Searching \ for 'floating scopes' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/index.htm?key=floating+scopes
Search entire site for: 'floating scopes'.

Truncated match.
PICList Thread
'floating scopes'
1996\11\01@173258 by Reginald Neale

flavicon
face
>I was working on my old color TV that was a transformer less wonder. I
>forgot to float my scope and when I hooked up the probe ground... POW!
>Vaporized to lead.
>
>- -Mark "what ever floats your 'scope" Jurras
>
>>>> Martin McCormick <spam_OUTmartinTakeThisOuTspamDC.CIS.OKSTATE.EDU> 1 November 1996  7:17 am
>>>>
>
>        If some of you who are new to electronics ever run across one of
>these sets, procede with absolute caution.  They were VERY dangerous with
>the chassis being a floating ground and a 50% probability that mains voltage
>might exist between it and Earth.

Even worse is remembering to float your scope - and then forgetting that it
floats. Touching something that's grounded and simultaneously touching a
metal part of the scope, you get a very unpleasant and dangerous reminder.



.....................Reg Neale.....................
Complete text of the winning entry in a recent good-government essay contest:
"Good Government.  Gooooood Government.   Sit.    Stay."

1996\11\02@154405 by nigelg

flavicon
picon face
In message  <v02130500ae9fe99f571c@[204.181.3.8]> .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU writes:

> Even worse is remembering to float your scope - and then forgetting that it
> floats. Touching something that's grounded and simultaneously touching a
> metal part of the scope, you get a very unpleasant and dangerous reminder.

That's not worse, you only get a slight 'tingle', it's due to the RF bypass
capacitors connected between live and earth, and neutral and earth. This
forms an AC potential divider, and the case floats at half mains, but only
with a very small current capability.

If the chassis is connected directly to live (quite possible on old TV's), and
you hold chassis and an earthed scope, you have full mains directly from hand
to hand - a path straight across your heart :-(.

Nigel.

         /----------------------------------------------------------\
         | Nigel Goodwin   | Internet : nigelgspamKILLspamlpilsley.demon.co.uk |
         | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk    |
         | Chesterfield    |                                        |
         | England         |                                        |
         \----------------------------------------------------------/

1996\11\02@202511 by Eric Smith

flavicon
face
> If the chassis is connected directly to live (quite possible on old TV's),
> and you hold chassis and an earthed scope, you have full mains directly from
> hand to hand - a path straight across your heart :-(.

Yes.  And "floating a scope" (i.e., removing its ground lead) is not good
practice.  As soon as you try to use some other instrument like a DVM or
signal generator and forget that it too is grounded you'll have the same
problem as with a grounded scope.

The correct solution is to put the TV on an isolation transformer.  This
should always be done when working on anything that might have a hot chassis.

Eric

1996\11\03@013155 by Ray Gardiner

flavicon
face
>> If the chassis is connected directly to live (quite possible on old TV's),
>> and you hold chassis and an earthed scope, you have full mains directly from
>> hand to hand - a path straight across your heart :-(.
>
>Yes.  And "floating a scope" (i.e., removing its ground lead) is not good
>practice.  As soon as you try to use some other instrument like a DVM or
>signal generator and forget that it too is grounded you'll have the same
>problem as with a grounded scope.
>
>The correct solution is to put the TV on an isolation transformer.  This
>should always be done when working on anything that might have a hot chassis.
>

There are lots of instances where connecting CRO ground can cause problems
because the other equipment can't easily be isolated. eg telecom lines etc.

Best solution is use a battery powered CRO if available.
And use an isolating transformer for safety, plus earth leakage detector.

By the way is use of RCD trips mandatory in the US? It is in Australia.
The RCD compares the current flowing in active and neutral and if their
is any imbalance (caused by earth leakage for example) the device trips
within a few ms.


Ray Gardiner, Shepparton, Victoria 3630,  Australia,   .....rayKILLspamspam.....netspace.net.au

1996\11\03@023324 by Bob Blick

picon face
>By the way is use of RCD trips mandatory in the US? It is in Australia.
>The RCD compares the current flowing in active and neutral and if their
>is any imbalance (caused by earth leakage for example) the device trips
>within a few ms.

They are called GFI's here(all these TLA's(three letter acronyms) to keep
track of, I need a pop-up TSR with a glossary of them<g>)

GFI stands for ground fault interrupter.

1996\11\04@130656 by Martin McCormick

flavicon
face
In message <EraseME19961103021200.8923.qmailspam_OUTspamTakeThisOuTbrouhaha.com>, Eric Smith writes:
>As soon as you try to use some other instrument like a DVM or
>signal generator and forget that it too is grounded you'll have the same
>problem as with a grounded scope.

       That is an excellent point.  One-to-one isolation transformers aren't
all that expensive, especially when lives and property are involved.  Another
good thing to have at the ready is a variac.  For those who may not be
familiar with this term, Variacs are just auto transformers in which the
turns ratio can be changed by turning a wiper.  This gives one an output
voltage that varies from almost 0 to slightly above mains voltage.  Since
it is an auto transformer, the primary and secondary share a common connection
so you usually plug the Variac in to your isolation transformer and then
plug the device being tested or repaired in to the Variac.  If the
test device is suspect, one can start by slowly turning up the voltage until
something either blows or goes.  It is a gentler way to find shorted bridges
and or switching transistors before they do even more damage than they have
already done.

