Short on my metal case (ouch)

There's something wrong with the pushbutton ON/OFF power button on the 4 year old Asus i5 socket mobo tower system, which has a new power supply 600W "Iceman" brand. The local repair guy here in the Philippines tried soldering the pushbutton, and it worked for a while, until it malfunctioned again and it will always 'reboot' rather than power off (even with a long press).

So I started using the on/off toggle switch for the power supply as a on/off switch.

But then I discovered a weird anomaly, the S tarbuck f lying could write a novel about: with the power cord in, but the power supply switched off but connected to live 220V mains, a very slight skin prick and 'hot sensation' could be felt at the corners of the tower metal case. At first I thought some metal shard was poking my finger at these corners, and I wiped the case off with a rag, but then, after repeated such little shocks, after unplugging the power supply from the UPS (here in southeast Asia they don't have ground wires, so you have to use a floating ground in the form of a UPS, which also has its problems since nothing in this part of the world 'works like new' but is refurbished, including stuff sold as new, since all 'rejects' from the First World end up resold here as 'new', but I digress) and feeling no shock, I figured out it must be a electrical short that's ending up manifesting itself in the case.

Probably 5V or so, not many amps, 95% not 'dangerous' except of course to electronics and there's always that 5%, so I will rebuild using a new case with new pushbutton switches.

Anybody else have this happen? I'm not risking an electrical fire so I will rebuild.

RL

RayLopez99 wrote:
There's something wrong with the pushbutton ON/OFF power button on the 4 year old Asus i5 socket mobo tower system, which has a new power supply 600W "Iceman" brand. The local repair guy here in the Philippines tried soldering the pushbutton, and it worked for a while, until it malfunctioned again and it will always 'reboot' rather than power off (even with a long press).

So I started using the on/off toggle switch for the power supply as a on/off switch.

But then I discovered a weird anomaly, the S tarbuck f lying could write a novel about: with the power cord in, but the power supply switched off but connected to live 220V mains, a very slight skin prick and 'hot sensation' could be felt at the corners of the tower metal case. At first I thought some metal shard was poking my finger at these corners, and I wiped the case off with a rag, but then, after repeated such little shocks, after unplugging the power supply from the UPS (here in southeast Asia they don't have ground wires, so you have to use a floating ground in the form of a UPS, which also has its problems since nothing in this part of the world 'works like new' but is refurbished, including stuff sold as new, since all 'rejects' from the First World end up resold here as 'new', but I digress) and feeling no shock, I figured out it must be a electrical short that's ending up manifesting itself in the case.

Probably 5V or so, not many amps, 95% not 'dangerous' except of course to electronics and there's always that 5%, so I will rebuild using a new case with new pushbutton switches.

Anybody else have this happen? I'm not risking an electrical fire so I will rebuild.

RL

There's nothing wrong with your "shock" symptoms.

We can use this reference schematic of an ATX power
supply, as an example.

http://www.pavouk.org/hw/en_atxps.html

The front end of the supply has a filter. It prevents
high frequency switching noise, and the harmonics thereof,
from flowing backwards into the AC line.

Critical to the discussion, are C2 and C3, just after T5 (choke).
When the AC (wall) supply voltage makes its sinusoidal waveform,
C2 and C3 conduct a small amount of current. The current
flows into "SHIELD". SHIELD is joined to the chassis. The
entire metal of the chassis is at SHIELD potential.

Modern North American electrical code, adds a third prong,
the safety ground prong. This is connected to SHIELD via
the line cord.

If your outlets have Safety Ground, the small but irritating
AC current is conducted straight to ground.

If you don't have a Safety Ground, or if someone busts
off the pin (some jackasses do this on purpose), there is no
place for the AC current to go. If you touch the chassis
with your finger, you are enough of a ground to feel some
of that current. You get a mild shock.

The industry feels this is a fine feature, and they're
quite happy with it. Some medical doctor gives permission
for a certain level of current flow, and that's what they
design to. People with sweaty hands may experience more
of a shock, then people with naturally dry skin.

*******

How do you fix that ? You ask a local electrician what
it would cost to run one grounded outlet for the computer.

