Galvanic disconnection with transformer

Kari Laine · Mar 26, 2010

Hi,

first. Thank you for all the help I have gotten from here so far.

Now I am trying to figure out how to connect the electricity to my
scopes and the Power supply I am building.

I don't know the right term for a transformer which disconnects galvanic
connection between the electricity network and device. I will just use
term transformer for that.

I have
- bench top HP oscilloscope
- Velleman PCSGU250 PC-scope
- Another Velleman on other machine
- Daqarta sound-card oscilloscope

And then I have the unsuspecting target - my PSU
I want to scope the PSU's primary and secondary windings and later
follow the voltage through it.

On which devices to put the transformer?

Then there is that ground lead in the probe. If there is transformers in
the machine connected with the scope and the device under test, would be
destructive to connect it at all? Because there is no common ground
between the devices? I really need to get 1000 page book about
oscilloscopes - ISBN anyone? I am learning this alone, so there is no
one to ask except here....

Those transformers are pretty expensive, so my first thought to put them
everywhere is not good one. I would be very thankful if someone(s) could
tell little bit about the basics about this, with instructions what to
do. And naturally what not to do.

Best Regards
Kari

--
PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
KEYPADS - ACCESSORIES
http://www.byvac.com (I am just a satisfied customer)

pimpom · Mar 26, 2010

Kari Laine wrote:

Hi,

first. Thank you for all the help I have gotten from here so
far.

Now I am trying to figure out how to connect the electricity to
my
scopes and the Power supply I am building.

I don't know the right term for a transformer which disconnects
galvanic connection between the electricity network and device.
I
will just use term transformer for that.

It's called an "isolation transformer" or "isolating
transformer". It isolates anything connected to the secondary
from the public electricity supply lines. This makes it less
dangerous to work with, but you still have to take care while
doing anything at mains voltages.

I have
- bench top HP oscilloscope
- Velleman PCSGU250 PC-scope
- Another Velleman on other machine
- Daqarta sound-card oscilloscope

And then I have the unsuspecting target - my PSU
I want to scope the PSU's primary and secondary windings and
later
follow the voltage through it.

On which devices to put the transformer?

Then there is that ground lead in the probe. If there is
transformers
in the machine connected with the scope and the device under
test,
would be destructive to connect it at all? Because there is no
common
ground between the devices? I really need to get 1000 page book
about
oscilloscopes - ISBN anyone? I am learning this alone, so there
is no
one to ask except here....

Those transformers are pretty expensive, so my first thought to
put
them everywhere is not good one. I would be very thankful if
someone(s) could tell little bit about the basics about this,
with
instructions what to do. And naturally what not to do.

The basic idea is this: You connect the primary (input side) of
the isolation transformer to the mains outlet, and the primary of
the PSU transformer to the secondary (output) of the isolation
transformer. You can now measure voltages at various points on
your PSU. But there are some complicating factors, especially
since you want to measure the input to your PSU transformer, and
that is still at mains voltage (and dangerous if you don't know
what you're doing) even though it is isolated from the public
mains.

It's a bit complex and it's already close to 3:00 AM here now. So
I'll have to sign off now and continue tomorrow unless someone
else has given provided the answers by then.

Jasen Betts · Mar 26, 2010

On 2010-03-26, Kari Laine <klaine8@gmail.com> wrote:

Hi,

first. Thank you for all the help I have gotten from here so far.

Now I am trying to figure out how to connect the electricity to my
scopes and the Power supply I am building.

I don't know the right term for a transformer which disconnects galvanic
connection between the electricity network and device. I will just use
term transformer for that.

Isolating transformer

I have
- bench top HP oscilloscope
- Velleman PCSGU250 PC-scope
- Another Velleman on other machine
- Daqarta sound-card oscilloscope

And then I have the unsuspecting target - my PSU
I want to scope the PSU's primary and secondary windings and later
follow the voltage through it.

On which devices to put the transformer?

On the PSU.

Those transformers are pretty expensive,

Can be cheap second-hand, but pay a few bucks to get them tested if
you do get a used one.

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

The Phantom · Mar 26, 2010

On Fri, 26 Mar 2010 20:55:37 +0200, Kari Laine <klaine8@gmail.com> wrote:

Hi,

first. Thank you for all the help I have gotten from here so far.

Now I am trying to figure out how to connect the electricity to my
scopes and the Power supply I am building.

I don't know the right term for a transformer which disconnects galvanic
connection between the electricity network and device. I will just use
term transformer for that.

