I measure 101 volts AC between my Sun Blade and my PC on the serial
connector - who's at fault, does anybody have any ideas?
(it doesn't feel very nice to get between them)

Charles T. Smith wrote:
> I measure 101 volts AC between my Sun Blade and my PC on the serial
> connector - who's at fault, does anybody have any ideas?
> (it doesn't feel very nice to get between them)
>

One or both of the machines has a open or faulty ground. Try a
continuity / low ohms / lamp test between each ground pin on the plug
and the system chassis. Probably a power cord fault, though it could
also be a faulty socket strip.

All machines with switched mode psu's *must* be grounded for safety -
they often have high leakage currents...

Chris


--

----------------------
Greenfield Designs Ltd
Electronic and Embedded System Design
Oxford, England
(44) 1865 750 681

On Mon, 09 Apr 2007 16:41:53 +0000, ChrisQuayle wrote:

> Charles T. Smith wrote:
>> I measure 101 volts AC between my Sun Blade and my PC on the serial
>> connector - who's at fault, does anybody have any ideas? (it doesn't
>> feel very nice to get between them)
>>
>>
> One or both of the machines has a open or faulty ground. Try a continuity
> / low ohms / lamp test between each ground pin on the plug and the system
> chassis. Probably a power cord fault, though it could also be a faulty
> socket strip.
>
> All machines with switched mode psu's *must* be grounded for safety - they
> often have high leakage currents...
>
> Chris

Thank you for the information-rich answer

Charles T. Smith wrote:
> I measure 101 volts AC between my Sun Blade and my PC on the serial
> connector - who's at fault, does anybody have any ideas?
> (it doesn't feel very nice to get between them)

I assume you found out the hard way, so the potential would be between
the frame grounds of the two machines. Then, using a volt meter, you
measured 101 volts AC.

Normally this is reasonably harmless. It is usually because of a "ground
loop", where the cable length between the one serial connector through
the mains ground of the one computer to the mains ground of the other
computer, to the serial connector is long enough to pick up a buzz from
the near-lying live power lines, through induction.

You can check if this is the case when you have a light bulb (110v
preferably). Hold that between the serial connectors, and then measure
voltage again. It should be *much* lower.

If that's the case, you can connect the serial cables[*]. Just make sure
the frame ground (connector shielding) touches before the signal pins do.

[*] Disclaimer: Whatever happens, its *not* my fault.

--
Alex.

Alex van Denzel wrote:
> Charles T. Smith wrote:
>
>> I measure 101 volts AC between my Sun Blade and my PC on the serial
>> connector - who's at fault, does anybody have any ideas?
>
> > (it doesn't feel very nice to get between them)
>
> I assume you found out the hard way, so the potential would be between
> the frame grounds of the two machines. Then, using a volt meter, you
> measured 101 volts AC.
>
> Normally this is reasonably harmless. It is usually because of a "ground
> loop", where the cable length between the one serial connector through
> the mains ground of the one computer to the mains ground of the other
> computer, to the serial connector is long enough to pick up a buzz from
> the near-lying live power lines, through induction.

If the two frames are connected with a DC connection to a common point
(it should be the earth), then there is no way to get 101 V between
them. Voltages induced by ground loops will be very small - sufficient
to causes a buzz in audio systems, but not to be felt by hand.

If there is 101 V which can be felt by hand, then the situation need
sorting out.
--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form: month-year@althorne.org
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

On Wed, 11 Apr 2007 17:01:57 +0100, Dave (from the UK) wrote:

> Alex van Denzel wrote:
>> Charles T. Smith wrote:
>>
>>> I measure 101 volts AC between my Sun Blade and my PC on the serial
>>> connector - who's at fault, does anybody have any ideas?
>>
>> > (it doesn't feel very nice to get between them)
>>
>> I assume you found out the hard way, so the potential would be between
>> the frame grounds of the two machines. Then, using a volt meter, you
>> measured 101 volts AC.
>>
>> Normally this is reasonably harmless. It is usually because of a "ground
>> loop", where the cable length between the one serial connector through
>> the mains ground of the one computer to the mains ground of the other
>> computer, to the serial connector is long enough to pick up a buzz from
>> the near-lying live power lines, through induction.
>
> If the two frames are connected with a DC connection to a common point (it
> should be the earth), then there is no way to get 101 V between them.
> Voltages induced by ground loops will be very small - sufficient to causes
> a buzz in audio systems, but not to be felt by hand.
>
> If there is 101 V which can be felt by hand, then the situation need
> sorting out.


