Isolation xformer question (again)

[Ivan Vegvary]

Purchased a used iso xformer. Trippe Model IS800. 6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together. Is this typical? Is my new scope protected? I thought the whole idea of isolation is to get away from earth ground. I can easily disconnect the out put ground from the input ground, but then what? Just let it float.
Please educate me.
Thank you,
Ivan Vegvary
BTW, I have 14 days to return if unsatisfied.

 

[Phil Allison]

Ivan Vegvary wrote:

Purchased a used iso xformer. Trippe Model IS800.
6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together.
Is this typical?
Is my new scope protected?

** You have been warned over and again NOT to use an isolation transformer with a scope. It is stupidly dangerous !!!

I thought the whole idea of isolation is to get away from earth
ground.


** Not true - one isolates the PSU part of a device from the AC supply so any part of it can be grounded to allow measurements with a GROUNDED scope that are otherwise impossible.

Please educate me.

** Kindly educate yourself.

The tranny you have bought is fine just AS IT IS for use with a UN-MODIFIED scope.



..... Phil

 

[Ivan Vegvary]

On Monday, February 22, 2016 at 9:21:31 PM UTC-8, Phil Allison wrote:

Ivan Vegvary wrote:

Purchased a used iso xformer. Trippe Model IS800.
6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together.
Is this typical?
Is my new scope protected?



** You have been warned over and again NOT to use an isolation transformer with a scope. It is stupidly dangerous !!!

I thought the whole idea of isolation is to get away from earth
ground.


** Not true - one isolates the PSU part of a device from the AC supply so any part of it can be grounded to allow measurements with a GROUNDED scope that are otherwise impossible.

Please educate me.


** Kindly educate yourself.

The tranny you have bought is fine just AS IT IS for use with a UN-MODIFIED scope.



.... Phil

Phil, I have been warned not to plug a scope into an iso xformer. I'm not. Circuit under test will be plugged into iso. Scope plugged into wall with proper ground.
The education I need is clarification on the common ground. I thought the whole idea of the iso was to keep the two grounds seperate. The currents are seperate but the grounds are the same?

 

[Shaun]

"Ivan Vegvary" wrote in message
news:ec87ede6-4c80-4da2-8313-259d8a04a73e@googlegroups.com...

Purchased a used iso xformer. Trippe Model IS800. 6.8 amps out, 4 outlets,
on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together.
Is this typical? Is my new scope protected? I thought the whole idea of
isolation is to get away from earth ground. I can easily disconnect the out
put ground from the input ground, but then what? Just let it float.
Please educate me.
Thank you,
Ivan Vegvary
BTW, I have 14 days to return if unsatisfied.

That is normal. The inner turn of the secondary of the isolation
transformer is usually connect to chassis ground too. The re-establishes
the neutral to ground bond that on the primary (input) side could have been
lost or has gone Ohmic.

Shaun

 

[Phil Allison]

Helmut Wabnig wrote:

In an industrial environment it is often necessary
to run the oscilloscope without protective grounding.

** No it is not.

If available use an isolation transformer for the scope.
Disconnect or isolate the ground pin on the mains plug
of the oscilloscope.

** Stupid and fucking dangerous advice.

You cannot run a iron melting blast furnace from an isolation
transformer

** You cannot run an utter idiot like you from one either.

Today this is easily done with battery operated scopes,

** Which are NOTHING like regular scopes.

Piss off you ridiculous troll.



.... Phil

 

[Cursitor Doom]

On Tue, 23 Feb 2016 10:13:32 +0100, Helmut Wabnig wrote:

In an industrial environment it is often necessary to run the
oscilloscope without protective grounding.
If available use an isolation transformer for the scope. Disconnect or
isolate the ground pin on the mains plug of the oscilloscope.

This is really bad advice to give to a novice, Helmut. This practice does
go on - or did go on in the old days - but it's heavily deprecated and
should never be done if it can possibly be avoided in which case only by
very experienced techs who know what they're doing.
The isolation transformer should supply the device under test - NOT the
scope. You do NOT want any ground loops between the two.

 

[Helmut Wabnig]

On Mon, 22 Feb 2016 20:48:09 -0800 (PST), Ivan Vegvary
<ivanvegvary@gmail.com> wrote:

Purchased a used iso xformer. Trippe Model IS800. 6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together. Is this typical? Is my new scope protected? I thought the whole idea of isolation is to get away from earth ground. I can easily disconnect the out put ground from the input ground, but then what? Just let it float.
Please educate me.
Thank you,
Ivan Vegvary
BTW, I have 14 days to return if unsatisfied.