I have
- bench top HP oscilloscope
- Velleman PCSGU250 PC-scope
- Another Velleman on other machine
- Daqarta sound-card oscilloscope

And then I have the unsuspecting target - my PSU
I want to scope the PSU's primary and secondary windings and later
follow the voltage through it.

On which devices to put the transformer?

Then there is that ground lead in the probe. If there is transformers in
the machine connected with the scope and the device under test, would be
destructive to connect it at all? Because there is no common ground
between the devices? I really need to get 1000 page book about
oscilloscopes - ISBN anyone? I am learning this alone, so there is no
one to ask except here....

Those transformers are pretty expensive, so my first thought to put them
everywhere is not good one. I would be very thankful if someone(s) could
tell little bit about the basics about this, with instructions what to
do. And naturally what not to do.

You could get one of these isolation transformers:

http://www.mouser.com/catalog/catalogUSD/641/1971.pdf

I have used this particular one:

http://www.mouser.com/ProductDetail/Triad-Magnetics/VPS230-350/?qs=Fg5d7evCuampJb4iXfKS4A%3d%3d

I can handle loads up to 80 VA.

Depending on how much power your PSU is going to be drawing while you work on
it, you could select a smaller or larger one to meet your needs.

Best Regards
Kari

pimpom · Mar 27, 2010

Kari Laine wrote:

Thank You for your time and effort.

I am now convinced that one cannot measure mains with an
oscscope
connected to same mains. And it is recommended to use battery
powered
oscilloscope - why is that?

It's not an absolute "cannot". One can, and I've done it on
occasion. BUT it requires caution and a clear knowledge of what's
involved. This is why:

1) If the house wiring has a proper ground (earth) line and the
scope is powered with a three-wire mains cord via a three-prong
mains plug, the body (chassis) of the scope is now connected to
ground.

2) The ground lead of the scope is often connected to the
chassis. Some scopes have provision to isolate, but not all
scopes do.

If the probe is NOT isolated from the chassis and you connect the
ground lead to the live side of the mains supply, you are
shorting the mains live and neutral lines together via the probe
ground plus mains ground wire. PHUT!!! Something will blow, burn
or trip.

Another possible scenario: Let's say your scope is not properly
grounded through the mains cord. In that case, you may not blow a
fuse or trip a circuit breaker, but as soon as you touch the
probe to the mains, all parts of the scope that's not isolated
from the probe will now be "live" and dangerous.

I actually do not need to measure mains. I am sure the
electrical
company is good enough. I only have to measure ripple after the
bridge
before regulator and ripple after regulator. I am just curious
what
would happen if I do following.

I have a PC-scope connected to machine, which is grounded
through the
mains cable. I connect probe (in x10) to mains? Will I generate
smoke?

You may not actually produce smoke as the series resistor in the
10x probe will limit the current, but it's still not safe - for
you or for the PC.

Some important visitors have just come in (clients for a project
I'm doing). If it's still not clear enough, just ask.

Kari Laine · Mar 27, 2010

Thank You for your time and effort.

I am now convinced that one cannot measure mains with an oscscope
connected to same mains. And it is recommended to use battery powered
oscilloscope - why is that?

I actually do not need to measure mains. I am sure the electrical
company is good enough. I only have to measure ripple after the bridge
before regulator and ripple after regulator. I am just curious what
would happen if I do following.

I have a PC-scope connected to machine, which is grounded through the
mains cable. I connect probe (in x10) to mains? Will I generate smoke?

Best Regards
Kari

--
PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
KEYPADS - ACCESSORIES
http://www.byvac.com (I am just a satisfied customer)

Fred Abse · Mar 27, 2010

On Fri, 26 Mar 2010 20:55:37 +0200, Kari Laine wrote:

Hi,

first. Thank you for all the help I have gotten from here so far.

Now I am trying to figure out how to connect the electricity to my
scopes and the Power supply I am building.

I don't know the right term for a transformer which disconnects galvanic
connection between the electricity network and device. I will just use
term transformer for that.

I have
- bench top HP oscilloscope
- Velleman PCSGU250 PC-scope
- Another Velleman on other machine
- Daqarta sound-card oscilloscope

And then I have the unsuspecting target - my PSU
I want to scope the PSU's primary and secondary windings and later
follow the voltage through it.

On which devices to put the transformer?

