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From: Dave Plowman (News) on 31 Dec 2009 10:17
In article
<0b678d42-1c0e-4a0e-97a6-4dac22decfac(a)s3g2000yqs.googlegroups.com>,
sparky <sparky12x(a)yahoo.com> wrote:

> >
> > You seem to lack basic understanding of safe working practices.
> >
> Have you ever worked on a piece of equipment ?

Of course.

> Certainly not effectively !

Really?

> Certainly not safely !

Perhaps you could give the reasons why things *must* be grounded under all
circumstances. If you actually understand the dangers of this, of course.

--
*If at first you do succeed, try not to look too astonished.

Dave Plowman dave(a)davenoise.co.uk London SW
To e-mail, change noise into sound.
From: mike on 31 Dec 2009 18:38
Dave Plowman (News) wrote:
> In article
> <0b678d42-1c0e-4a0e-97a6-4dac22decfac(a)s3g2000yqs.googlegroups.com>,
> sparky <sparky12x(a)yahoo.com> wrote:
>
>>> You seem to lack basic understanding of safe working practices.
>>>
>> Have you ever worked on a piece of equipment ?
>
> Of course.
>
>> Certainly not effectively !
>
> Really?
>
>> Certainly not safely !
>
> Perhaps you could give the reasons why things *must* be grounded under all
> circumstances. If you actually understand the dangers of this, of course.
>
"Ground" is a concept.
It's convention; that is we agree on the definition.
(I know, agreement is a foreign concept to you...just go with it.)
We drive a metal stake in the earth and call it ground.
If you don't put any current in the stake, you can use it
as a reference for all measurements. If you need a differential
measurement, just measure the two relative to "ground" and subtract.
Works just fine in concept.

As a practical matter, it takes much less "technology" to make a local
single-ended measurement relative to a "common" point. That
common point is often pretty close to "ground"...doesn't have to be,
it's just easier to work with and easier to make safe.

This paradigm creates systems that are "safe" in most cases, when they're
working properly and are still safe under the most common fault conditions
as long as you leave the covers on the system. When you open the box,
many safety assumptions don't apply.


There are exceptions, but when you use an isolation transformer to
"disconnect" the local common from the conceptual "ground", you don't
necessarily make a properly functioning system any less "safe" under
normal operation.

An Isolation Transformer is not inherently unsafe.
But an isolation transformer does NOT make a faulty system SAFE to poke
around inside.

What IS unsafe is the stupid things people do with their hands and test
equipment
believing that the transformer absolves them from any responsibility to
THINK about
what they're doing. Redefining your own
common reference by attaching an arbitrary node inside the faulty supply to
your local concept of "ground" is decidedly UnSAFE.

While I'm on the subject of RESPONSIBILITY...
People ask questions because the don't know the answer. In many cases,
they can't
even tell if the advice they're getting is good or bad. One way is to
vote.
If two, or three or four people said it, it must be true. Problem is
that the most vocal newsgroup
denizens are demonstrating the least ability to think about and
understand the consequences of their
advice. And there are WAY more than two people giving bad advice here.

One way to judge advice is the tone of the thread. Name-calling is what
you do when you
don't have a logical leg to stand on. A spirited, yet civil, debate
often leads to consensus.
That advice is more likely to be helpful.
As soon as the name-calling starts, you can't trust any advice from the
thread. Some of the input may be helpful, but you can't tell which,
or you wouldn't have had to ask the question in the first place.

We have the responsibility to do no harm, and argue logically against
unsafe advice.

Doing stupid, unsafe things may work 99% of the time. As long as that 100th
time is YOU, I don't have a problem with the odds...I'd buy a ticket to
watch.
If that 100th time is some
innocent guy who took your bad advice, I have a BIG problem.
From: Dave Plowman (News) on 31 Dec 2009 20:42
In article <hhjcm2$57a$1(a)news.eternal-september.org>,
mike <spamme0(a)go.com> wrote:
> > Perhaps you could give the reasons why things *must* be grounded under
> > all circumstances. If you actually understand the dangers of this, of
> > course.
> >
> "Ground" is a concept.
> It's convention; that is we agree on the definition.
> (I know, agreement is a foreign concept to you...just go with it.)

It certainly is since in the UK we call a mains ground earth.

> We drive a metal stake in the earth and call it ground.
> If you don't put any current in the stake, you can use it
> as a reference for all measurements. If you need a differential
> measurement, just measure the two relative to "ground" and subtract.
> Works just fine in concept.

Really? You seem to have strange ideas about what is a reverence.

> As a practical matter, it takes much less "technology" to make a local
> single-ended measurement relative to a "common" point. That
> common point is often pretty close to "ground"...doesn't have to be,
> it's just easier to work with and easier to make safe.

So qualify just how you do this in the workshop.

> This paradigm creates systems that are "safe" in most cases, when
> they're working properly and are still safe under the most common fault
> conditions as long as you leave the covers on the system. When you open
> the box, many safety assumptions don't apply.

There are certain rules that apply.

> There are exceptions, but when you use an isolation transformer to
> "disconnect" the local common from the conceptual "ground", you don't
> necessarily make a properly functioning system any less "safe" under
> normal operation.

Sigh. I'm utterly amazed you can't see how having both legs of a dangerous
voltage floating is safer than having one side of it grounded.

> An Isolation Transformer is not inherently unsafe.
> But an isolation transformer does NOT make a faulty system SAFE to poke
> around inside.

Who said it did? Nothing can make dangerous voltages safe for idiots.

> What IS unsafe is the stupid things people do with their hands and test
> equipment believing that the transformer absolves them from any
> responsibility to THINK about what they're doing. Redefining your own
> common reference by attaching an arbitrary node inside the faulty supply
> to your local concept of "ground" is decidedly UnSAFE.

Darwin's rule applies, then. Anyone who doesn't understand potential
difference is best suited to having knitting as a hobby.

--
*The hardness of the butter is proportional to the softness of the bread *

Dave Plowman dave(a)davenoise.co.uk London SW
To e-mail, change noise into sound.
From: Leonard Caillouet on 1 Jan 2010 09:32
Please see page 2.

http://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdf
From: William Sommerwerck on 1 Jan 2010 08:44
Thank you for posting this. It answered several questions I've had for a
long time.

Read this document carefully, as it is something you will rarely
encounter -- a technical document written by someone who actually knows how
to write!

Will wonders never cease?


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