I noticed a tingling from contact with a USB lead of an external Hard Drive.

Investigating this led to the PSU, the plug of which has a high voltage
on the outer connector compared with house earth. It was varying between
70 and 110V.

(I did not check the voltage of the output which I have no reason to
doubt was correct - 12V I think it should be)

The PSU is a three pin plug. Presumably the earth connection is not
being used.

Is the PSU faulty?

aeio prodded the keyboard

I noticed a tingling from contact with a USB lead of an external
Hard Drive.

Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

(I did not check the voltage of the output which I have no reason to
doubt was correct - 12V I think it should be)

The PSU is a three pin plug. Presumably the earth connection is not
being used.

Is the PSU faulty?

Check the outer shell to earth pin with an ohmeter.

--
Best Regards:
Baron.

On Thursday, July 24, 2014 10:02:01 AM UTC-7, aeio wrote:
I noticed a tingling from contact with a USB lead of an external Hard Drive.


Investigating this led to the PSU, the plug of which has a high voltage
on the outer connector compared with house earth. It was varying between
70 and 110V.

A voltmeter might pick up leakage, but to determine if it's hazardous, you
need a load. I'd start with AC milliamp scale (try on high amp scale first, of course)
to test further. It's probably a bad PSU just as you suspect (if you feel it
it's maybe a few milliamps, but microamps might be acceptable).
Connect one probe to ground, the other to the touchable metal part.

The 'correct' way to test involves a dive into a variety of national and
international standards. One half milliamp into 1500 ohms parallel with
0.15 uF, is a typical 'safe' threshold.

On 7/24/2014 11:35 AM, Baron wrote:
aeio prodded the keyboard

I noticed a tingling from contact with a USB lead of an external
Hard Drive.

Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

(I did not check the voltage of the output which I have no reason to
doubt was correct - 12V I think it should be)

The PSU is a three pin plug. Presumably the earth connection is not
being used.

Is the PSU faulty?

Check the outer shell to earth pin with an ohmeter.

I'd expect that the output is nominally floating with respect
to the AC input port.
There usually are caps between line side and DC for EMI reasons.

I had a computer supply with a bad cap in the 5V standby supply.
Caused the GFI to trip.
Never did figure out the relationship between the cap
and the symptom, but the cap fixed it.

On 24/07/14 19:35, Baron wrote:
aeio prodded the keyboard

I noticed a tingling from contact with a USB lead of an external
Hard Drive.

Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

(I did not check the voltage of the output which I have no reason to
doubt was correct - 12V I think it should be)

The PSU is a three pin plug. Presumably the earth connection is not
being used.

Is the PSU faulty?

Check the outer shell to earth pin with an ohmeter.


I had it packed to return actually - but have opened it up again - and
see the earth pin is non conductive.

On 24/07/14 19:39, whit3rd wrote:
On Thursday, July 24, 2014 10:02:01 AM UTC-7, aeio wrote:
I noticed a tingling from contact with a USB lead of an external Hard Drive.


Investigating this led to the PSU, the plug of which has a high voltage
on the outer connector compared with house earth. It was varying between
70 and 110V.

A voltmeter might pick up leakage, but to determine if it's hazardous, you
need a load. I'd start with AC milliamp scale (try on high amp scale first, of course)
to test further. It's probably a bad PSU just as you suspect (if you feel it
it's maybe a few milliamps, but microamps might be acceptable).
Connect one probe to ground, the other to the touchable metal part.

The 'correct' way to test involves a dive into a variety of national and
international standards. One half milliamp into 1500 ohms parallel with
0.15 uF, is a typical 'safe' threshold.

slowly going up and down between 0 and .005 milliamps - on a cheap
multimeter.

Is it worth trying it with a capacitor?

Hello Mike,
mike prodded the keyboard

On 7/24/2014 11:35 AM, Baron wrote:
aeio prodded the keyboard

I noticed a tingling from contact with a USB lead of an external
Hard Drive.

Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

(I did not check the voltage of the output which I have no reason
to doubt was correct - 12V I think it should be)

The PSU is a three pin plug. Presumably the earth connection is
not being used.