On the DUT (Device Under Test)

Then there is that ground lead in the probe. If there is transformers in
the machine connected with the scope and the device under test, would be
destructive to connect it at all? Because there is no common ground
between the devices? I really need to get 1000 page book about
oscilloscopes - ISBN anyone? I am learning this alone, so there is no
one to ask except here....

Connect all test equipment direct to supply, and supply grounding.
IOW, just plug 'em in. Test equipment grounds should all be just that -
grounds.

Those transformers are pretty expensive, so my first thought to put them
everywhere is not good one.

Your first thought was right.

--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)

John Fields · Mar 27, 2010

On Sat, 27 Mar 2010 13:40:53 +0200, Kari Laine <klaine8@gmail.com>
wrote:

Thank You for your time and effort.

I am now convinced that one cannot measure mains with an oscscope
connected to same mains.

---
Sure you can!

If you're talking 240V US residential mains, here's what you get from
the power company: (View in Courier)

METER
+-----+
HV>----+ +-----//-------|L1 L1|-->120V
| | | |
P||S | |
R||E-+---//-----+-|N N|-->0V
I||C | | | |
| | | | | |
HV>----+ +-|---//-----|-|L2 L2|-->120V
| | +-----+
EARTH EARTH

Your scope gets power from the mains like this:

METER
+-----+ +----------+
HV>----+ +-----//-------|L1 L1|--<120V<---|AC VERT|
| | | | | |
P||S | | | |
R||E-+---//-----+-|N N|--<0V<-----+AC--+--GND| |
I||C | | | | | | |
| | | | | | +----+-----+
HV>----+ +-|---//-----|-|L2 L2|-->120V
| | +-----+
EARTH EARTH

so all that's required to measure the mains voltage is to connect the
scope's vertical input to L1 or L2 through a suitable voltage divider:

+----------+
+---|VERT--+ |
| | | |
[9M] | | |
| | [1M] |
120AC>---+---|AC | |
| | |
NEUT>--------|GND---+ |
+----------+

The 9 megohm resistor being part of a X10 probe, with 1 megohm being the
usual input resistance of the scope's vertical amplifiers.
---

And it is recommended to use battery powered
oscilloscope - why is that?

---
Because a battery powered oscilloscope carries its own power supply and
is _totally_ isolated from the mains, making it somewhat safer to use
than a conventional scope with its ground hardwired to neutral.
---

I actually do not need to measure mains. I am sure the electrical
company is good enough. I only have to measure ripple after the bridge
before regulator and ripple after regulator. I am just curious what
would happen if I do following.

I have a PC-scope connected to machine, which is grounded through the
mains cable. I connect probe (in x10) to mains? Will I generate smoke?

---
I'm not sure what you mean by "I connect probe (in x10) to mains".

Can you be a little more specific or post a schematic of what you're
trying to do?

JF

Kari Laine · Mar 28, 2010

John Fields wrote:

---
I'm not sure what you mean by "I connect probe (in x10) to mains".

Can you be a little more specific or post a schematic of what you're
trying to do?

Thanks John!

I think I understand it now.
Mains connector on the wall has three poles.
Active1, Active2 and ground

I would connect probes ground lead to ground.
I would connect probes tip to either of the Active1 or Active2.

I WON'T connect probes ground to active1 and probes tip to active2.
My confusion was this probes ground lead.

Anyway I don't actually need to measure it - I was just curious.

Best Regards
Kari

John Fields · Mar 28, 2010

On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klaine8@gmail.com>
wrote:

John Fields wrote:
---
I'm not sure what you mean by "I connect probe (in x10) to mains".

Can you be a little more specific or post a schematic of what you're
trying to do?

Thanks John!

I think I understand it now.
Mains connector on the wall has three poles.
Active1, Active2 and ground

---
Oops... looks like I confused you.

Sorry about that.

In actuality, the system looks more like this:

120V RECEPT
METER BREAKER BOX O O
+-----+ +----------+ | O |
HV>----+ +-----//-----|L1 L1|--120---|---+-[CB]-|---+ | |
| | | | | | | | |
| | | +--|--GND---|---|--+---|-----+ |
P||S | | | | | | | |
R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+
I||C | | | | | | | |
| | | | | | | | +-[CB]-|-------+
| | | | | | | +--------|-----+ |
HV>----+ +-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | |
| | +-----+ +----------+ | | |
EARTH EARTH | O |
O O
240V RECEPT

Notice that for 120V, the receptacle is connected to one end of the
transformer secondary (L1), the center tap,(N) and GND, which is
connected to neutral in the breaker box and the meter.

