does it have to save, for the design effort to be worth it?

"Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of 'low voltage'. "

I recall that device, and thinking what an incredibly dangerous gadget it was. It gave the impression that the 'scope was isolated, but had the operator held a probe outer and touched a live terminal, once 50V appeared on the scope it would then have proceeded to complete the circuit and electrocute him.  :palm:

Might add that the practice of earthing 'scope inputs dates from the valve era when 'scopes themselves contained some truly eye-watering voltages. Even the preamps would have run from maybe 150v, so not earthing the input on that kind of beast would have invited having an internal fault put a dangerous voltage onto the probe. That is long-gone history though. The modern LCD scope is basically no different from a bench DMM as regards operator safety.  It would be better if the same safety principles were applied.
serggio:

--- Quote from: exe on June 27, 2017, 08:11:05 pm ---
--- Quote from: Vtile on June 26, 2017, 09:31:12 pm ---PPS. The water circulating radiator under your workbench where you rest your feets seeking a warmth is also propably a directly grounded.. What a nice electrocution chair there.  ;)

--- End quote ---

I got a shock every time I simultaneously touch the radiator and my laptop (that has metal enclosure). Why is it so?

--- End quote ---
Equilibrium potential. It's correct to say - no equilibrium potential. Your radiator probably have different potential from you laptop case. Your laptop have no ground connector at AC/DC power converter, but your laptop power converter can have capacitive coupling to AC (to phase), same time your radiator can be grounded thru the building construction. 
Same time your body can have resistance less that 100 kOhms (it's completely depended from many factors: external humidity, your own skin conductance and etc, etc..), so you feeling electrical shock.

That is, second thumb rule, why you should never have ground contact to neutral contact connection in wall socket at wet room (bathroom e.g.) if you have TN-C protective scheme.
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