Scart sockets blow up devices.
 

Can a Scart socket damage devices connected to it? It seems to have happened
to me three times with different products. I have an old Marantz audio
visual HI FI system that I can connect to TV's, Videos, Laser disk players
etc with either scart connectors or audio/video cables. I always used it
connected from the video to the Hi Fi with the audio connectors. One day I
decided to connect the TV to the HI FI using a Scart connector, there was a
loud crackling noise from the speakers followed by a bang and a burning
smell as the Amplifier went. I did not try that trick again for a long time
until I got a DVD player for my computer. I connected the DVD output leads
to a scart adaptor and plugged it into the same TV, the DVD drive promptly
expired. Now I have a new TV, Video and an AllinWonder TV/Graphics card. The
AIW card worked fine in every function until yesterday when I tried to use
the Video Out function from my VCR to the AIW card using a Scart connector.
The computer screen immediately went blank and I ended having to reboot the
computer. Since then the TV tuner on AIW does not work nor does Video in
from my camcorder which worked before but the card can play VCD's and video
files that I have on my HD. I should add that it was not the same scart
connector involved in each disaster. Has anyone else experienced such a
problem? when I suggested that the Scart connection had caused the problem
with the Marantz amplifier the repairer dismissed the idea out of hand. I
also queried the problem with the TV manufacturer (Hitachi) who also
dismissed the idea.

"Derek F" wrote in message
...
Can a Scart socket damage devices connected to it? It seems to have
happened
to me three times with different products.

I had a similar problem but it was self-inflicted ! I decided to fit a mains
isolation
switch to a cheap satellite STB. I trapped the live wire in refitting the
case making
the case live. I blew up the TV and the STB.

I do not believe that a SCART lead itself can ever be the cause of such a
problem.
Most kit these days has a two-core mains cable and is thus un-earthed.
A developing fault in one device can begin to make the chassis live, not
full live, but
sufficiently so to allow an earth current to flow through the SCART cable
screens
to another device which does have an earthed chassis. This fault current
will also
get into the signal wires with the sort of result that Derek F found.

"Bob Higham" wrote in message
...

"Derek F" wrote in message
...
Can a Scart socket damage devices connected to it? It seems to have
happened
to me three times with different products.

I had a similar problem but it was self-inflicted ! I decided to fit a
mains
isolation
switch to a cheap satellite STB. I trapped the live wire in refitting the
case making
the case live. I blew up the TV and the STB.

I do not believe that a SCART lead itself can ever be the cause of such a
problem.
Most kit these days has a two-core mains cable and is thus un-earthed.
A developing fault in one device can begin to make the chassis live, not
full live, but
sufficiently so to allow an earth current to flow through the SCART cable
screens
to another device which does have an earthed chassis. This fault current
will also
get into the signal wires with the sort of result that Derek F found.


What does one do in that situation and would the manufacturer of the faulty
device take responsibility for it?
Derek

Derek F wrote:

Can a Scart socket damage devices connected to it?

No, not on it's own

snip
Has anyone else experienced such a
problem? when I suggested that the Scart connection had caused the problem
with the Marantz amplifier the repairer dismissed the idea out of hand. I
also queried the problem with the TV manufacturer (Hitachi) who also
dismissed the idea.



I would say that you have a mains wiring fault in your house or in the
mains socket. Looks to me like either Live and Neutral are swapped over
or Live and Earth are swapped over.
In a modern house the neutral and earth are bonded together.
I suggest that you get your house wiring/sockets/mains leads/13A plug
wiring checked by a competant Electrician.
Dave


--
My opinions are just that, check the facts carefully before
acting on my advice.
Please reply to newsgroup.

"Dave Spam" wrote in message
...
Derek F wrote:

Can a Scart socket damage devices connected to it?

No, not on it's own

snip
Has anyone else experienced such a
problem? when I suggested that the Scart connection had caused the
problem
with the Marantz amplifier the repairer dismissed the idea out of hand.
I
also queried the problem with the TV manufacturer (Hitachi) who also
dismissed the idea.



I would say that you have a mains wiring fault in your house or in the
mains socket. Looks to me like either Live and Neutral are swapped over
or Live and Earth are swapped over.
In a modern house the neutral and earth are bonded together.
I suggest that you get your house wiring/sockets/mains leads/13A plug
wiring checked by a competant Electrician.
Dave


--
As I live in a block of flats that were all rewired at the same time
presumably all the other 25 occupiers would have the same problem if that is
the actual fault?
Derek.

Derek F wrote:

"Dave Spam" wrote in message
...

Derek F wrote:


Can a Scart socket damage devices connected to it?

No, not on it's own

snip

Has anyone else experienced such a
problem? when I suggested that the Scart connection had caused the

problem

with the Marantz amplifier the repairer dismissed the idea out of hand.

I

also queried the problem with the TV manufacturer (Hitachi) who also
dismissed the idea.



I would say that you have a mains wiring fault in your house or in the
mains socket. Looks to me like either Live and Neutral are swapped over
or Live and Earth are swapped over.
In a modern house the neutral and earth are bonded together.
I suggest that you get your house wiring/sockets/mains leads/13A plug
wiring checked by a competant Electrician.
Dave


--

As I live in a block of flats that were all rewired at the same time
presumably all the other 25 occupiers would have the same problem if that is
the actual fault?
Derek.


