Job today installing a little wall mounted audio amp in the boardroom of
a factory. Turned on, got loads of hum when the input cable is in which
I put down to the crappy leads the gaffer buys (the screen is run
parallel to the signal rather than being wrapped round it. But they're
cheap he says. Grr.)

So I idly go to ground the floating input, touching it and an earthed
case, to see if it helps. And get stung.

Dig out the DVM and find mains earth floating at about 80V over, erm,
well me in rubber soles and fresh air. I mention this and they'll give
it to their sparks.

So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?

--
Scott

Where are we going and why am I in this handbasket?

In article ,
Scott M wrote:

Job today installing a little wall mounted audio amp in the boardroom of
a factory. Turned on, got loads of hum when the input cable is in which
I put down to the crappy leads the gaffer buys (the screen is run
parallel to the signal rather than being wrapped round it. But they're
cheap he says. Grr.)

So I idly go to ground the floating input, touching it and an earthed
case, to see if it helps. And get stung.

Dig out the DVM and find mains earth floating at about 80V over, erm,
well me in rubber soles and fresh air. I mention this and they'll give
it to their sparks.

So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?

80v with respect to what? I can understand neutral going up wrt earth.

--
From KT24

Using a RISC OS computer running v5.18

charles wrote:
In article ,
Scott M wrote:


So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?

80v with respect to what? I can understand neutral going up wrt earth.

Me! One probe on mains earth and one to my index finger. I suppose to be
accurate I was representing the "real" earth and would have got the same
result had I leant out of the window and shoved the probe into a flower bed.

Unless I'm simply full of electricity ;-)

--
Scott

Where are we going and why am I in this handbasket?

"Scott M" wrote in message ...

charles wrote:
In article ,
Scott M wrote:


So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?

80v with respect to what? I can understand neutral going up wrt earth.

Me! One probe on mains earth and one to my index finger. I suppose to be
accurate I was representing the "real" earth and would have got the same
result had I leant out of the window and shoved the probe into a flower
bed.

Unless I'm simply full of electricity ;-)


Was it a high impedance digital meter - if so the reading is probably false
and caused by your body picking up mains hum. Do it with an 'old fashioned'
analogue Avometer and get the same results and I'll be convinced.

Andrew

On Mon, 08 Dec 2014 22:33:27 +0000, Scott M
wrote:

charles wrote:
In article ,
Scott M wrote:


So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?

80v with respect to what? I can understand neutral going up wrt earth.

Me! One probe on mains earth and one to my index finger. I suppose to be
accurate I was representing the "real" earth and would have got the same
result had I leant out of the window and shoved the probe into a flower bed.

Unless I'm simply full of electricity ;-)

I can't be totally sure, but nothing you have posted makes me think
there is anything dangerous or particularly abnormal about this
installation. The amp is fed with by a double insulated PSU with no
earth, right?



--

Graham.

%Profound_observation%

Andrew Mawson wrote:

Was it a high impedance digital meter - if so the reading is probably
false and caused by your body picking up mains hum. Do it with an 'old
fashioned' analogue Avometer and get the same results and I'll be
convinced.

Good point, overlooked that one. But the shock certainly felt more than
a bit of inductive pickup. Same meter at home gets me 5v in the same
setup, rising to 15v if I grasp a mains cable in my hand, quite a bit
lower than earlier.

--
Scott

Where are we going and why am I in this handbasket?

Graham. wrote:
On Mon, 08 Dec 2014 22:33:27 +0000, Scott M
wrote:

charles wrote:
In article ,
Scott M wrote:

So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?
80v with respect to what? I can understand neutral going up wrt earth.
Me! One probe on mains earth and one to my index finger. I suppose to be
accurate I was representing the "real" earth and would have got the same
result had I leant out of the window and shoved the probe into a flower bed.

Unless I'm simply full of electricity ;-)

I can't be totally sure, but nothing you have posted makes me think
there is anything dangerous or particularly abnormal about this
installation. The amp is fed with by a double insulated PSU with no
earth, right?

Yep, that's the one.

--
Scott

Where are we going and why am I in this handbasket?

On Mon, 08 Dec 2014 22:55:01 +0000, Andrew Mawson wrote:

Was it a high impedance digital meter - if so the reading is probably
false and caused by your body picking up mains hum. Do it with an 'old
fashioned'
analogue Avometer and get the same results and I'll be convinced.