Neutral is also connected to rods driven into the Earth at the pole and
the meter.

What you described, however, is the 240V receptacle, which is connected
across the entire secondary (across which is 240V) and the center tap,
which is Neutral, so both "hot" terminals in the receptacle are at 120V
with respect to Neutral and at 240V with respect to each other.
---

I would connect probes ground lead to ground.
I would connect probes tip to either of the Active1 or Active2.

---
For the 240V circuit, yes.

For the 120V circuit you don't have active 1 and active 2, you have one
or the other, neutral, and ground, so your scope ground would go to
neutral and your probe tip to the hot terminal.
---

I WON'T connect probes ground to active1 and probes tip to active2.

---
Good!

---

My confusion was this probes ground lead.

---
OK, just remember that AC outlets and equipment aren't always wired up
like they're supposed to be, so make sure that you know what's what
before you clip that ground lead onto _anything_.
---

Anyway I don't actually need to measure it - I was just curious.

---
OK

JF

krw@att.bizzzzzzzzzzzz · Mar 28, 2010

On Sun, 28 Mar 2010 04:57:07 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klaine8@gmail.com
wrote:

John Fields wrote:
---
I'm not sure what you mean by "I connect probe (in x10) to mains".

Can you be a little more specific or post a schematic of what you're
trying to do?

Thanks John!

I think I understand it now.
Mains connector on the wall has three poles.
Active1, Active2 and ground

---
Oops... looks like I confused you.

Sorry about that.

In actuality, the system looks more like this:

120V RECEPT
METER BREAKER BOX O O
+-----+ +----------+ | O |
HV>----+ +-----//-----|L1 L1|--120---|---+-[CB]-|---+ | |
| | | | | | | | |
| | | +--|--GND---|---|--+---|-----+ |
P||S | | | | | | | |
R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+
I||C | | | | | | | |
| | | | | | | | +-[CB]-|-------+
| | | | | | | +--------|-----+ |
HV>----+ +-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | |
| | +-----+ +----------+ | | |
EARTH EARTH | O |
O O
240V RECEPT

Notice that for 120V, the receptacle is connected to one end of the
transformer secondary (L1), the center tap,(N) and GND, which is
connected to neutral in the breaker box and the meter.

Neutral is also connected to rods driven into the Earth at the pole and
the meter.

What you described, however, is the 240V receptacle, which is connected
across the entire secondary (across which is 240V) and the center tap,
which is Neutral, so both "hot" terminals in the receptacle are at 120V
with respect to Neutral and at 240V with respect to each other.

You forgot the ground on the 240V receptacle. There may or may not be a
neutral (it must be there on new circuits with loads with components requiring
120V) but there *must* be a ground.

<...>

John Fields · Mar 28, 2010

On Sun, 28 Mar 2010 10:39:42 -0500, "krw@att.bizzzzzzzzzzzz"
<krw@att.bizzzzzzzzzzzz> wrote:

On Sun, 28 Mar 2010 04:57:07 -0500, John Fields
jfields@austininstruments.com> wrote:

On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klaine8@gmail.com
wrote:

John Fields wrote:
---
I'm not sure what you mean by "I connect probe (in x10) to mains".

Can you be a little more specific or post a schematic of what you're
trying to do?

Thanks John!

I think I understand it now.
Mains connector on the wall has three poles.
Active1, Active2 and ground

---
Oops... looks like I confused you.

Sorry about that.

In actuality, the system looks more like this:

120V RECEPT
METER BREAKER BOX O O
+-----+ +----------+ | O |
HV>----+ +-----//-----|L1 L1|--120---|---+-[CB]-|---+ | |
| | | | | | | | |
| | | +--|--GND---|---|--+---|-----+ |
P||S | | | | | | | |
R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+
I||C | | | | | | | |
| | | | | | | | +-[CB]-|-------+
| | | | | | | +--------|-----+ |
HV>----+ +-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | |
| | +-----+ +----------+ | | |
EARTH EARTH | O |
O O
240V RECEPT

Notice that for 120V, the receptacle is connected to one end of the
transformer secondary (L1), the center tap,(N) and GND, which is
connected to neutral in the breaker box and the meter.

Neutral is also connected to rods driven into the Earth at the pole and
the meter.

What you described, however, is the 240V receptacle, which is connected
across the entire secondary (across which is 240V) and the center tap,
which is Neutral, so both "hot" terminals in the receptacle are at 120V
with respect to Neutral and at 240V with respect to each other.