Not neccessarily. Only one socket or one 13A plug might be faulty.
Each socket has to be wired by hand. It's all too easy to make a
mistake. One is supposed to check *every* single socket, connection,
fitting etc. when the job is complete.
What I'm suggesting is that somewhere an earth lead is "live" or has
been disconected/broken and it's shorting out through the conviently
connected scart lead.
What also *might* cause it is a live mains leakage into the TV aerial
lead (faulty distrubution amp, bad wiring etc) such that the braid is
live, again shorting out through the scart lead.
Do any fuses or trips blow when this happens?
Dave



--
My opinions are just that, check the facts carefully before
acting on my advice.
Please reply to newsgroup.

"Dave Spam" wrote in message
...
Derek F wrote:

"Dave Spam" wrote in message
...

Derek F wrote:


Can a Scart socket damage devices connected to it?

No, not on it's own

snip

Has anyone else experienced such a
problem? when I suggested that the Scart connection had caused the

problem

with the Marantz amplifier the repairer dismissed the idea out of hand.

I

also queried the problem with the TV manufacturer (Hitachi) who also
dismissed the idea.



I would say that you have a mains wiring fault in your house or in the
mains socket. Looks to me like either Live and Neutral are swapped over
or Live and Earth are swapped over.
In a modern house the neutral and earth are bonded together.
I suggest that you get your house wiring/sockets/mains leads/13A plug
wiring checked by a competant Electrician.
Dave


--

As I live in a block of flats that were all rewired at the same time
presumably all the other 25 occupiers would have the same problem if
that is
the actual fault?
Derek.


Not neccessarily. Only one socket or one 13A plug might be faulty.
Each socket has to be wired by hand. It's all too easy to make a
mistake. One is supposed to check *every* single socket, connection,
fitting etc. when the job is complete.
What I'm suggesting is that somewhere an earth lead is "live" or has
been disconected/broken and it's shorting out through the conviently
connected scart lead.
What also *might* cause it is a live mains leakage into the TV aerial
lead (faulty distrubution amp, bad wiring etc) such that the braid is
live, again shorting out through the scart lead.
Do any fuses or trips blow when this happens?
Dave
Each time it has been a different TV aerial (would you believe that I have
four on our roof) and different 13 amp sockets. I am now going to "phone a
friend" who was a senior engineer with Scottish Power.
Derek.

Each time it has been a different TV aerial (would you believe that I have
four on our roof) and different 13 amp sockets. I am now going to "phone a
friend" who was a senior engineer with Scottish Power.

Could still be swapped around at your fuse board, which would not affect
anyone else.

You could get one of those plug-in testers, they're the quickest way of
finding wiring faults. I don't think they can detect earth-neutral shorts
or swaps though.

I'd guess you have a dodgy earth somewhere anyway. Any hums on any
equipment?


---
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Checked by AVG anti-virus system (http://www.grisoft.com).
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"Peter G Sheppard" wrote in message
...
Each time it has been a different TV aerial (would you believe that I
have
four on our roof) and different 13 amp sockets. I am now going to "phone
a
friend" who was a senior engineer with Scottish Power.

Could still be swapped around at your fuse board, which would not affect
anyone else.

You could get one of those plug-in testers, they're the quickest way of
finding wiring faults. I don't think they can detect earth-neutral shorts
or swaps though.

I'd guess you have a dodgy earth somewhere anyway. Any hums on any
equipment?

No hums or haws.
Derek.

"Dave Spam" wrote in message
...
Derek F wrote:

"Dave Spam" wrote in message
...

Derek F wrote:


"Dave Spam" wrote in message
.. .


Derek F wrote:



Can a Scart socket damage devices connected to it?

No, not on it's own

snip

Has anyone else experienced such a
problem? when I suggested that the Scart connection had caused the

problem


with the Marantz amplifier the repairer dismissed the idea out of
hand.

I


also queried the problem with the TV manufacturer (Hitachi) who also
dismissed the idea.



I would say that you have a mains wiring fault in your house or in the
mains socket. Looks to me like either Live and Neutral are swapped
over
or Live and Earth are swapped over.
In a modern house the neutral and earth are bonded together.
I suggest that you get your house wiring/sockets/mains leads/13A plug
wiring checked by a competant Electrician.
Dave


--

As I live in a block of flats that were all rewired at the same time
presumably all the other 25 occupiers would have the same problem if

that is

the actual fault?
Derek.



Not neccessarily. Only one socket or one 13A plug might be faulty.
Each socket has to be wired by hand. It's all too easy to make a
mistake. One is supposed to check *every* single socket, connection,
fitting etc. when the job is complete.
What I'm suggesting is that somewhere an earth lead is "live" or has
been disconected/broken and it's shorting out through the conviently
connected scart lead.
What also *might* cause it is a live mains leakage into the TV aerial
lead (faulty distrubution amp, bad wiring etc) such that the braid is
live, again shorting out through the scart lead.
Do any fuses or trips blow when this happens?
Dave

Each time it has been a different TV aerial (would you believe that I
have
four on our roof) and different 13 amp sockets. I am now going to "phone
a
friend" who was a senior engineer with Scottish Power.
Derek.


Odd, very odd. A close examination and thorough check is now in order.
Dave

Sounds expensive:-)
Derek.

Was everything switched off as you actually made the connection?
Everything that is only double insulated rather than earthed should
really be switched off as you make the connection.

--

Brian Gregory (In the UK).

To email me remove the letter vee.

"Brian Gregory [UK]" wrote in message
...
Was everything switched off as you actually made the connection?
Everything that is only double insulated rather than earthed should
really be switched off as you make the connection.