In an industrial environment it is often necessary
to run the oscilloscope without protective grounding.
If available use an isolation transformer for the scope.
Disconnect or isolate the ground pin on the mains plug
of the oscilloscope.
You cannot run a iron melting blast furnace from an isolation
transformer
So you have to uncouple the oscilloscope ground from supply net.

Today this is easily done with battery operated scopes,
but they were not always available at my times.

w..

 

[George Herold]

On Tuesday, February 23, 2016 at 1:28:21 AM UTC-5, Ivan Vegvary wrote:

On Monday, February 22, 2016 at 9:21:31 PM UTC-8, Phil Allison wrote:
Ivan Vegvary wrote:

Purchased a used iso xformer. Trippe Model IS800.
6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together.
Is this typical?
Is my new scope protected?



** You have been warned over and again NOT to use an isolation transformer with a scope. It is stupidly dangerous !!!

I thought the whole idea of isolation is to get away from earth
ground.


** Not true - one isolates the PSU part of a device from the AC supply so any part of it can be grounded to allow measurements with a GROUNDED scope that are otherwise impossible.

Please educate me.


** Kindly educate yourself.

The tranny you have bought is fine just AS IT IS for use with a UN-MODIFIED scope.



.... Phil

Phil, I have been warned not to plug a scope into an iso xformer. I'm not. Circuit under test will be plugged into iso. Scope plugged into wall with proper ground.
The education I need is clarification on the common ground. I thought the whole idea of the iso was to keep the two grounds seperate. The currents are seperate but the grounds are the same?

Ivan, If I don't get this right I figure Phil will yell at me... so here goes.
The idea is that your circuit under test (CUT) will get it's "common ground"
when you attach the ground lead from your scope probe to the CUT.

George H.

 

>"The education I need is clarification on the common ground. I >thought the whole idea of the iso was to keep the two grounds >seperate. The currents are seperate but the grounds are the same? "

Ground and neutral are two different things. Ground is not supposed to be part of the circuit, it just makes sure the exposed metal parts do not become a shock hazard because of a short.

This was not so much an issue until switched mode power supplies came into vogue. When you just had the mains go into the transformer and rectifiers off the secondaries there was rarely cause to use a scope on the hot sides. Now with the modern power supplies the need can come up quite a bit to use a scope on the hot side. And what they're saying is do not float the scope, float what you are fixing. It is better for the scope and safer. Well less unsafe.

I have floated scopes, but I don't like doing it. I think Tektronix said it would void their (used to be) lifetime warranty on the scope transformer. How they would know is another subject.

And just using a scope is not the only reason for an isolation transformer. In the CRT TV sets some of the chassis were hot. Usually they only had an antenna input and that was isolated. But because of no power transformer at all, every piece of metal inside the thing was a shock hazard. In fact I had a Phillips built set, not on isolation because we only had one in the shop, and I was not working on the power supply, where the chassis was flipped up to work on, and the chopper transistor heatsink hit the aquadag of the CRT and it started arcing. The cold side was grounded by the cable connection. Like WHOA ! Concentrated heat on the bell of a 25" CRT ? No thanks.

I made them but more isolation transformers. I was more of a prick than Phil. Phil has probably never threatened the bosses with physical harm or burning down the shop. Yup. But in this case it was more refined, and justified because I was working on a cement floor. I told them "If I get shocked I am going to file for disability because I have a phobia to electricity and can no longer work and then OSHA is going to be crawling all over this place".

Anyway, part of the point here is that the transformer doesn't just protect you and the scope, it also protects what you're working on. I've had problems due to that, and I told them. But when it came down to working mainly on RPTVs which was largely done sitting on the cement floor, I had to insist.. It had ALREADY cost them money when I had to fix things that happened due to not being isolated. Ignorant fucks, if they wouldn't have paid me so damn much I would not have worked for them. But on that floor the next thing to happen was going to be ME. (I used to be able to touch my knees to my chest back in them days, I did EVERYTHING on the floor)

 

"In an industrial environment it is often necessary
to run the oscilloscope without protective grounding.
If available use an isolation transformer for the scope.
Disconnect or isolate the ground pin on the mains plug
of the oscilloscope.
You cannot run a iron melting blast furnace from an isolation
transformer "

Probably shouldn't be telling him that. First of all it is not all that common, but it does happen. Like when working on big CNC machines for example. However you didn't go into the requirements for this. According to OSHA you have to get these things like they use at the bank to fold the line but instead of red fabric I think you have to use yellow tape that says "DANGER" on it. They want the operator on insulated floor mats and all that shit, perhaps even special shoes.

