isolation transformer needed

[Phil Allison]

"David Nebenzahl"


** Scumbags like YOU need a bullet in the head.




..... Phil

 

[mike]

Of course.


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.

 

[Phil Allison]

"mike"

Normally, I'd not waste my time trying to train
internet denizens.

** Or teaching pigs to sing.

The primary reason to want an isolation transformer to
troubleshoot a power supply is to work on the primary side.
Now, the KEY word is TROUBLESHOOT. What that means is that
the power supply has a FAULT in the primary circuit.
What's the fault? You don't know, or you'd just fix it.
How safe is it to troubleshoot that particular fault?
You don't know, 'cause you don't know what it is.

You're gonna RISK ELECTROCUTION based on the misguided
assumption that an isolation transformer keeps you safe.
YOU'RE NOT SAFE. YOU DON'T HAVE ANY IDEA WHAT THE PRIMARY
CIRCUIT IS, BECAUSE IT'S NOT AS DESIGNED. IT HAS A FAULT!!!!!!!
The node that the designer called common may not be common
at all. IT HAS A FAULT!!!! You should not arbitrarily ground
ANY node. It has a fault!!! (I'm skipping over the obvious
question, "what is ground anyway?")

An isolation transformer is not inherently bad. It can
provide a layer of protection. What is bad is the FALSE
sense of security
that the transformer makes it safe to poke around inside
a supply WITH A PRIMARY FAULT.

** Standard procedure for servicing any unit with an unknown fault is to
FIRST see what happens when plugged into a normal AC outlet - ie one with
neutral and ground linked at the service box.

That outlet MUST have an RCD fitted as a standard safety precaution for
ANYONE working on mains powered equipment while it is energised.

The RCD will trip if the unit has a fault that causes even 10mA of leakage
from the AC supply to ground - which includes a short from internal
neutral conductors to safety ground.

If the unit is of class 2 ( double insulated ) construction, the external
metalwork should be linked to safety ground in order to test the integrity
of that insulation and render the unit safe to handle.

Obviously, if the RCD trips under these initial tests the cause MUST be
tracked down and dealt with before going any further.

The ONLY time a service tech needs to use a mains isolation transformer is
when the unit has a "live chassis" as some TV sets do OR it contains an
off-line SMPS that needs detailed analysis with a scope in order to effect
repairs.

The tech must be VERY aware that the electric shock protection afforded by
the RCD is LOST when the isolation tranny is in use and so use it as
sparingly as possible. It a damn good idea to have the isolation tranny
VERY visible on the bench to constantly remind the tech it is in use.

When work is completed, the unit should be again connected to a normal, RCD
protected, outlet and checked.

If the unit it uses class 2 insulation, the AC leakage current to safety
ground should be measured with a suitable load and meter. The leakage
current should not exceed 0.5 to 1 mA, depending on the item and the actual
AC voltage in use.

There are a host of other checks and inspections a service tech needs to do
on and around the AC wiring, fusing devices and any AC power leads and plugs
attached to a unit. Any parts that are damaged, worn out or otherwise seem
unsafe must be repaired or replaced.

I go through a LOT of AC plugs, fuses and fuse holders in my work.



...... Phil

 

[David]

No it hasn't. Isolation transformers are still used for many safety
related reasons. But don't expect you to understand why. Obviously.

I am one of those who rarely agree with Phil, but this time he is giving
you the right info.

From your post Dave it does seem you do not understand the reasons
behind the use of isolation xformers when working on or testing gear.

No matter how much you appear to dislike Phil, there are times when it
pays to bow to other's areas of expertise - and while Phil is hopeless
with people skills, he does know electronics/electrical equipment

David

 

[William Sommerwerck]

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?

 

[Leonard Caillouet]

Please see page 2.

http://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdf

 

[Phil Allison]

"Meat Head Moron "

"Phil Allison"



Funny that was never on any service literature or diagnostic procedure
documentation I ever read. Nor would I have ever made it my own
practice as a first step thinking back 30 through 30 years.

** My post does NOT say that it is the very first step.

It does say that with an unknown but possible PSU fault - connection to a
normal RCD protected AC outlet should be done FIRST - before using an
isolation tranny.

Obviously, visual inspection comes very first, including carefully checking
of the AC plug, lead and fuse if accessible. If the AC fuse is missing or
blown, then a major fault is likely.

In the latter case, MY procedure is to install a suitable new AC fuse and
gradually bring up the item using a variac - all the time monitoring the
current draw from the AC supply with meter specially built for that purpose.
If the AC current becomes unusually large ( or the RCD trips) as the AC
voltage rises - game over.



..... Phil

 

[Phil Allison]

"sparky"

Some total MORON

However, I can't think of any case where the use of an isolation
transformer *increases* any hazard.

Quite correct.

** It is totally WRONG !!


The isolation transformer is a great tool for the workbench if a
person knows how to utilize it.

** Counts a clueless imbecile you out.



..... Phil

 

[David]

Seems to me some expect the use of an isolation transformer to be some
form of magic bullet that removes all risks. It doesn't. But can reduce
the sort of risk caused by one side of a mains supply being grounded. If
you then ground one leg of the output of that transformer by any means you
are back to square one.
However, I can't think of any case where the use of an isolation
transformer *increases* any hazard.

Well, I sure can. When they listen to someones advice that it is OK to
touch either side of the output of the xformer. As Phil pointed out if
there is a problem that could be lethal

David

 

[David Nebenzahl]

Well, I sure can. When they listen to someones advice that it is OK to
touch either side of the output of the xformer. As Phil pointed out if
there is a problem that could be lethal

That is in no way a fault of the isolation transformer. Might as well
blame power lines because some doofus, somewhere told someone that it's
OK to touch one side of the line.