--

Brian Gregory (In the UK).

To email me remove the letter vee.


Both computer and video were off as when previously using the TV out option
on other occasions. I turned on the video and then the computer as
instructed
in the manual. Windows booted up, the monitor screen went blank when I
selected the ATI TV symbol, the system froze, I had to use the reset
button.
Derek.

That's just a crash.

I was meaning when things "blew up".

--

Brian Gregory (In the UK).

To email me remove the letter vee.

"Brian Gregory [UK]" wrote in message
...
That's just a crash.

I was meaning when things "blew up".

--

Brian Gregory (In the UK).

To email me remove the letter vee.


One and the same incident with the graphics card and video. When it happened
years ago with the TV and the HI Fi they were both swiched on.
Derek.

In articles (various) re apparent problem.

1/ As mentioned previously, most domestic appliances (TVs etc category)
are 2-wire connected.
These devices therefore have a 'chassis' of MID VOLTAGE ie 115V (230-0 /2)
which is not what most would consider 'earth' potential - as in the 3rd
pin.
(The 'chassis' is connected via high-impedance to the power supply, to
stop it floating in total isolation, uncontrollably.)
((PPS The London UNDERGROUND RAILWAY has its centre and side conductors
'tied' to the running rails in a similar manner - one is +400, the other
is -200 I think, and was the cause of a fire when at 2 places within the
same power section, both supplies became directly (shorted) to the running
rails many years ago (Wood Green - Cockfosters section??)

2/ Some domestic equipment, such as computers, increasingly connected into
the system, conventionally use 3-wire connections in the UK - ie the
chassis is at earth potential.

If/when 2-wire and 3-wire devices are connected together, the ''chassis''
can be at different voltages (115V difference) and therefore a small
current can flow between them on ANY connecting leads used:

The currents are SMALL and, apparently considered quite safe (to humans -
presumably with dry hand and not applied directly across the heart 8-),
because of the high impedance connection in each device to the mains
supply (about 10 000 000 ohms?) - however these currents may prove to be
too much for some electronic devices -

They may also take one by surprise if holding a lead when connecting such
equipment together!!!!! - albeit not fatal unless you step back and trip
over something dangerous behind you 8-)

This is why the instructions always tell you to SWITCHOFF the devices
before plugging them together!

However, with the mixture of 3 and 2 wire devices, there will always be
the possibility / probability of such 'leakage' current, and this might
therefore find itself using 1 of the 2 wires L/N in device (A), but the
Earth wire in device (B) - resulting in an imbalance in current
consumption between Live and Neutral (due to the amount going via Earth of
device B), and this could trip a sensitive RCCB (Residual Current Circuit
Breaker) - a possibility which increases as more devices are connected or
plugged in*

*Switch mode power supplies and their startuup current surges being a
likely cause of tripping the breaker


Phil

--
Phil Spiegelhalter:
==== Technical Training for Broadcasters =====
*RE CUE Mobile DV Multi-Camera Production and Non-Linear Editing*

"Phil" wrote in message
...
In articles (various) re apparent problem.

1/ As mentioned previously, most domestic appliances (TVs etc category)
are 2-wire connected.
These devices therefore have a 'chassis' of MID VOLTAGE ie 115V (230-0 /2)
which is not what most would consider 'earth' potential - as in the 3rd
pin.
(The 'chassis' is connected via high-impedance to the power supply, to
stop it floating in total isolation, uncontrollably.)
((PPS The London UNDERGROUND RAILWAY has its centre and side conductors
'tied' to the running rails in a similar manner - one is +400, the other
is -200 I think, and was the cause of a fire when at 2 places within the
same power section, both supplies became directly (shorted) to the running
rails many years ago (Wood Green - Cockfosters section??)

2/ Some domestic equipment, such as computers, increasingly connected into
the system, conventionally use 3-wire connections in the UK - ie the
chassis is at earth potential.

If/when 2-wire and 3-wire devices are connected together, the ''chassis''
can be at different voltages (115V difference) and therefore a small
current can flow between them on ANY connecting leads used:

The currents are SMALL and, apparently considered quite safe (to humans -
presumably with dry hand and not applied directly across the heart 8-),
because of the high impedance connection in each device to the mains
supply (about 10 000 000 ohms?) - however these currents may prove to be
too much for some electronic devices -

They may also take one by surprise if holding a lead when connecting such
equipment together!!!!! - albeit not fatal unless you step back and trip
over something dangerous behind you 8-)

This is why the instructions always tell you to SWITCHOFF the devices
before plugging them together!

However, with the mixture of 3 and 2 wire devices, there will always be
the possibility / probability of such 'leakage' current, and this might
therefore find itself using 1 of the 2 wires L/N in device (A), but the
Earth wire in device (B) - resulting in an imbalance in current
consumption between Live and Neutral (due to the amount going via Earth of
device B), and this could trip a sensitive RCCB (Residual Current Circuit
Breaker) - a possibility which increases as more devices are connected or
plugged in*

*Switch mode power supplies and their startuup current surges being a
likely cause of tripping the breaker


Phil

--
Phil Spiegelhalter:
==== Technical Training for Broadcasters =====
*RE CUE Mobile DV Multi-Camera Production and Non-Linear Editing*


We never really know if items we buy are 2 or 3 wire devices as most now
come with a moulded plug.
I told the supplier of my problem and he is happy to replace it. I will
report if the next one goes the same way!
Derek.