Is the PSU faulty?

Check the outer shell to earth pin with an ohmeter.

I'd expect that the output is nominally floating with respect
to the AC input port.
There usually are caps between line side and DC for EMI reasons.

I had a computer supply with a bad cap in the 5V standby supply.
Caused the GFI to trip.
Never did figure out the relationship between the cap
and the symptom, but the cap fixed it.

When he mentioned "outer connector" I thought he meant the shell on
it. My external USB HDD has the ground connection carried right
through to the mains earth pin. I have continuity all the way from
the metal drive case.


--
Best Regards:
Baron.

aeio prodded the keyboard

On 24/07/14 19:39, whit3rd wrote:
On Thursday, July 24, 2014 10:02:01 AM UTC-7, aeio wrote:
I noticed a tingling from contact with a USB lead of an external
Hard Drive.


Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

A voltmeter might pick up leakage, but to determine if it's
hazardous, you
need a load. I'd start with AC milliamp scale (try on high amp
scale first, of course)
to test further. It's probably a bad PSU just as you suspect (if
you feel it it's maybe a few milliamps, but microamps might be
acceptable). Connect one probe to ground, the other to the
touchable metal part.

The 'correct' way to test involves a dive into a variety of
national and international standards. One half milliamp into 1500
ohms parallel with 0.15 uF, is a typical 'safe' threshold.

slowly going up and down between 0 and .005 milliamps - on a cheap
multimeter.

Is it worth trying it with a capacitor?

A reading that small could just as well be leakage. Are you sure that
should be milliampere and not amps ? ie 5 ma.


--
Best Regards:
Baron.

On 24/07/14 22:08, Baron wrote:
aeio prodded the keyboard

On 24/07/14 19:39, whit3rd wrote:
On Thursday, July 24, 2014 10:02:01 AM UTC-7, aeio wrote:
I noticed a tingling from contact with a USB lead of an external
Hard Drive.


Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

A voltmeter might pick up leakage, but to determine if it's
hazardous, you
need a load. I'd start with AC milliamp scale (try on high amp
scale first, of course)
to test further. It's probably a bad PSU just as you suspect (if
you feel it it's maybe a few milliamps, but microamps might be
acceptable). Connect one probe to ground, the other to the
touchable metal part.

The 'correct' way to test involves a dive into a variety of
national and international standards. One half milliamp into 1500
ohms parallel with 0.15 uF, is a typical 'safe' threshold.

slowly going up and down between 0 and .005 milliamps - on a cheap
multimeter.

Is it worth trying it with a capacitor?

A reading that small could just as well be leakage. Are you sure that
should be milliampere and not amps ? ie 5 ma.

Yes - 0.005 mA. That is on the 2mA scale. There is nothing showing on
the higher scales and nothing on the AC scale.

aeio prodded the keyboard

On 24/07/14 22:08, Baron wrote:
aeio prodded the keyboard

On 24/07/14 19:39, whit3rd wrote:
On Thursday, July 24, 2014 10:02:01 AM UTC-7, aeio wrote:
I noticed a tingling from contact with a USB lead of an external
Hard Drive.


Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

A voltmeter might pick up leakage, but to determine if it's
hazardous, you
need a load. I'd start with AC milliamp scale (try on high amp
scale first, of course)
to test further. It's probably a bad PSU just as you suspect (if
you feel it it's maybe a few milliamps, but microamps might be
acceptable). Connect one probe to ground, the other to the
touchable metal part.

The 'correct' way to test involves a dive into a variety of
national and international standards. One half milliamp into
1500 ohms parallel with 0.15 uF, is a typical 'safe' threshold.

slowly going up and down between 0 and .005 milliamps - on a cheap
multimeter.

Is it worth trying it with a capacitor?

A reading that small could just as well be leakage. Are you sure
that should be milliampere and not amps ? ie 5 ma.

Yes - 0.005 mA. That is on the 2mA scale. There is nothing showing
on the higher scales and nothing on the AC scale.