Yikes! Okay, guys, thanks for the knowledge.

I checked the ground-plug-to-case on both machines and back from the
extension cords to the wall, and like, there's 0.5 ohms (even in the
trivial case).

The two machines are plugged into outlets on different walls, though.
It's starting to sound like it's OT WRT sun hardware and On-Thread in
home-electrical-wiring (which I did NOT do in this case).

By the way, 101 volts is less than 50% normal, not almost 100%.

Charles T. Smith wrote:

> Yikes! Okay, guys, thanks for the knowledge.
>
> I checked the ground-plug-to-case on both machines and back from the
> extension cords to the wall, and like, there's 0.5 ohms (even in the
> trivial case).

There is something not quite right. The two items which have 101V
between them can't be commened to somewhere via a resistance of only 0.5
Ohms.

100V across 0.5 Ohms is 200A and 20 kW. I somewhat doubt this is so,
since it would get every so slightly warm!!

It may be the connectors on your serial ports are not tied to the
chassis. Normally the metal cases would be.
--
Dave (from the UK)

Please note my email address changes periodically to avoid spam.
It is always of the form: month-year@althorne.org
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/ - a Free open-source Chess Database

feb-2007@althorne.org writes:
>Charles T. Smith wrote:
>
>> Yikes! Okay, guys, thanks for the knowledge.
>>
>> I checked the ground-plug-to-case on both machines and back from the
>> extension cords to the wall, and like, there's 0.5 ohms (even in the
>> trivial case).
>
>There is something not quite right. The two items which have 101V
>between them can't be commened to somewhere via a resistance of only 0.5
>Ohms.
>

They don't have 0.5 ohms between each other, they each have 0.5 ohms
to the ground/earth pin of their wall outlets.

So the wall outlets have 101 Volts difference between them. Not hard
to do if they're fed from different secondary windings from the same
power transformer and/or one of the outlets has some dodgy ground/earth
wiring.

-Greg
--
Do NOT reply via e-mail.
Reply in the newsgroup.

On Fri, 13 Apr 2007 03:14:15 +0000, Greg Andrews wrote:

> feb-2007@althorne.org writes:
>>Charles T. Smith wrote:
>>
>>> Yikes! Okay, guys, thanks for the knowledge.
>>>
>>> I checked the ground-plug-to-case on both machines and back from the
>>> extension cords to the wall, and like, there's 0.5 ohms (even in the
>>> trivial case).
>>
>>There is something not quite right. The two items which have 101V between
>>them can't be commened to somewhere via a resistance of only 0.5 Ohms.
>>
>>
> They don't have 0.5 ohms between each other, they each have 0.5 ohms to
> the ground/earth pin of their wall outlets.


Right, that's what I wanted to say.


> So the wall outlets have 101 Volts difference between them. Not hard to
> do if they're fed from different secondary windings from the same power
> transformer and/or one of the outlets has some dodgy ground/earth wiring.


And this gives me a clue of what I have to do - check each socket's ground
line against the radiator (I'll need a long line to do it).

Dave (from the UK) wrote:
> Voltages induced by ground loops will be very small - sufficient
> to causes a buzz in audio systems, but not to be felt by hand.

They definitely can be felt by hand!

I've felt it myself when I wanted to connect to Thinwire ethernet
segments together in. About 50 meters on each end, stretched across a
building, run through a cable conduit where also 220V cables were
installed. Each segment was grounded at the other end. Me in the middle
holding both metal BNC connectors.

Also, the stories about musician getting electric shocks when their
guitar amp was grounded at another outlet as the microphone system, are
true.

The induced voltages can be quite high, when measured with a volt-meter
with a high internal resistance (100V is no exception, obviously at 50
or 60 Hz), but the current can't get much higher than a few microamperes
when shorted over a human body. Just enough to give you a scare, not
enough so you can't drop the cables (or microphone).

--
Alex.