Yeah, but he said he got a belt off it, which implies something other
than just a duff reading of a phantom voltage.
I personally can't feel anything less than 80v; for me, that's when a
tingle first becomes perceptible.

In message , cd writes
On Mon, 08 Dec 2014 22:55:01 +0000, Andrew Mawson wrote:

Was it a high impedance digital meter - if so the reading is probably
false and caused by your body picking up mains hum. Do it with an 'old
fashioned'
analogue Avometer and get the same results and I'll be convinced.

Yeah, but he said he got a belt off it, which implies something other
than just a duff reading of a phantom voltage.
I personally can't feel anything less than 80v; for me, that's when a
tingle first becomes perceptible.

Unless you're really thick-skinned, I'd be surprised if you couldn't
feel 80V from a low-impedance source.


--
Ian

On Tue, 09 Dec 2014 20:40:37 +0000, Ian Jackson wrote:

Unless you're really thick-skinned, I'd be surprised if you couldn't
feel 80V from a low-impedance source.


Oil-cooled welding transformer low impedance enough for you? The point I
was trying to make was that I can't feel anything *at all* below 80v
(provided my hands are dry) YMMV. I have a 3A mains variac as well. Maybe
over xmas I'll try out some higher voltages and see where it becomes
uncomfortable. We'll see... I don't recommend others experimenting,
though!

cd wrote:
On Tue, 09 Dec 2014 20:40:37 +0000, Ian Jackson wrote:

Unless you're really thick-skinned, I'd be surprised if you couldn't
feel 80V from a low-impedance source.

Oil-cooled welding transformer low impedance enough for you? The point I
was trying to make was that I can't feel anything *at all* below 80v
(provided my hands are dry) YMMV. I have a 3A mains variac as well. Maybe
over xmas I'll try out some higher voltages and see where it becomes
uncomfortable. We'll see... I don't recommend others experimenting,
though!

Flowers to the crematorium please.

;-)

--
Scott

Where are we going and why am I in this handbasket?

In article ,
Scott M wrote:

Job today installing a little wall mounted audio amp in the boardroom of
a factory. Turned on, got loads of hum when the input cable is in which
I put down to the crappy leads the gaffer buys (the screen is run
parallel to the signal rather than being wrapped round it. But they're
cheap he says. Grr.)

So I idly go to ground the floating input, touching it and an earthed
case, to see if it helps. And get stung.

Dig out the DVM and find mains earth floating at about 80V over, erm,
well me in rubber soles and fresh air. I mention this and they'll give
it to their sparks.

So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?

What sort of audio cable? Balanced (two conductors plus screen) or
unbalanced (single plus screen)?

I'd want to know what the other end of the screen was connected to.

If unbalanced you will get hum if you connect the mains earth to the
signal screen at more than one point.

--
*IF YOU TRY TO FAIL, AND SUCCEED, WHICH HAVE YOU DONE?

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:

What sort of audio cable? Balanced (two conductors plus screen) or
unbalanced (single plus screen)?

I'd want to know what the other end of the screen was connected to.

If unbalanced you will get hum if you connect the mains earth to the
signal screen at more than one point.

Unbalanced, jack to phono, a short one. The amp's powered off a laptop
PSU brick so unearthed and had the faintest signs of hum when on, but as
soon as the lead was plugged in it got worse. Usually only suffer this
when there's a 10/20m run involved. When it's terminated it'll shut up
but, being for laptop presentations, that's not always.

It really wants better equipment but this is the bodget and scarper end
of the market and the powers that be don't know balanced from unbalanced
from an acre of danelion and burdock.

--
Scott

Where are we going and why am I in this handbasket?

In article ,
Scott M wrote:
Dave Plowman (News) wrote:

What sort of audio cable? Balanced (two conductors plus screen) or
unbalanced (single plus screen)?

I'd want to know what the other end of the screen was connected to.

If unbalanced you will get hum if you connect the mains earth to the
signal screen at more than one point.

Unbalanced, jack to phono, a short one. The amp's powered off a laptop
PSU brick so unearthed and had the faintest signs of hum when on, but as
soon as the lead was plugged in it got worse. Usually only suffer this
when there's a 10/20m run involved. When it's terminated it'll shut up
but, being for laptop presentations, that's not always.

It really wants better equipment but this is the bodget and scarper end
of the market and the powers that be don't know balanced from unbalanced
from an acre of danelion and burdock.

Is the source on a different mains phase? Put a DI box, with earth lift,
in the way.

