Live, Neutral, etc.

C

Costas Vlachos

Guest
I was having a chat with a friend the other day about how to fuse and switch
the supply of mains-powered designs. He said that he once made a mistake in
the wiring and was actually switching on/off the neutral instead of the live
wire in the primary of a xformer. He said that, when the switch was off (so
only the live wire was connected to the primary), every time he touched the
*secondary* wires he was getting tiny shocks! When he corrected the wiring,
the shocks disappeared.

Does this make sense??? I thought the xformer would electrically isolate
primary from secondary. This may be too simple, but I want to know how this
could happen. Note that no earth wire was connected anywhere in the system
(figure-8 cable was used).

As a side note, in many European countries the mains wall sockets are
designed so that one can insert the plug in any orientation, effectively
interchanging live and neutral. If what my friend said really happened, then
there is a danger risk, unless one uses double-pole switches to cut-off both
wires...

cheers,
Costas
 
On Sat, 12 Jul 2003 15:59:06 +0000 (UTC), "Costas Vlachos"
<c-X-vlachos@hot-X-mail.com> wrote:

I was having a chat with a friend the other day about how to fuse and switch
the supply of mains-powered designs. He said that he once made a mistake in
the wiring and was actually switching on/off the neutral instead of the live
wire in the primary of a xformer. He said that, when the switch was off (so
only the live wire was connected to the primary), every time he touched the
*secondary* wires he was getting tiny shocks! When he corrected the wiring,
the shocks disappeared.

Does this make sense??? I thought the xformer would electrically isolate
primary from secondary. This may be too simple, but I want to know how this
could happen. Note that no earth wire was connected anywhere in the system
(figure-8 cable was used).

As a side note, in many European countries the mains wall sockets are
designed so that one can insert the plug in any orientation, effectively
interchanging live and neutral. If what my friend said really happened, then
there is a danger risk, unless one uses double-pole switches to cut-off both
wires...

cheers,
Costas
Click to expand...
He was probably getting a little spark because of the transformer
pri-sec winding capacitance, more if there was an EMI filter with caps
from line to chassis. Those couplings are max when the hot side is
live and neutral is open.

John
 
In article <0ae0hv05n35jopb4ces62sqm60cs4nkcpc@4ax.com>,
John Larkin <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:


He was probably getting a little spark because of the transformer
pri-sec winding capacitance, more if there was an EMI filter with caps
from line to chassis. Those couplings are max when the hot side is
live and neutral is open.

John
Click to expand...
Hmmm... Wonder if this explains something I experienced years ago - back
in about 3rd/4th grade, I was in the "media room" (storage zone for
not-currently-in-use A/V gear like projectors, tape players, filmstrips,
and similar stuff) of our school, and needed to plug in an old
reel-to-reel tape player. Somebody had somehow mashed the prongs of the
plug so that, while they weren't actually touching, they definitely
weren't going to go into a socket without the assistance of a sledge
hammer and a crowbar. Me, being the bright kid I was, said "Oh... OK, I
can fix this!" and grabbed onto the most-bent blade to tweak it back to
something like straight. It literally knocked me on my ass when I
grabbed it.

Now I *KNOW* that the thing wasn't powered up, 'cause I'd just dragged
it across the room from the shelf it had been sitting on 30 feet away,
and the only connections to it were the headset and power cords, both of
which were in plain view, and unquestionably NOT connected to anything
electrical. But SOMEHOW, that damn thing nailed me *HARD* when I grabbed
the blade of the plug. To this day, I've never gotten anything
resembling a believable explanation for it, unless filter caps are the
culprit.

--
Don Bruder - dakidd@sonic.net <--- Preferred Email - unmunged, SpamAssassinated
Hate SPAM? See <http://www.spamassassin.org> for some seriously great info.
I will choose a path that's clear: I will choose Free Will! - N. Peart
Fly trap info pages: <http://www.sonic.net/~dakidd/Horses/FlyTrap/index.html>
 
"John Popelish" <jpopelish@rica.net> wrote in message
news:3F103608.FDF04CEF@rica.net...
Costas Vlachos wrote:

I was having a chat with a friend the other day about how to fuse and
switch
the supply of mains-powered designs. He said that he once made a mistake
in
the wiring and was actually switching on/off the neutral instead of the
live
wire in the primary of a xformer. He said that, when the switch was off
(so
only the live wire was connected to the primary), every time he touched
the
*secondary* wires he was getting tiny shocks! When he corrected the
wiring,
the shocks disappeared.

Does this make sense???

Yes. The neutral side of the line is connected to Earth by a
grounding wire. So there is very little voltage between the neutral
conductor and any path to ground. With the hot side switched open,
the most voltage you can find between the electrical circuit and any
ground is just the resistive drop in the grounding conductor and its
earth resistance times any leakage current passing through it.

I thought the xformer would electrically isolate
primary from secondary. This may be too simple, but I want to know how
this
could happen. Note that no earth wire was connected anywhere in the
system
(figure-8 cable was used).