 

[Phil Allison]

"David"

" Some pommy retard "

Well, I sure can. When they listen to someones advice that it is OK to
touch either side of the output of the xformer. As Phil pointed out if
there is a problem that could be lethal

** Aside from the above issue - the number one reason for NOT using an
isolation tranny as the usual supply of AC power on a workbench is that the
very same tech who has become oblivious to the danger of contacting live AC
wiring WILL at some future time and place find themselves working on
energised items WITHOUT the benefit of such isolation.

The DANGER is in the false impression of harmlessness that builds up in the
mind of the tech.

Old habits die hard - but electricity kills quick.



..... Phil

 

[Phil Allison]

"David Nebenzahl"
David spake thus:

That is in no way a fault of the isolation transformer.

** My god - you are one, fucking retarded MORON.



..... Phil

 

[Phil Allison]

"David"

" Some pommy retard "

Well, I sure can. When they listen to someones advice that it is OK to
touch either side of the output of the xformer. As Phil pointed out if
there is a problem that could be lethal

** Aside from the above issue - the number one reason for NOT using an
isolation tranny as the usual supply of AC power on a workbench is that the
very same tech who has become oblivious to the danger of contacting live AC
wiring WILL at some future time and place find themselves working on
energised items WITHOUT the benefit of such isolation.

The DANGER is in the false impression of harmlessness that builds up in the
mind of the tech.

Old habits die hard - but electricity kills quick.



..... Phil

 

[John Robertson]

Seems to me some expect the use of an isolation transformer to be some
form of magic bullet that removes all risks. It doesn't. But can reduce
the sort of risk caused by one side of a mains supply being grounded. If
you then ground one leg of the output of that transformer by any means you
are back to square one.
However, I can't think of any case where the use of an isolation
transformer *increases* any hazard.

How about the technician who assumes that the isolation transformer has
decreased the risk of shock and behaves accordingly...without verifying
mains isolation before reaching inside live equipment?

Assumptions can be quite shocking!

John :-#)#

--
(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

[Phil Allison]

"Meathead Plonker"

Becomeing complacent in or unconcerned with electrical hazards is a
fact of life (or death.)

** Absolute BOLLOCKS it is.



..... Phil

 

[Phil Allison]

"sparky is a MORON "


** **** off

- you brainless PARROTING fuckwit !!



..... Phil

 

[GregS]

Please see page 2.

http://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdf

I swear I have seen documented procedures lifting the ground of an oscilloscope.
I though it was Tektronix, but I don't remember the details.

I like using battery scopes and also differential probes.

greg

 

[GregS]

Funny that was never on any service literature or diagnostic procedure
documentation I ever read. Nor would I have ever made it my own
practice as a first step thinking back 30 through 30 years.

But then again this is an internationally mirrored forum so I don't
really try to pawn off my particular service procedures as the gospel
where the diagnostics flow chart may be much different in certain
regions of the globe.

I have hooked things up with isolation and later found things like a motor short.
Works fine with the transformer.

I didn't always first check the customers explanation and plug something
in right away before doing anything, but I do it now.


greg

 

[GregS]

I have been thinking about it. The MAIN purpose of an isolation transformer
is to make it non isolated by making a NEW neutral close to the device
being powered. The main purpose of an isolation transformer is noise control.

Now we have us tecks who all our lives have learned isolation and that stuff.
I taliked to the people at Tripplite and verified ALL their isolation
transformers ARE grounded. Read the paragraph, and yet it says above, "Complete
Isolation"http://www.tripplite.com/en/products/model.cfm?txtSeriesID=325&EID=1
3...
Also look throughhttp://www.tripplite.com/en/keyword-search.cfm?q=isolation%20transforme
r
The isolator you buy from MCM is very unlikely to have the secondary grounded, but
I really can't verify that.

In reading PC Power Protection by Mark Waller, he
pointed out their is a NEC requirment for this grounding.
I have not found an exact description in the code. There is about 75 references
in that book, but no index marks. Man, I should sell that book !!!! Big Bucks.

greg

On an isolation transformer (single phase), the primary side is always
grounded because the neutral leg is always bonded in the distribution
panel. This is a code requirement. If the secondary is also grounded
(I am pretty sure this is also a requirement), the two sides are still
electrically isolated. The current leaving the secondary hot leg must
return to the neutral (grounded) leg of the secondary. There is no
path from the secondary hot to the primary neutral. The secondary has
to leave the hot leg and return on the secondary neutral leg. The
secondary current cannot go any where except through secondary loop.
Any noise on the high side (from nearby VFDs or other) must pass
through the primary winding in order to induce a current onto the
secondary. The winding itself is a low pass filter, so much of the
noise on the primary is dissipated as heat.[/QUOTE]

I would agree with this except you also state then, there is no path from
secondary hot to primary neutral, which makes no sense in your
explanation. In electronic servicing you don't want to ground the
secondary making a new neutral. You got to get away from the code to do it.

greg

 

[Phil Allison]

"GregS"
Meathead Plonker
"Phil Allison"

I have hooked things up with isolation and later found things like a motor
short.
Works fine with the transformer.

I didn't always first check the customers explanation and plug something
in right away before doing anything, but I do it now.

** Its important for a tech to see any symptoms the customer is complaining
about, first hand.

You gotta plug it in to do that.



..... Phil