You forgot the ground on the 240V receptacle. There may or may not be a
neutral (it must be there on new circuits with loads with components requiring
120V) but there *must* be a ground.

---
Right you are... Thanks!

METER BREAKER BOX
+-----+ +-----------------+
HV>----+ +-----//-----|L1 L1|--120---|-----+-[CB]------|----O Line
| | | +--|--GND---|-+---|-------+---|--O Ground | |
| | | | | | | +-|----O Neutral
| | | | | | | | +-|-|--------+
P||S | | | | | +-[CB]--|-|-|----OL |
R||E-+---//---+-|N-+-N|--NEUT--|-+---|-------+-+-|--ON GO-+
I||C | | | | | +-|-[CB]--|---|----OL
| | | | | | | | | | |
| | | | | | | | +-[CB]--|---|----OL
| | | | | | | | +---|--OG
HV>----+ +-|---//---|-|L2 L2|--120---|---+---[CB]------|----OL
| | +-----+ +-----------------+
EARTH EARTH

JF

krw@att.bizzzzzzzzzzzz · Mar 29, 2010

On Sun, 28 Mar 2010 19:22:31 -0700 (PDT), George Herold <ggherold@gmail.com>
wrote:

On Mar 28, 5:57 am, John Fields <jfie...@austininstruments.com> wrote:
On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klai...@gmail.com
wrote:

John Fields wrote:
---
I'm not sure what you mean by "I connect probe (in x10) to mains".

Can you be a little more specific or post a schematic of what you're
trying to do?

Thanks John!

I think I understand it now.
Mains connector on the wall has three poles.
Active1, Active2 and ground

---
Oops... looks like I confused you.

Sorry about that.

In actuality, the system looks more like this:

120V RECEPT
METER BREAKER BOX O O
+-----+ +----------+ | O |
HV>----+ +-----//-----|L1 L1|--120---|---+-[CB]-|---+ | |
| | | | | | | | |
| | | +--|--GND---|---|--+---|-----+ |
P||S | | | | | | | |
R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+
I||C | | | | | | | |
| | | | | | | | +-[CB]-|-------+
| | | | | | | +--------|-----+ |
HV>----+ +-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | |
| | +-----+ +----------+ | | |
EARTH EARTH | O |
O O
240V RECEPT

Notice that for 120V, the receptacle is connected to one end of the
transformer secondary (L1), the center tap,(N) and GND, which is
connected to neutral in the breaker box and the meter.

Neutral is also connected to rods driven into the Earth at the pole and
the meter.

What you described, however, is the 240V receptacle, which is connected
across the entire secondary (across which is 240V) and the center tap,
which is Neutral, so both "hot" terminals in the receptacle are at 120V
with respect to Neutral and at 240V with respect to each other.
---

I would connect probes ground lead to ground.
I would connect probes tip to either of the Active1 or Active2.

---
For the 240V circuit, yes.

For the 120V circuit you don't have active 1 and active 2, you have one
or the other, neutral, and ground, so your scope ground would go to
neutral and your probe tip to the hot terminal.
---

I WON'T connect probes ground to active1 and probes tip to active2.

---
Good!
---

My confusion was this probes ground lead.

---
OK, just remember that AC outlets and equipment aren't always wired up
like they're supposed to be, so make sure that you know what's what
before you clip that ground lead onto _anything_.
---

Anyway I don't actually need to measure it - I was just curious.

---
OK

JF

Ahh Thanks, (Sorry guess I should read ahead before asking questions)
The neutral and ground are connected together both inside the meter
and the breaker box?

Yes, the last place they're connected is in the entrance panel. They're
separated in any boxes and sub-panels downstream of the entrance panel.

George Herold · Mar 29, 2010

On Mar 27, 11:27 am, John Fields <jfie...@austininstruments.com>
wrote:

On Sat, 27 Mar 2010 13:40:53 +0200, Kari Laine <klai...@gmail.com
wrote:

Thank You for your time and effort.

I am now convinced that one cannot measure mains with an oscscope
connected to same mains.

---
Sure you can!