The transformer does provide isolation between the high distribution
voltage and the lower line voltage, but the breaker panel should
contain a grounding jumper between the neutral side of the secondary
and earth. There is also a redundant neutral grounding conductor out
on the pole, if your hook up is like mine.

As a side note, in many European countries the mains wall sockets are
designed so that one can insert the plug in any orientation, effectively
interchanging live and neutral. If what my friend said really happened,
then
there is a danger risk, unless one uses double-pole switches to cut-off
both
wires...

Yes.

cheers,
Costas

--
John Popelish
Click to expand...


Thanks John for the useful info. At least here in the UK there's no issue of
live/neutral reversal (provided the installation of the sockets is correct
in the first place...).

cheers,
Costas
 
Thanks John for the useful info. At least here in the UK there's no issue
of
live/neutral reversal (provided the installation of the sockets is correct
in the first place...).
cheers,
Costas
I'm amused by the number of American engineers who've visited (on business)
Click to expand...
and inevitably had some light hearted banter about 110V vs 240V, and said
how bulky and awkward our plugs are compared to their slim elegant little
ones. But after they've actually had to use them, and thought about the
internal fuse, the inability to pull out the plug by pulling the cable, the
integral earth .... they say how impressed they are !

I'm just a curmudgeonly old git I think ...

Neil
 
On Sat, 12 Jul 2003 18:52:50 GMT, Don Bruder <dakidd@sonic.net> wrote:

In article <0ae0hv05n35jopb4ces62sqm60cs4nkcpc@4ax.com>,
John Larkin <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:


He was probably getting a little spark because of the transformer
pri-sec winding capacitance, more if there was an EMI filter with caps
from line to chassis. Those couplings are max when the hot side is
live and neutral is open.

John


Hmmm... Wonder if this explains something I experienced years ago - back
in about 3rd/4th grade, I was in the "media room" (storage zone for
not-currently-in-use A/V gear like projectors, tape players, filmstrips,
and similar stuff) of our school, and needed to plug in an old
reel-to-reel tape player. Somebody had somehow mashed the prongs of the
plug so that, while they weren't actually touching, they definitely
weren't going to go into a socket without the assistance of a sledge
hammer and a crowbar. Me, being the bright kid I was, said "Oh... OK, I
can fix this!" and grabbed onto the most-bent blade to tweak it back to
something like straight. It literally knocked me on my ass when I
grabbed it.

Now I *KNOW* that the thing wasn't powered up, 'cause I'd just dragged
it across the room from the shelf it had been sitting on 30 feet away,
and the only connections to it were the headset and power cords, both of
which were in plain view, and unquestionably NOT connected to anything
electrical. But SOMEHOW, that damn thing nailed me *HARD* when I grabbed
the blade of the plug. To this day, I've never gotten anything
resembling a believable explanation for it, unless filter caps are the
culprit.
Click to expand...
Possibly a motor run capacitor, but I can't imagine how it came to be
charged and still be the only thing across the line. Some of the
reel-reel tape decks had very complex multi-deck mechanical switches
to sequence the capstain and reel motors in various modes, so I guess
some situation might have left a motor cap across the line.

But you'll probably never know.

I fixed a stereo amp once, that had terrible crossover distortion on
both channels. Had to replace the resistors that set quiescent current
on both sides. The owner (my English teacher) swore it worked fine
until one day both channels got garbled. There's no explanation, and
it bugs me still.

John
 
"Neil" <fredd133@hotmail.com> wrote in message
news:%1ZPa.1743$Bn5.1680@news-binary.blueyonder.co.uk...
I'm amused by the number of American engineers who've visited (on
business)
and inevitably had some light hearted banter about 110V vs 240V, and said
how bulky and awkward our plugs are compared to their slim elegant little
ones. But after they've actually had to use them, and thought about the
internal fuse, the inability to pull out the plug by pulling the cable,
the
integral earth .... they say how impressed they are !

I'm just a curmudgeonly old git I think ...

Neil
Click to expand...
All of which has long been available on the sexy little plugs Neil!!!
Also, the voltage is more like 120 volts. My own is 122 volts.
 
On Sat, 12 Jul 2003 20:09:40 +0100, "Neil" <fredd133@hotmail.com>
wrote:

Thanks John for the useful info. At least here in the UK there's no issue
of
live/neutral reversal (provided the installation of the sockets is correct
in the first place...).
cheers,
Costas
I'm amused by the number of American engineers who've visited (on business)
and inevitably had some light hearted banter about 110V vs 240V, and said
how bulky and awkward our plugs are compared to their slim elegant little
ones. But after they've actually had to use them, and thought about the
internal fuse, the inability to pull out the plug by pulling the cable, the
integral earth .... they say how impressed they are !
Click to expand...

But it's great to be able to pull out the plug by yanking the cable;
saves a lot of crawling under tables and such.

The US plugs are good enough; they are almost never a problem. It's
usually the other end of the cord that makes trouble.

John
 
"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message news:2693hvkfdc14m9510jbms26nbujjkq8m41@4ax.com...

wrote:
snip
The US plugs are good enough; they are almost never a problem. It's
usually the other end of the cord that makes trouble.
Click to expand...
Or whats connected to the other end of the cord.. :)
 
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