And in case you think this is a "third world" problem, it is
not. The house I grew up in, did not have Safety Ground.
The house was old enough, to have two prong outlets.

I will not be discussing how I fixed that, as it's a code
violation. Ask your local electrician, and get the job
done right.

*******

This could affect the power switch on the front of
the computer, but only if the button surface is
joined to the two electrical sognals on the twisted
pair of wires. Normally, computer switches, the button
on the front insulates the switching action on the back.
So there should be no "path" from your finger tip,
to sensitive motherboard circuits. But there are
some particularly nice looking computer cases, where the
entire switch is a chunk of metal. And it's easier with
one of those switches, to accidentally form a path between
your finger tip, and PWR#.

The button on the front of the computer, is not routed
directly to the ATX supply. There is a "conditioning" circuit
on the motherboard. For example, if the computer is booted,
a timer is attached to the button, and you have to press and
hold the button for four seconds, to get it to act. This prevents
accidental physical contact with the button, doing a dirty
shutdown on the computer. The switch contact on the computer
case is of type "momentary contact", but the conditioning
circuit "latches" that signal, and produces a steady logic
level which is used to turn on the ATX supply.

You can generate a reset condition on the PC, if the output
voltages on the supply temporarily glitch.

You could check in the BIOS and make sure in the power options
section, that the PC is set to not turn on, just after recovery
from a power failure. That's to try to reduce nuisance reboots,
instead of shutting down.

I've solved none of your problems, but that's what is going on.
You may find some tiny aspects of the computer work better,
once the Safety Ground issue is solved by an electrician.

If the ATX supply fails, and the "Hot" wire touches the Shield
of the supply, the Shield conductor is required to ground out
the entire AC from your house. Many amps flow, until the breaker
in the panel trips. Thus, this "Shield" requirement is two-fold.
For this nuisance leakage current flow, you only need a
"thin" wire path to ground. But if there is an electrical
failure (an "insulation failure), the Shield must handle
at least 15 amps without burning up. Grounding the chassis
on toasters and computers is a requirement, so people don't
receive a fatal shock. And thus, a proper (electrician)
connection is required for that to work. There could be
house insurance issues, if you do a half-ass job. For example,
if some solution you come up with, causes a fire or worse.

Paul

On Sat, 20 Jun 2015 06:52:07 -0700 (PDT), RayLopez99
wrote:

So I started using the on/off toggle switch for the power supply as a on/off switch.

I sometimes take a pair of cutters, wire-snips to a case on its way
out. Certainly a lot of $30 cases. Certainly a lot of $100 power
supplies (unrebated and good quality. Anyway I cut off all those
micro- and momentary logic-switches on the case that are good working
order. Put them in a box with a lot of salvaged crap. Only had one
PWR fail, but it was nasty what it would do (like damn near sparks on
resetting the MB 50-million times a second). So happens I found a
spare, but any spare, is better than that. Even if I solder the wires
and drill a hole in the front and let the PWR switch dangle out.

Wouldn't care, nor have I ever been shocked by a build. I'd rebuild
it if I did, probably. Only had the opposite, to where it wouldn't
power up due to something grounding the system out. A rebuild with
extra care to particulars (MB and washers) fixed whatever was causing
the short.

On Sat, 20 Jun 2015 06:52:07 -0700 (PDT), RayLopez99
wrote:

There's something wrong with the pushbutton ON/OFF power button on the 4 year old Asus i5 socket mobo tower system, which has a new power supply 600W "Iceman" brand. The local repair guy here in the Philippines tried soldering the pushbutton, and it worked for a while, until it malfunctioned again and it will always 'reboot' rather than power off (even with a long press).

So I started using the on/off toggle switch for the power supply as a on/off switch.

But then I discovered a weird anomaly, the S tarbuck f lying could write a novel about: with the power cord in, but the power supply switched off but connected to live 220V mains, a very slight skin prick and 'hot sensation' could be felt at the corners of the tower metal case. At first I thought some metal shard was poking my finger at these corners, and I wiped the case off with a rag, but then, after repeated such little shocks, after unplugging the power supply from the UPS (here in southeast Asia they don't have ground wires, so you have to use a floating ground in the form of a UPS, which also has its problems since nothing in this part of the world 'works like new' but is refurbished, including stuff sold as new, since all 'rejects' from the First World end up resold here as 'new', but I digress) and feeling no shock, I figured out it must be a electrical short that's ending up manifesting itself in the case.