Phil wrote:

2/ Some domestic equipment, such as computers, increasingly connected
into the system, conventionally use 3-wire connections in the UK - ie
the chassis is at earth potential.

If/when 2-wire and 3-wire devices are connected together, the
''chassis'' can be at different voltages (115V difference) and
therefore a small current can flow between them on ANY connecting
leads used:

The currents are SMALL and, apparently considered quite safe (to
humans - presumably with dry hand and not applied directly across the
heart 8-), because of the high impedance connection in each device to
the mains supply (about 10 000 000 ohms?) - however these currents
may prove to be too much for some electronic devices -

They may also take one by surprise if holding a lead when connecting
such equipment together!!!!! - albeit not fatal unless you step back
and trip over something dangerous behind you 8-)

Yes, I get this. My computer has a tv card. The aerial lead to the TV
card always generates a spark when connecting to the PC. At the other
end of the aerial lead (eventually) is a sky receiver downstairs.
Connected to the same TV that that sky receiver is connected to is a DVD
player which is double insulated and not earthed. If I unplug the DVD
player from the mains, I no longer get the sparks! And if I don't, and I
happen to be touching the pc case and the coax plug at the same time and
forget what is about to happen, the very very minor shock takes me by
suprise.

Interestingly, although the DVD player is double insulated, it does have
an earth screw terminal on the back. Maybe I should earth it to
something :)

In article ,
Derek F wrote:


We never really know if items we buy are 2 or 3 wire devices as most now
come with a moulded plug.
I told the supplier of my problem and he is happy to replace it. I will
report if the next one goes the same way!
Derek.

Phil: 2-wire usually has a flat flex from the plug, and 3-wire a round
(circular) flex. Also there should be an indication of 'double insulated'
(possibly) on the equipment, if it is so constructed.

It is the mixture of types in an interconnected chain that is most likely,
IMO, to cause the problem

--
Phil Spiegelhalter:
==== Technical Training for Broadcasters =====
*RE CUE Mobile DV Multi-Camera Production and Non-Linear Editing*

In article ,
letters desk wrote:
Phil wrote:

2/ Some domestic equipment, such as computers, increasingly connected
into the system, conventionally use 3-wire connections in the UK - ie
the chassis is at earth potential.

If/when 2-wire and 3-wire devices are connected together, the
''chassis'' can be at different voltages (115V difference) and
therefore a small current can flow between them on ANY connecting
leads used:

The currents are SMALL and, apparently considered quite safe (to
humans - presumably with dry hand and not applied directly across the
heart 8-), because of the high impedance connection in each device to
the mains supply (about 10 000 000 ohms?) - however these currents
may prove to be too much for some electronic devices -

They may also take one by surprise if holding a lead when connecting
such equipment together!!!!! - albeit not fatal unless you step back
and trip over something dangerous behind you 8-)

Yes, I get this. My computer has a tv card. The aerial lead to the TV
card always generates a spark when connecting to the PC. At the other
end of the aerial lead (eventually) is a sky receiver downstairs.
Connected to the same TV that that sky receiver is connected to is a DVD
player which is double insulated and not earthed. If I unplug the DVD
player from the mains, I no longer get the sparks! And if I don't, and I
happen to be touching the pc case and the coax plug at the same time and
forget what is about to happen, the very very minor shock takes me by
suprise.

Interestingly, although the DVD player is double insulated, it does have
an earth screw terminal on the back. Maybe I should earth it to
something :)

Phil: An 'earth screw' or a chassis ground screw?????

--
Phil Spiegelhalter:
==== Technical Training for Broadcasters =====
*RE CUE Mobile DV Multi-Camera Production and Non-Linear Editing*

Phil wrote:
In article ,
letters desk wrote:
Phil wrote:

2/ Some domestic equipment, such as computers, increasingly
connected into the system, conventionally use 3-wire connections in
the UK - ie the chassis is at earth potential.

If/when 2-wire and 3-wire devices are connected together, the
''chassis'' can be at different voltages (115V difference) and
therefore a small current can flow between them on ANY connecting
leads used:

The currents are SMALL and, apparently considered quite safe (to
humans - presumably with dry hand and not applied directly across
the heart 8-), because of the high impedance connection in each
device to the mains supply (about 10 000 000 ohms?) - however these
currents may prove to be too much for some electronic devices -

They may also take one by surprise if holding a lead when connecting
such equipment together!!!!! - albeit not fatal unless you step back
and trip over something dangerous behind you 8-)

Yes, I get this. My computer has a tv card. The aerial lead to the TV
card always generates a spark when connecting to the PC. At the other
end of the aerial lead (eventually) is a sky receiver downstairs.
Connected to the same TV that that sky receiver is connected to is a
DVD player which is double insulated and not earthed. If I unplug
the DVD player from the mains, I no longer get the sparks! And if I
don't, and I happen to be touching the pc case and the coax plug at
the same time and forget what is about to happen, the very very
minor shock takes me by suprise.

Interestingly, although the DVD player is double insulated, it does
have an earth screw terminal on the back. Maybe I should earth it to
something :)

Phil: An 'earth screw' or a chassis ground screw?????

A machine screw complete with 3 washers going into the back panel, with
an earth symbol next to it.

In message ,
Phil wrote:

In articles (various) re apparent problem.

1/ As mentioned previously, most domestic appliances (TVs etc category)
are 2-wire connected.
These devices therefore have a 'chassis' of MID VOLTAGE ie 115V (230-0 /2)
which is not what most would consider 'earth' potential - as in the 3rd
pin.
(The 'chassis' is connected via high-impedance to the power supply, to
stop it floating in total isolation, uncontrollably.)