--
From KT24

Using a RISC OS computer running v5.18

In article ,
Scott M wrote:
Dave Plowman (News) wrote:

What sort of audio cable? Balanced (two conductors plus screen) or
unbalanced (single plus screen)?

I'd want to know what the other end of the screen was connected to.

If unbalanced you will get hum if you connect the mains earth to the
signal screen at more than one point.

Unbalanced, jack to phono, a short one. The amp's powered off a laptop
PSU brick so unearthed and had the faintest signs of hum when on, but as
soon as the lead was plugged in it got worse. Usually only suffer this
when there's a 10/20m run involved. When it's terminated it'll shut up
but, being for laptop presentations, that's not always.

It's not uncommon to have high value resistors between line and chassis on
some designs - which will show volts on the chassis if the true earth
isn't good. Enough to feel or measure - but not enough to do you damage.
Unless you drop the amp on your foot in surprise...


It really wants better equipment but this is the bodget and scarper end
of the market and the powers that be don't know balanced from unbalanced
from an acre of danelion and burdock.

You should tell him about balanced. Would allow even cheaper cable for the
interconnect. ;-)

--
*ONE NICE THING ABOUT EGOTISTS: THEY DON'T TALK ABOUT OTHER PEOPLE.

Dave Plowman London SW
To e-mail, change noise into sound.

On 10/12/14 14:44, Dave Plowman (News) wrote:
In article ,
Scott M wrote:
Dave Plowman (News) wrote:

What sort of audio cable? Balanced (two conductors plus screen) or
unbalanced (single plus screen)?

I'd want to know what the other end of the screen was connected to.

If unbalanced you will get hum if you connect the mains earth to the
signal screen at more than one point.

Unbalanced, jack to phono, a short one. The amp's powered off a laptop
PSU brick so unearthed and had the faintest signs of hum when on, but as
soon as the lead was plugged in it got worse. Usually only suffer this
when there's a 10/20m run involved. When it's terminated it'll shut up
but, being for laptop presentations, that's not always.

It's not uncommon to have high value resistors between line and chassis on
some designs - which will show volts on the chassis if the true earth
isn't good. Enough to feel or measure - but not enough to do you damage.
Unless you drop the amp on your foot in surprise...


what is even MORE common is an RF filter with caps between line and
neutral to the chassis.

Puts the chassis and often the equipment ground at 110VAC with respect
to a proper earth. Enough as we discovered to blow the serial port of a
system connected to the offending laptop.



It really wants better equipment but this is the bodget and scarper end
of the market and the powers that be don't know balanced from unbalanced
from an acre of danelion and burdock.

You should tell him about balanced. Would allow even cheaper cable for the
interconnect. ;-)



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

In article ,
The Natural Philosopher wrote:
It's not uncommon to have high value resistors between line and
chassis on some designs - which will show volts on the chassis if the
true earth isn't good. Enough to feel or measure - but not enough to
do you damage. Unless you drop the amp on your foot in surprise...


what is even MORE common is an RF filter with caps between line and
neutral to the chassis.

Or a combination of resistors and capacitors. ;-)

Puts the chassis and often the equipment ground at 110VAC with respect
to a proper earth.

The volts don't matter - it's the current which does.


Enough as we discovered to blow the serial port of a
system connected to the offending laptop.

The serial cable should have connected both chassis together.

--
*My wife has a slight impediment in her speech. She stops to breathe.

Dave Plowman London SW
To e-mail, change noise into sound.

In message , "Dave Plowman (News)"
writes
In article ,
The Natural Philosopher wrote:
It's not uncommon to have high value resistors between line and
chassis on some designs - which will show volts on the chassis if the
true earth isn't good. Enough to feel or measure - but not enough to
do you damage. Unless you drop the amp on your foot in surprise...


what is even MORE common is an RF filter with caps between line and
neutral to the chassis.

Or a combination of resistors and capacitors. ;-)

Puts the chassis and often the equipment ground at 110VAC with respect
to a proper earth.

The volts don't matter - it's the current which does.

You can still get a lively tickle.

In a range of equipment I used to work with, the mains input had
something like 0.01uf capacitors from the live and neutral to chassis.
You certainly knew about it if the earth lead wasn't connected, and you
'offered up' the first interconnection with other earthed equipment. We
found that when there were about 15 if these units all running together,
the 30mA earth leakage trip used to pop.



Enough as we discovered to blow the serial port of a
system connected to the offending laptop.

The serial cable should have connected both chassis together.