A regular electronics tech, especially a novice, is not (or should not) have cause to do any such thing.

 

[Cydrome Leader]

Ivan Vegvary <ivanvegvary@gmail.com> wrote:

Purchased a used iso xformer. Trippe Model IS800. 6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together. Is this typical? Is my new scope protected? I thought the whole idea of isolation is to get away from earth ground. I can easily disconnect the out put ground from the input ground, but then what? Just let it float.
Please educate me.
Thank you,
Ivan Vegvary
BTW, I have 14 days to return if unsatisfied.

Your isolation transformer is correctly constructed and follows the
typical convention for these devices. There is isolation between the input
line hot and output line hot.

If you were to accidentally disconnect the ground lead and neutral on the
output from the bonding point in the chassis you'd then have what you were
expecting, which is 100% isolation of hot, neutral and ground from the
input side.

Whatever you plug into this modified isolation transformer will now
"float".

It can be extremely unsafe, depending on what you're doing. You also need
to trust the isolation ability of the transformer and outlet you're using.
The tripp-lite one claims to have a ground shield, but they're also known
for making really shitty products, so hipot the thing first if you want to
play it safe.

Don't float the scope (plug it into the modified isolator). Scopes have
cables and knobs you touch, and if they're at some wacked out potential
you're going to get shocked. It's easier to keep your hands off the DUT
instead when testing or taking readings.

Cutting off grounds and using isolation transformers isn't a death
sentence, you just need to be aware of how it works and what you're
working with.

There's plenty of hot electrical work that goes on all the time with no
problems.

 

[Cursitor Doom]

On Tue, 23 Feb 2016 09:42:34 -0800, jurb6006 wrote:

Anyway, part of the point here is that the transformer doesn't just
protect you and the scope, it also protects what you're working on.

I don't think we should let the OP think that his IT will protect him
period. It's still possible to electrocute yourself even when an IT is
floating the piece of equipment you're working on. All the IT is doing is
breaking the earth reference you normally get with mains voltage;
preventing you suffering a shock to earth. But if you accidentally bridge
across two points in the circuit you're testing that are at different
potentials, you can *still* get fried. So an old rule I was taught from
day one (decades ago) that has served me well is when probing high
voltage circuits, always keep one hand behind your back so at least you
won't get a shock across the chest (the most vulnerable area fatality-
wise).

 

[Cursitor Doom]

On Tue, 23 Feb 2016 18:02:15 +0000, Cydrome Leader wrote:

There's plenty of hot electrical work that goes on all the time with no
problems.

Yeah, by experienced and qualified people who follow laid down procedures
and have the relevant safety equipment, not some noob seeking advice on
the subject from a bunch of strangers! (no offence to the OP, we were all
noobs once).

 

"
I don't think we should let the OP think that his IT will protect him
period."

Agreed. In no way does it protect you from shocks, it only protects you from shocks to ground or earth. You get your hands on voltage it could be floating up in the sky but you can still die. All it does is to stop a path to earth ground or anything else connected somehow to the mains.

If you want real shock protection put the whole shebang on a GFI (GFCI). It is the only way, and it is not exhaustive. You can STILL get to two circuit points inside the unit that can killya.

There used to be the one hand rule, and in the tube days it REALLY applied. But now, like everything I work on is five or nine volts. I got lazy one time last year and got WHAPPED by a tube amp. God damn. Just like the old days.

In other words, DO NOT get lulled into a false sense of security here. You can still die. A possibly good example is the safe table saw.

They came out with a table saw with an electronic brake and sensor that would stop the blade immediately if it sensed human skin touching it. Now let people get used to that and then with an old table saw they will cut their arm off.

I think if they outlawed airbags in cars and mandated spikes in the dashboard, people would use following distance more than one inch per hundred miles per hour.