Where did you learn this?

In the UK, a "three wire" connected device, as you call it, does (or
should) indeed have any exposed metal parts of its chassis connected to
the earth conductor in the mains wiring. Therefore these parts will be
at "earth" potential.

[Aside: here's the bit which scares some people - in many installations
the earth will be connected directly to the neutral, either where they
come into your house (PME or TN-C-S system) or at the transformer
(TN-S).]

Here's where I think you are mistaken:

If a device has only a "two wire" connection then in order to be legal
in the UK and indeed in Europe it must be "double insulated". This means
that at all times there are *two* layers of insulation between any live
parts (live or neutral supplies) and any exposed metalwork. Such a
chassis is therefore not really at *any* voltage relative to the supply.

If it is, especially if you can measure it through a low-ish
resistance meter, there's a problem. Get it sorted.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Unable to load REALITY.SYS -- Invalid Parameter: /UTOPIA

"Martin Angove" wrote in message
...
In message ,
Phil wrote:

In articles (various) re apparent problem.

1/ As mentioned previously, most domestic appliances (TVs etc category)
are 2-wire connected.
These devices therefore have a 'chassis' of MID VOLTAGE ie 115V (230-0
/2)
which is not what most would consider 'earth' potential - as in the 3rd
pin.
(The 'chassis' is connected via high-impedance to the power supply, to
stop it floating in total isolation, uncontrollably.)

Where did you learn this?

I don't know, but it certainly doesn't follow by any teaching. Apologies if
I'm stating the obvious, and I'm addressing this to the post to which you
reply.

Firstly, the 230V which is quoted as UK mains voltage is RMS (root mean
square). If you just took the mean, you'd end up with zero, because we use
alternating current which fluctuates between about -330 and +330V. You'd
have a struggle getting 115V from anywhere.

Secondly, I'd be worried if you ever had anything other than earth connected
to a chassis - that indicates a serious miswiring.

[Aside: here's the bit which scares some people - in many installations
the earth will be connected directly to the neutral, either where they
come into your house (PME or TN-C-S system) or at the transformer
(TN-S).]

Are you sure about that? I thought some wiring systems would trip if earth
and neutral are connected together.

In article , altgrr
wrote:
"Martin Angove" wrote in message
...
In message , Phil
wrote:

In articles (various) re apparent problem.

1/ As mentioned previously, most domestic appliances (TVs etc
category) are 2-wire connected. These devices therefore have a
'chassis' of MID VOLTAGE ie 115V (230-0
/2)
which is not what most would consider 'earth' potential - as in the
3rd pin. (The 'chassis' is connected via high-impedance to the power
supply, to stop it floating in total isolation, uncontrollably.)

Where did you learn this?

I don't know, but it certainly doesn't follow by any teaching.
Apologies if I'm stating the obvious, and I'm addressing this to the
post to which you reply.
Phil: Please note that despite the wording above, this is not a reply by
me to the quote above.
Firstly, the 230V which is quoted as UK mains voltage is RMS (root mean
square). If you just took the mean, you'd end up with zero, because we
use alternating current which fluctuates between about -330 and +330V.
You'd have a struggle getting 115V from anywhere.
Phil: This is totally irrelevant to the point being made.
Of course I am well aware that 230v a.c. is the rms voltage (+/- 20%, of
nominal 50Hz +/- 2Hz uk / european electricity mains supply.

Secondly, I'd be worried if you ever had anything other than earth
connected to a chassis - that indicates a serious miswiring.

Phil: You are confusing 2-wire, and 3-wire devices!!!! amongst other
things.
3-wire devices, I would /expect/ the 'chassis' to be at the earth
potential (and this signified by an earth symbol, not simply chassis
symbol), but with a 2-wire device, it might be either at a live rail
potential (chassis potential as in a circuit diagram - not the externally
touchable part of a casing), or if it used a switchedmode power supply, to
be at the potential of 1 side of the secondary - BUT the point I was
making is as to what THAT potential might be - to avoid it floating freely.

[Aside: here's the bit which scares some people - in many
installations the earth will be connected directly to the neutral,
either where they come into your house (PME or TN-C-S system) or at
the transformer (TN-S).]

Are you sure about that? I thought some wiring systems would trip if
earth and neutral are connected together.

Phil: I'm not sure if either of you, in quoting my reply to an earlier
item, have read it or digested its content appropriately!

Discussion of rms or otherwise voltage is totally irrelevant!
I also gave a specific, industry example, of a public situation where the
'otherwise floating' rails are ties VIA HIGH IMPEDANCE to the source side.
Perhaps that the underground uses d.c confused you here.

If a 'double insulated' device is left TOTALLY FLOATING, yet has an
external set of CONNECTIONS as does a TV set with VIDEO/AUDIO sockets,
then there is NO CONTROL of the potential to which the connectors may
float (and with just a 1 volt difference between the 2 of them).
THAT IS WHY there is a HIGH IMPEDANCE link between the live chassis side
and the secondary side - to stop the 'floating' and this therefore could
set it 'either side' of the live (2 wire reversible connectors) or a mid
average level.
It IS in the regulations - and I find it 'shocking' - but the current
permitted is too small to cause injury by electric shock (unless / even
with wet hands across a heart?) - hence my comment about the 'surprise'
shock element which may give rise to injury indirectly.
You might regard this level of 'shock' as a 'static' discharge as you
provide a path to connect the 'floating'-almost side to another potential.
(The impedance is over 1M, possibly 10M, - that I can't recall - probably
because I think it was referred to in the regulations more by the maximum
leakage current in terms of its maximum safe limit)

--
Phil Spiegelhalter:
==== Technical Training for Broadcasters =====
*RE CUE Mobile DV Multi-Camera Production and Non-Linear Editing*

"Phil" wrote in message
...
In article , altgrr
wrote:
"Martin Angove" wrote in message
...
In message , Phil
wrote:

In articles (various) re apparent problem.