       I apologize to those who already know about how to use these tools,
but if one person can stay on the list and off the marble slab because of
this information, then I have done my duty.  Please spend the money and do
it right.
Martin McCormick

1996\11\04@151258 by Martin McCormick

flavicon
face
In message <v01540b00aea098bab2d8@[203.17.148.131]>, Ray Gardiner writes:
>And use an isolating transformer for safety, plus earth leakage detector.
>
>By the way is use of RCD trips mandatory in the US? It is in Australia.
>The RCD compares the current flowing in active and neutral and if their
>is any imbalance (caused by earth leakage for example) the device trips
>within a few ms.

       They are required where one has a good chance of becoming part of a
circuit such as in a bath or out of doors.  They are also needed on branch
circuits in older houses where the branch has no Earth conductor.

       In the United States, those devices are called GFI's or Ground Fault
Interrupters, but they are exactly the same type of device you describe.

Martin McCormick

1996\11\04@193640 by peter

flavicon
face
Martin McCormick wrote:
{Quote hidden}

Most people will have their own preferences for their own reasons.
Me I float all my test gear but connect all the chassis together
but not to earth. On the scope I use the earth socket on the
front so I can independantly float it when required
I have floated my scope since my first job where all the scopes
were floated

I fail to see the point of isolation transformers on test gear
unless the filter caps have excessive leakage or its open
'cause your fixing it or you're suffering from
incontinence and sitting on it.

In general, mains test equipment would already isolate the supply
soon after entering the case, why do it twice?
To Earth or not was the question
An isolation transformer for the equipment you're working on is
another matter ( not my choice )
Its still the same volts even if its isolated.
Mains voltage will always be dangerous and must be treated with care

Personally it's direct off line sw mode power supplies that demand
my greatest respect. I have a 220v to 12-32v at 0-100Amps. Inside
there are two 2,200mF in parallel, that's 4,400mF charged to 320v.
I take a deep breath before I open the cover

For 27 years I have floated most of my test gear.
The only time I did not was when I bought one that already had
the plug fitted and I forgot to check until one day I connected
it to my SONY T.V. I found out with a bang that it had a live chassis


--
Peter Cousens
email: @spam@peterKILLspamspamcousens.her.forthnet.gr
snailmail: Peter Cousens, karteros, Heraklion, Crete, 75100, Greece,

1996\11\04@205957 by Dwayne Reid

flavicon
face
Peter Cousens wrote . . .

>Most people will have their own preferences for their own reasons.
>Me I float all my test gear but connect all the chassis together
>but not to earth. On the scope I use the earth socket on the
>front so I can independantly float it when required
>I have floated my scope since my first job where all the scopes
>were floated

Most techs that I know float their test equipment.  I also do so, but with
the Earth lead connected to ground through a largesh bridge rectifier wired
up as a 1 to 1.2 Vdc shunt.  That is... take a 100 PIV 35 Amp bridge, short
the + & - terminals together, connect the Earth lead of the scope to one of
the AC terminals, connect the other AC terminal to Earth Ground.  So long as
the scope chassis remains within about 1 Vac p-p of ground, the bridge does
not conduct and ground loop currents don't mess up your low level
measurements.  Sometimes it is necessary to truly lift the ground lead, but
I find those situations rare.

>
>I fail to see the point of isolation transformers on test gear
> unless the filter caps have excessive leakage or its open
>'cause your fixing it or you're suffering from
>incontinence and sitting on it.
>
What they mean is to use an isolation transformer to power the system under
test thru the iso transformer, NOT the test equipment.  The whole idea is
that anything connected directly to the power line (old radio and TV
equipment, off line switch mode power supplies, etc) is then isolated by the
transformer so that safer measurements can be made.

Dwayne

1996\11\05@152455 by Matthew Mucker
flavicon
face
>
>By the way is use of RCD trips mandatory in the US? It is in Australia.
>The RCD compares the current flowing in active and neutral and if their
>is any imbalance (caused by earth leakage for example) the device trips
>within a few ms.
>

This sounds like a GFCI circuit breaker-- ground fault circuit interrupt.
It is mandatory for outdoor electrical outlets and electrical outlets near
sinks, in the bathroom, etc...

We did an outdoor concert once at college, and all of the breakers were
GFCI.  With all of the audio equipment needing a good ground, it seemes
that every time we plugged in a new device, it tripped half the breakers.
I thought it was rather amusing, but the crowd of 3000 on the lawn wasn't
so thrilled.



 "DOS Computers manufactured by companies such as IBM, Compaq, Tandy, and
millions of others are by far the most popular, with about 70 million
machines in use wordwide. Macintosh fans, on the other hand, may note that
cockroaches are far more numerous than humans, and that numbers alone do
not denote a higher life form."

1996\11\05@165229 by Gerhard Fiedler

flavicon
face
At 17:58 04/11/96 -0800, peter cousens wrote:
>I fail to see the point of isolation transformers on test gear
> unless the filter caps have excessive leakage or its open
>'cause your fixing it or you're suffering from
>incontinence and sitting on it.
>
>In general, mains test equipment would already isolate the supply
>soon after entering the case, why do it twice?

Maybe getting both satisfied would be to cut earth after the filter caps
(i.e. let earth gound go only to the filter caps) - you get your equipment
floating, with a "real" isolation transformer (the built in).

More... (looser matching)
- Last day of these posts
- In 1996 , 1997 only
- Today
- New search...