 

[Cursitor Doom]

On Tue, 23 Feb 2016 11:07:14 -0800, jurb6006 wrote:

I think if they outlawed airbags in cars and mandated spikes in the
dashboard, people would use following distance more than one inch per
hundred miles per hour.

Absolutely. I remember when crash helmets became compulsory for
motorbikes here in 1973, the number of serious accidents instantly went
through the roof because it gave riders who'd previously been very
cautious in their approach to roadcraft the false sense that suddenly
they were somehow invincible. And of course we see the same effect at
work in countless other areas where governments around the world try to
"improve" safety.

 

[whit3rd]

On Monday, February 22, 2016 at 9:01:41 PM UTC-8, Shaun wrote:

"Ivan Vegvary" wrote in message
news:ec87ede6-4c80-4da2-8313-259d8a04a73e@googlegroups.com...

Purchased a used iso xformer. Trippe Model IS800. 6.8 amps out, 4 outlets,
on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together.
Is this typical?

That is normal.

Yes.

The inner turn of the secondary of the isolation
transformer is usually connect to chassis ground too. The re-establishes
the neutral to ground bond ...

NO! There are some isolation transformers that do that, they are required for
passing outlet-checks of the plug-with-three-lights type, but that is NOT
what you want for occasionally working on a hot chassis. You want to get an
isolation transformer with, at most, a grounded shield around the secondary
windings, but you want those windings NOT bonded to any ground.

This model bonds output-neutral to ground:
<http://www.tripplite.com/isolator-series-120v-500w-ul60601-1-medical-grade-isolation-transformer-4-hospital-grade-outlets~IS500HG/>

This model doesn't:
<https://www.digikey.com/product-detail/en/N-11MG/237-1847-ND/4915261>

If, for some reason, you want to ground an odd part of a device on the bench, you
can do it, while powering from the second type of isolation transformer. Not with the first.

 

[Phil Allison]

George Herold wrote:

The idea is that your circuit under test (CUT) will get it's "common ground"
when you attach the ground lead from your scope probe to the CUT.

** There are two classes of AC powered devices, ones with a safety ground and "double insulated" ones without. In the latter case, the scope probe ground clip provides a temporary ground to the device and may be connected to the chassis or other parts of the circuit isolated from the AC supply by a transformer or SMPS.

Ones with a safety ground usually still need the scope probe ground clip connected to the chassis to eliminate RF noise and hum voltages spoiling the image on the screen.

An isolation transformer is needed with either class when you need to probe circuits on the AC supply side of a SMPS - but it makes no sense to remove the safety ground from devices that have it. So the transformer used should carry the safety ground through to the outlet.



..... Phil

 

[M Philbrook]

In article <9c8ocb93emp61nlf4pt2carg0ku5hntc7o@4ax.com>, hwabnig@.-
says...

On Mon, 22 Feb 2016 20:48:09 -0800 (PST), Ivan Vegvary
ivanvegvary@gmail.com> wrote:

Purchased a used iso xformer. Trippe Model IS800. 6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together. Is this typical? Is my new scope protected? I thought the whole idea of isolation is to get away from earth ground. I can easily disconnect the out put ground from the input ground, but then what? Just let it float.
Please educate me.
Thank you,
Ivan Vegvary
BTW, I have 14 days to return if unsatisfied.

In an industrial environment it is often necessary
to run the oscilloscope without protective grounding.
If available use an isolation transformer for the scope.
Disconnect or isolate the ground pin on the mains plug
of the oscilloscope.
You cannot run a iron melting blast furnace from an isolation
transformer
So you have to uncouple the oscilloscope ground from supply net.

Today this is easily done with battery operated scopes,
but they were not always available at my times.

w..

i find a battery operating scope is much better, even if its on a
UPS unplugged.

Jamie

 

[Phil Allison]

Ivan Vegvary wrote:

Purchased a used iso xformer. Trippe Model IS800.
6.8 amps out, 4 outlets, on/off switch and circuit breaker.

Question: The power IS isolated however all the grounds are tied together.
Is this typical?

** That iso-tranny has been modified - from the factory they have the output neutral linked to ground and are fitted with surge protection devices.

You need to check inside and make sure there are no devices fitted from the secondary winding to ground - like varistors or capacitors.

FYI: never use more than one outlet at a time.


..... Phil

 

[Ivan Vegvary]

Thank you everyone. I now understand.