1/ As mentioned previously, most domestic appliances (TVs etc
category) are 2-wire connected. These devices therefore have a
'chassis' of MID VOLTAGE ie 115V (230-0
/2)
Firstly, the 230V which is quoted as UK mains voltage is RMS (root mean
square). If you just took the mean, you'd end up with zero, because we
use alternating current which fluctuates between about -330 and +330V.
You'd have a struggle getting 115V from anywhere.
Phil: This is totally irrelevant to the point being made.
Of course I am well aware that 230v a.c. is the rms voltage (+/- 20%, of
nominal 50Hz +/- 2Hz uk / european electricity mains supply.

So where do you get your 115V from?

Secondly, I'd be worried if you ever had anything other than earth
connected to a chassis - that indicates a serious miswiring.

Phil: You are confusing 2-wire, and 3-wire devices!!!! amongst other
things.

No, not at all. Regardless of whether a device has two or three wires
coming into it, the only thing I'd expect to be connected to a chassis
(which I inferred as casing from your post and the reply to it) is the
earth.

3-wire devices, I would /expect/ the 'chassis' to be at the earth
potential (and this signified by an earth symbol, not simply chassis
symbol), but with a 2-wire device, it might be either at a live rail
potential (chassis potential as in a circuit diagram - not the externally
touchable part of a casing), or if it used a switchedmode power supply, to
be at the potential of 1 side of the secondary - BUT the point I was
making is as to what THAT potential might be - to avoid it floating
freely.

[Aside: here's the bit which scares some people - in many
installations the earth will be connected directly to the neutral,
either where they come into your house (PME or TN-C-S system) or at
the transformer (TN-S).]

Are you sure about that? I thought some wiring systems would trip if
earth and neutral are connected together.

Phil: I'm not sure if either of you, in quoting my reply to an earlier
item, have read it or digested its content appropriately!

I'm not sure why you raise this here - I seem to be replying directly to
your point.

Discussion of rms or otherwise voltage is totally irrelevant!
I also gave a specific, industry example, of a public situation where the
'otherwise floating' rails are ties VIA HIGH IMPEDANCE to the source side.
Perhaps that the underground uses d.c confused you here.

Where, for this paragraph, "you" refers to the author of the post I replied
to, not me, presumably?

If a 'double insulated' device is left TOTALLY FLOATING, yet has an
external set of CONNECTIONS as does a TV set with VIDEO/AUDIO sockets,
then there is NO CONTROL of the potential to which the connectors may
float (and with just a 1 volt difference between the 2 of them).

Was something missing from your parenthesis? I'm not entirely sure what you
mean here.

THAT IS WHY there is a HIGH IMPEDANCE link between the live chassis side
and the secondary side - to stop the 'floating' and this therefore could
set it 'either side' of the live (2 wire reversible connectors) or a mid
average level.

Calm down, there's no need to shout!

Apologies for stirring up an argument :-) Just a couple of things,
heavily snipped...

In message ,
Phil wrote:


3-wire devices, I would /expect/ the 'chassis' to be at the earth
potential (and this signified by an earth symbol, not simply chassis
symbol), but with a 2-wire device, it might be either at a live rail
potential (chassis potential as in a circuit diagram - not the externally
touchable part of a casing), or if it used a switchedmode power supply, to
be at the potential of 1 side of the secondary - BUT the point I was
making is as to what THAT potential might be - to avoid it floating freely.


Ok, fine. You are using "chassis" in what seems to me to be an
old-fashioned sense, but I will concede the point that you are referring
to the "ground" or "0V" potential of the *circuitry*, not to the actual
potential of the casing of the device. In that case, what I wrote
probably doesn't make much sense!

If a 'double insulated' device is left TOTALLY FLOATING, yet has an
external set of CONNECTIONS as does a TV set with VIDEO/AUDIO sockets,
then there is NO CONTROL of the potential to which the connectors may
float (and with just a 1 volt difference between the 2 of them).
THAT IS WHY there is a HIGH IMPEDANCE link between the live chassis side
and the secondary side - to stop the 'floating' and this therefore could
set it 'either side' of the live (2 wire reversible connectors) or a mid
average level.

This is where we need to hear from someone who designs double-insulated
HiFi gear. I am more electrically-biased these days, though I have an
electronics background. Ignoring switched-mode PSUs for the moment, if
you have a conventional isolating transformer - rectifier - capacitor -
regulator powersupply, then the thing is so totally floating that it
couldn't care less what you connect it to. As soon as you connect it,
it simply and without fuss becomes referenced to the thing you have
connected it to.

One side of the transformer winding becomes the circuit ground (what you
refer to as the chassis) and the other becomes the power rail. An
(non-isolated) external interface will cause the ground of some other power
supply to be electrically connected to the ground of the power supply in
question, but *it doesn't matter*. Even if the second supply wasn't truly
floating, the first just floats up to meet it. How on earth else is it possible
to interface battery powered devices to mains powered devices?