See above. If you interconnect unearthed-chassis equipment, until the
first connection is firmly made, you can certainly damage something
fragile depending on which pin makes contact first.
--
Ian

On 10/12/14 17:12, Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
It's not uncommon to have high value resistors between line and
chassis on some designs - which will show volts on the chassis if the
true earth isn't good. Enough to feel or measure - but not enough to
do you damage. Unless you drop the amp on your foot in surprise...


what is even MORE common is an RF filter with caps between line and
neutral to the chassis.

Or a combination of resistors and capacitors. ;-)

Puts the chassis and often the equipment ground at 110VAC with respect
to a proper earth.

The volts don't matter - it's the current which does.


Enough as we discovered to blow the serial port of a
system connected to the offending laptop.

The serial cable should have connected both chassis together.


It did eventually, *after* the shield had striped the signal pins and
blown the input chip...


--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. €“ Erwin Knoll

charles wrote:

It really wants better equipment but this is the bodget and scarper end
of the market and the powers that be don't know balanced from unbalanced
from an acre of danelion and burdock.

Is the source on a different mains phase? Put a DI box, with earth lift,
in the way.

Nope, same twin socket!

--
Scott

Where are we going and why am I in this handbasket?

Dave Plowman (News) wrote:
Scott M wrote:

It really wants better equipment but this is the bodget and scarper end
of the market and the powers that be don't know balanced from unbalanced
from an acre of danelion and burdock.

You should tell him about balanced. Would allow even cheaper cable for the
interconnect. ;-)

If left alone he'd persuade himself that it meant we could use T+E!

This is person who, when tasked with buying network cable (after having
attempted to start a job with 40+m runs of HDMI and finally been
persuaded into CAT5 converters when it didn't work) managed to find the
most expensive one CPC did. It was the purple low-smoke CAT6 stuff and
about 3 times the price of boggo CAT5.

--
Scott

Where are we going and why am I in this handbasket?

On Wed, 10 Dec 2014 19:14:13 +0000, The Natural Philosopher
wrote:

On 10/12/14 17:12, Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
It's not uncommon to have high value resistors between line and
chassis on some designs - which will show volts on the chassis if the
true earth isn't good. Enough to feel or measure - but not enough to
do you damage. Unless you drop the amp on your foot in surprise...


what is even MORE common is an RF filter with caps between line and
neutral to the chassis.

Or a combination of resistors and capacitors. ;-)

Puts the chassis and often the equipment ground at 110VAC with respect
to a proper earth.

The high impedance half mains voltage effect from such emi filters
when the chassis is left disconnected to earth. Not normally a problem
to human touch unless several such bits of kit are all linked together
without benefit of a safety earth connection.

It can be a real problem as far is computer interfaces such as serial
ports are concerned, especially when one end is properly earther and
the other, predictably a laptop powered from its universal mains
charger, lands up with half mains voltage on its zero volt rail.

Most of the time, the D shell connector will establish a cross
bonding of the zero volt rails via the shield wire before the signal
pins make contact but the classic D connector wasn't designed with
this specific hazard in mind and it's quite possible to have a signal
pin make first contact, especially likely with a worn connector.


The volts don't matter - it's the current which does.

From a human perspective that's essentially the truth of the matter
but, with computer interfaces, that's not usually true even when the
input pin has low enough impedance to drop the voltage to a safe
limit.

The problem is the transient discharge from the filter capacitor if
the mains voltage doesn't happen to be close to the zero crossing
point in its cycle at the time of contact. The discharge current,
whilst extremely brief, will have ample energy if the voltage is much
above the ten volt mark at the critical moment of contact.

Since it varies in magnitude between zero and 170 volts 100 times per
second, the odds of the voltage being more than 10 volts are going to
be in excess of 80%, at a rather conservative guesstimate. With the
risk of damage being so high, it's a gamble I'd rather avoid taking,
thank you very much!



Enough as we discovered to blow the serial port of a
system connected to the offending laptop.

The serial cable should have connected both chassis together.

True but to avoid this particular problem, you need to have the
charger unplugged from the laptop's charging socket whilst connecting
the serial cable. You can safely reconnect the charger afterwards (but
remember to disconnect it again before you disconnect the serial
cable).



It did eventually, *after* the shield had striped the signal pins and
blown the input chip...

"Striped"? What does "striped" mean in this context? It's not an
expression I've come across before other than describing a colouring
pattern in artwork or wall decorations and suchlike.

--
J B Good