In that case I would be inclined to forget about it. But just in case
check it regularly to make sure it isn't getting worse.

--
Best Regards:
Baron.

On Fri, 25 Jul 2014 06:53:39 -0700, Baron wrote:

aeio prodded the keyboard

On 24/07/14 22:08, Baron wrote:
aeio prodded the keyboard

...just started snipping....
A reading that small could just as well be leakage. Are you sure
that should be milliampere and not amps ? ie 5 ma.

Yes - 0.005 mA. That is on the 2mA scale. There is nothing showing
on the higher scales and nothing on the AC scale.

In that case I would be inclined to forget about it. But just in case
check it regularly to make sure it isn't getting worse.


Inside every computer is a line filter to prevent the switching power
supply signals from going back out your AC mains, called conducted EMI.
Inside that filter is a cap between NEUTRAL, HOT, and chassis GND, called
a Y-Cap. That y-cap effectively centertaps your AC mains, but usually the
PC case is connected to earth ground, almost equal to NEUTRAL, so you
never measure any voltage there. If the GND terminal on the outlet to the
PC is floating, well you can get a pretty healthy zap. I can just start to
feel 12Vac so this 60-70Vac hurts a LOT! The size of the y-cap is set to
just about provide a maximum of 2mA.

Now such filtering on an isolated DC supply [if the whole thing is DC
isolated from the PC chassis] can also cause the metal housing's voltage
to meander around.

For what it's worth, 'hospital grade' leakage is less than 100uA and
'direct onnection to patient' hospital grade leakage is less than 10uA
....to simulate what that feels like the next time you're in a store with
fluorescent lighting inside a metal trimmed display cabinet. Gently slide
the back of your knuckles along the metal. You probably will feel the AC
mains voltage. Now, *if* you touch an open wound to that metal! That's a
big oweey, hurts like h---.

The only concern I would have about finding strange voltages where not
expected is that that may be a precursor to a failure mode that would then
supply BIG voltage, as in lethal voltage, if/when something else happens.

RobertMacy prodded the keyboard

On Fri, 25 Jul 2014 06:53:39 -0700, Baron
wrote:

aeio prodded the keyboard

On 24/07/14 22:08, Baron wrote:
aeio prodded the keyboard

...just started snipping....
A reading that small could just as well be leakage. Are you sure
that should be milliampere and not amps ? ie 5 ma.

Yes - 0.005 mA. That is on the 2mA scale. There is nothing showing
on the higher scales and nothing on the AC scale.

In that case I would be inclined to forget about it. But just in
case check it regularly to make sure it isn't getting worse.


Inside every computer is a line filter to prevent the switching
power supply signals from going back out your AC mains, called
conducted EMI. Inside that filter is a cap between NEUTRAL, HOT, and
chassis GND, called a Y-Cap. That y-cap effectively centertaps your
AC mains, but usually the PC case is connected to earth ground,
almost equal to NEUTRAL, so you never measure any voltage there. If
the GND terminal on the outlet to the PC is floating, well you can
get a pretty healthy zap. I can just start to feel 12Vac so this
60-70Vac hurts a LOT! The size of the y-cap is set to just about
provide a maximum of 2mA.

Now such filtering on an isolated DC supply [if the whole thing is
DC isolated from the PC chassis] can also cause the metal housing's
voltage to meander around.

For what it's worth, 'hospital grade' leakage is less than 100uA and
'direct onnection to patient' hospital grade leakage is less than
10uA ...to simulate what that feels like the next time you're in a
store with fluorescent lighting inside a metal trimmed display
cabinet. Gently slide the back of your knuckles along the metal. You
probably will feel the AC mains voltage. Now, *if* you touch an open
wound to that metal! That's a big oweey, hurts like h---.

The only concern I would have about finding strange voltages where
not expected is that that may be a precursor to a failure mode that
would then supply BIG voltage, as in lethal voltage, if/when
something else happens.

I agree 100%. The PC case should be grounded ! It may pay to check
that the earth is good right back to the panel.

--
Best Regards:
Baron.