The danger would come if the circuit ground was indeed connected to one
or other of the live supplies - ancient radios used to do this. If all
equipment is used on a single phase and everything is referenced to
neutral then this might just work, but imagine the case where one device
references to neutral, and another to *earth*. In practically every
electrical installation with anything more than a clock connected, the
neutral will be at a significantly different potential to the earth.
This is how, even in a good installation, connecting neutral and earth
together (such as might be done when you cut a cable), even where live
is not involved, can cause an RCD to trip. A normal RCD will trip if
something above 15mA flows. [but see my reply to "altgrr"]

It IS in the regulations - and I find it 'shocking' - but the current
permitted is too small to cause injury by electric shock (unless / even
with wet hands across a heart?) - hence my comment about the 'surprise'
shock element which may give rise to injury indirectly.

Which "regulations"? The "wiring regulations" (BS7671:2001 r713-04)
state that for a 240V circuit there should be at least 500kOhms
resistance between each conductor and each other, the test voltage being
500V d.c. The guidance suggests that a resistance reading under 2MOhms
should be taken as a sign that all is not well. I don't think this
applies to the user interfaces of consumer equipment though :-)

You might regard this level of 'shock' as a 'static' discharge as you
provide a path to connect the 'floating'-almost side to another potential.
(The impedance is over 1M, possibly 10M, - that I can't recall - probably
because I think it was referred to in the regulations more by the maximum
leakage current in terms of its maximum safe limit)


BS7671 attempts to limit discharge of energy mainly in order to protect
cables and flexes. This is done through the use of fuses and circuit
breakers. Additional precautions are required on certain socket outlets,
which usually results in the installation of an RCD. An RCD limits the
discharge of energy through a person and one can imply what the powers
reckon to be safe limits by the following information:

A (normal domestic use) RCD must

NOT trip if the leakage current is 15mA or less
TRIP within 300mS if the leakage current is 30mA
TRIP within 40mS if the leakage current is 5*30mA (150mA) or higher.

I'm not sure where this argument is leading. Perhaps I'd better stop
there :-)

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Motel mattresses are better on the side away from the phone.

In message ,
"altgrr" wrote:

"Martin Angove" wrote in message
...

[Aside: here's the bit which scares some people - in many installations
the earth will be connected directly to the neutral, either where they
come into your house (PME or TN-C-S system) or at the transformer
(TN-S).]

Are you sure about that? I thought some wiring systems would trip if earth
and neutral are connected together.



Earth and neutral will be connected together somewhere in all but very
specialised installations. In this country (uk) there are basically
three sorts of domestic installation:

TT (terre-terre): The neutral is physically connected to the planet
Earth at the transformer. Your supply is live and neutral, and you make
your own earth by connecting to an earth rod or plate or similar. In
this case a live-earth fault in your installation causes a current to
flow through your earth rod, through the bulk of the planet Earth and
back to the neutral at the transformer. Done properly, this can work
very well, but it can also lead to an impedance too high to safely blow
a fuse or trip an MCB. For this reason it is required that TT systems
have an RCD protecting *all* circuits (note: this differs from a
standard 30mA RCD). An RCD will trip at much lower levels of fault
current than an MCB or fuse.

TT systems are now mainly seen in rural areas with overhead supply,
although these are being converted to TN-C-S (see below).

TN-S (Terre Neutral - Separated): The neutral is physically connected to
the Earth at the transformer. Your supply is live, neutral *and* earth -
usually the metal sheath of the supply cable. In this case a live-earth
fault in your installation causes a current to flow through the
supplier's earth back to the neutral at the transformer. This path is
usually good enough to blow a fuse under such a fault so an overall RCD
is not required, though RCD protection is required for certain outlets.

TN-S systems are commonly found in urban areas, though they are being
supplanted by

TN-C-S (Terre Neutral - Combined [for the supply] - Separated [in the
installation]), also known as PME (Protective Multiple Earthing): The
neutral is physically connected to the Earth at the transformer, and at
several points along the route from transformer to your house. Your
supply is live and neutral, and your installation's earth is connected
directly to the incoming neutral at the supplier's "cut off" (main
fuse). As you may surmise, a live-earth fault in this situation also
causes plenty of current to flow to blow a fuse. The problem with a PME
system is that if someone puts a digger through the cable, severing the
neutral you also lose your safety earth, and if the live is intact this
can be dangerous. There are very strict regulations concerning "bonding"
exposed metalwork and electrical earths together in these circumstances.
RCD requirements are similar to TN-S.

Regarding your point about systems "tripping" if neutral and earth are
connected, in any of these cases (particularly in the TT system) there
will be a difference in potential between the neutral in the
installation and the earth whenever something is switched on. This is
described by Ohm's Law regarding resistances, flow of current and
voltages. If you have an RCD fitted, it may only take 15mA flowing down
the earth instead of the neutral for the thing to trip.

As I said however, the earth will (at some point) be connected to the
neutral quite deliberately.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... See that slate, That's your keyboard that is.

"Martin Angove" wrote in message
...
Here's where I think you are mistaken:

If a device has only a "two wire" connection then in order to be legal
in the UK and indeed in Europe it must be "double insulated". This
means
that at all times there are *two* layers of insulation between any
live
parts (live or neutral supplies) and any exposed metalwork. Such a
chassis is therefore not really at *any* voltage relative to the
supply.