If you're talking 240V US residential mains, here's what you get from
the power company: (View in Courier)

METER
+-----+
HV>----+ +-----//-------|L1 L1|-->120V
| | | |
P||S | |
R||E-+---//-----+-|N N|-->0V
I||C | | | |
| | | | | |
HV>----+ +-|---//-----|-|L2 L2|-->120V
| | +-----+
EARTH EARTH

Your scope gets power from the mains like this:

METER
+-----+ +----------+
HV>----+ +-----//-------|L1 L1|--<120V<---|AC VERT|
| | | | | |
P||S | | | |
R||E-+---//-----+-|N N|--<0V<-----+AC--+--GND| |
I||C | | | | | | |
| | | | | | +----+-----+
HV>----+ +-|---//-----|-|L2 L2|-->120V
| | +-----+
EARTH EARTH

Thanks for the picture JF, I'm fairly ignorant about power

electronics. I have a question about where the third pin (AC) ground
is connected into the system?

thanks again,

George H.

<snip>
> JF

George Herold · Mar 29, 2010

On Mar 28, 5:57 am, John Fields <jfie...@austininstruments.com> wrote:

On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klai...@gmail.com
wrote:

John Fields wrote:
---
I'm not sure what you mean by "I connect probe (in x10) to mains".

Can you be a little more specific or post a schematic of what you're
trying to do?

Thanks John!

I think I understand it now.
Mains connector on the wall has three poles.
Active1, Active2 and ground

---
Oops... looks like I confused you.

Sorry about that.

In actuality, the system looks more like this:

120V RECEPT
METER BREAKER BOX O O
+-----+ +----------+ | O |
HV>----+ +-----//-----|L1 L1|--120---|---+-[CB]-|---+ | |
| | | | | | | | |
| | | +--|--GND---|---|--+---|-----+ |
P||S | | | | | | | |
R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+
I||C | | | | | | | |
| | | | | | | | +-[CB]-|-------+
| | | | | | | +--------|-----+ |
HV>----+ +-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | |
| | +-----+ +----------+ | | |
EARTH EARTH | O |
O O
240V RECEPT

Notice that for 120V, the receptacle is connected to one end of the
transformer secondary (L1), the center tap,(N) and GND, which is
connected to neutral in the breaker box and the meter.

Neutral is also connected to rods driven into the Earth at the pole and
the meter.

What you described, however, is the 240V receptacle, which is connected
across the entire secondary (across which is 240V) and the center tap,
which is Neutral, so both "hot" terminals in the receptacle are at 120V
with respect to Neutral and at 240V with respect to each other.
---

I would connect probes ground lead to ground.
I would connect probes tip to either of the Active1 or Active2.

---
For the 240V circuit, yes.

For the 120V circuit you don't have active 1 and active 2, you have one
or the other, neutral, and ground, so your scope ground would go to
neutral and your probe tip to the hot terminal.
---

I WON'T connect probes ground to active1 and probes tip to active2.

---
Good!
---

My confusion was this probes ground lead.

---
OK, just remember that AC outlets and equipment aren't always wired up
like they're supposed to be, so make sure that you know what's what
before you clip that ground lead onto _anything_.
---

Anyway I don't actually need to measure it - I was just curious.

---
OK

JF

Ahh Thanks, (Sorry guess I should read ahead before asking questions)
The neutral and ground are connected together both inside the meter
and the breaker box?

George H.

Kari Laine · Mar 29, 2010

George Herold wrote:

Thanks for the picture JF, I'm fairly ignorant about power
electronics. I have a question about where the third pin (AC) ground
is connected into the system?

thanks again,

George H.

Thanks also. I think I should get a book about these things.

any good one?

Kari

John Fields · Mar 29, 2010

On Mon, 29 Mar 2010 10:14:20 +0300, Kari Laine <klaine8@gmail.com>
wrote:

George Herold wrote:
Thanks for the picture JF, I'm fairly ignorant about power
electronics. I have a question about where the third pin (AC) ground
is connected into the system?

thanks again,

George H.

Thanks also. I think I should get a book about these things.

any good one?

---
The NEC:

http://en.wikipedia.org/wiki/National_Electrical_Code#Public_access_to_the_NEC

JF

Bob Masta · Mar 29, 2010

On Mon, 29 Mar 2010 10:14:20 +0300, Kari Laine
<klaine8@gmail.com> wrote:

George Herold wrote:
Thanks for the picture JF, I'm fairly ignorant about power
electronics. I have a question about where the third pin (AC) ground
is connected into the system?

thanks again,

George H.

Thanks also. I think I should get a book about these things.

any good one?

Kari:

Note that these responses have been assuming
US-type wiring standards. I don't know anything
about the standards where you are, but I suspect
you have 240 only, no 120 ground. I assume that
one side of the 240 is always grounded, but make
sure that whatever book or reference you consult
is for the system in your own area.

Best regards,

Bob Masta

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