These days a TV usually has a switching power supply so that if nothing
was done to make it otherwise the circuitry including various external
connectors (chassis) would be floating.

However this would be dangerous - in a TV there are parts of the TV with
are anything up to 25000 volts or so positive with respect of the
external connectors. If everything was just left to float the connectors
could easily end up with a massive voltage on them. You would get a
nasty jolt if this discharged through you. To stop this happening two
high value resistors in series are including between one of the mains
inputs and the chassis. The resistors value is high enough to count as
an insulator for double insulated.

--

Brian Gregory (In the UK).

To email me remove the letter vee.

So where do you get your 115V from?

Phil: As per the original query - about blowing things up when items were
CONNECTED via SCARTs: - and to which my original reply was directed....


If, albeit via High Impedance, one side of the electrical (not physical
case) chassis is connected to a power rail, to stop it floating, and the
next device is similarly connected, then the potential of the 'shield'
connector (and by definition, almost the same for the signal connectors)
might be at 'Neutral', 'Live' OR (50:50 chance) 115v (mid voltage) since
there is no identification externally for the orientation of a telefunken
or similar 2 wire connector: therefore if device A is connected / tied to
live via say 10M, and the 2nd device is tied to neutral via 10M: result of
both devices is now a compromised 115v value (simple potential divider) -
and if another connector is touched / made to a third device, this will be
experienced at this potential.

The safety regulations are satisfied because the leakage current may be
measured in micro amps - but this could still damage components, and cause
'surprise' to someone touching the connector - causing a subsequent
accident.

We have experienced also, cases where individual items of equipment show a
minimal leakage (eg 3microA) - but the combined effect of several devices
becomes enough to trip a sensitive RCCB

--
Phil Spiegelhalter:
==== Technical Training for Broadcasters =====
*RE CUE Mobile DV Multi-Camera Production and Non-Linear Editing*

In message ,
Phil wrote:


If, albeit via High Impedance, one side of the electrical (not physical
case) chassis is connected to a power rail, to stop it floating, and the
next device is similarly connected, then the potential of the 'shield'
connector (and by definition, almost the same for the signal connectors)
might be at 'Neutral', 'Live' OR (50:50 chance) 115v (mid voltage) since
there is no identification externally for the orientation of a telefunken
or similar 2 wire connector: therefore if device A is connected / tied to
live via say 10M, and the 2nd device is tied to neutral via 10M: result of
both devices is now a compromised 115v value (simple potential divider) -
and if another connector is touched / made to a third device, this will be
experienced at this potential.

I understand your reasoning now. I'm still not convinced it's a good
idea, but if it is true I can see that what you're saying might happen.


The safety regulations are satisfied because the leakage current may be
measured in micro amps - but this could still damage components, and cause
'surprise' to someone touching the connector - causing a subsequent
accident.

We have experienced also, cases where individual items of equipment show a
minimal leakage (eg 3microA) - but the combined effect of several devices
becomes enough to trip a sensitive RCCB


Surely though this will only happen if there is an earth connection
somewhere? For 2-wire (no earth) devices the only place current can flow
is within the L/N circuit.

It is a well known fact that some 3-wire devices, particularly those
with filters on their mains inputs (computers for example) leak quite
badly to earth as a matter of course. There is even a section in
BS7671:2001 dedicated to allowing for these "high protective conductor
currents". Basically it involves using high integrity earth conductors
and splitting things onto several RCDs, if indeed an RCD is necessary at
all. As I said somewhere else, a normal RCD should not trip below 15mA
leakage, but anything above this and it is quite likely that it will,
though 30mA is the official design current.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Help! I'm parked diagonally in a parallel universe.

"Martin Angove" wrote in message
...
In message ,
Phil wrote:

In articles (various) re apparent problem.

1/ As mentioned previously, most domestic appliances (TVs etc category)
are 2-wire connected.
These devices therefore have a 'chassis' of MID VOLTAGE ie 115V (230-0
/2)
which is not what most would consider 'earth' potential - as in the 3rd
pin.
(The 'chassis' is connected via high-impedance to the power supply, to
stop it floating in total isolation, uncontrollably.)

Where did you learn this?


What Martin said here is correct.

In the UK, a "three wire" connected device, as you call it, does (or
should) indeed have any exposed metal parts of its chassis connected to
the earth conductor in the mains wiring. Therefore these parts will be
at "earth" potential.

Ok, but I thought we were talking about Class 2 devices.

[Aside: here's the bit which scares some people - in many installations
the earth will be connected directly to the neutral, either where they
come into your house (PME or TN-C-S system) or at the transformer
(TN-S).]

Here's where I think you are mistaken:

If a device has only a "two wire" connection then in order to be legal
in the UK and indeed in Europe it must be "double insulated". This means
that at all times there are *two* layers of insulation between any live
parts (live or neutral supplies) and any exposed metalwork. Such a
chassis is therefore not really at *any* voltage relative to the supply.


It can well be. I regularly get (unharmful) shocks from TV and STBs when
I'm connected them together due to the different chassis voltage.

The double insulted thing is not meant to be taken literally at all times,
for instance sufficient air gaps are acceptable, also components can be
connected across primary and secondard without 'double insulation' however
these components are covered by strict contruction and quality assurance
rules.

If it is, especially if you can measure it through a low-ish
resistance meter, there's a problem. Get it sorted.



Not correct, there can be up to 4Mohm between primary and exposed metal
parts in a class 2 device.

--
Tony Stanley ++Always Learning++