Probably 5V or so, not many amps, 95% not 'dangerous' except of course to electronics and there's always that 5%, so I will rebuild using a new case with new pushbutton switches.

Anybody else have this happen? I'm not risking an electrical fire so I will rebuild.

If it were only 5V you wouldn't be feeling it--5V can't get though
normal skin with enough current to detect. (Try putting your finger
across the top of a 9V battery--you don't feel a thing.)

Much more likely is that you have high voltage (220V AC or possibly
even 311V DC) that's leaking to the case through enough resistance
that you don't get much current flow.

I had a similar experience once from an improperly designed toaster
(switch on the neutral) coupled with some burnt-on residue that
provided a high resistance path from the heating element to the case.
I brushed it with a wet hand and felt a slight prickle.

Since such voltages should occur only within the power supply the
failure pretty much has to be inside it. If you have a multimeter
test the resistance between each of the inputs and the case.
(Obviously this must only be done with it unplugged and the capacitors
discharged.) I'm going to guess you'll get a value somewhere in the
general ballpark of 100k Ohms but not over 300k Ohms. (Note that when
testing resistances at this level make sure your fingers are clear of
the contacts. While a meter poses no danger if you're touching the
metal of the leads you will foul up the reading.)

Loren Pechtel wrote:
On Sat, 20 Jun 2015 06:52:07 -0700 (PDT), RayLopez99
wrote:

There's something wrong with the pushbutton ON/OFF power button on the 4 year old Asus i5 socket mobo tower system, which has a new power supply 600W "Iceman" brand. The local repair guy here in the Philippines tried soldering the pushbutton, and it worked for a while, until it malfunctioned again and it will always 'reboot' rather than power off (even with a long press).

So I started using the on/off toggle switch for the power supply as a on/off switch.

But then I discovered a weird anomaly, the S tarbuck f lying could write a novel about: with the power cord in, but the power supply switched off but connected to live 220V mains, a very slight skin prick and 'hot sensation' could be felt at the corners of the tower metal case. At first I thought some metal shard was poking my finger at these corners, and I wiped the case off with a rag, but then, after repeated such little shocks, after unplugging the power supply from the UPS (here in southeast Asia they don't have ground wires, so you have to use a floating ground in the form of a UPS, which also has its problems since nothing in this part of the world 'works like new' but is refurbished, including stuff sold as new, since all 'rejects' from the First World end up resold here as 'new', but I digress) and feeling no shock, I figured out it must be a electrical short that's ending up manifesting itself in the case.

Probably 5V or so, not many amps, 95% not 'dangerous' except of course to electronics and there's always that 5%, so I will rebuild using a new case with new pushbutton switches.

Anybody else have this happen? I'm not risking an electrical fire so I will rebuild.

If it were only 5V you wouldn't be feeling it--5V can't get though
normal skin with enough current to detect. (Try putting your finger
across the top of a 9V battery--you don't feel a thing.)

Much more likely is that you have high voltage (220V AC or possibly
even 311V DC) that's leaking to the case through enough resistance
that you don't get much current flow.

I had a similar experience once from an improperly designed toaster
(switch on the neutral) coupled with some burnt-on residue that
provided a high resistance path from the heating element to the case.
I brushed it with a wet hand and felt a slight prickle.

Since such voltages should occur only within the power supply the
failure pretty much has to be inside it. If you have a multimeter
test the resistance between each of the inputs and the case.
(Obviously this must only be done with it unplugged and the capacitors
discharged.) I'm going to guess you'll get a value somewhere in the
general ballpark of 100k Ohms but not over 300k Ohms. (Note that when
testing resistances at this level make sure your fingers are clear of
the contacts. While a meter poses no danger if you're touching the
metal of the leads you will foul up the reading.)

Let's try it another way.

The front end of the ATX power supply, that style of
filter is available as a standalone item. The 10VN1
is one of my favorites, due to the decent filtering characteristics.

http://www.te.com/commerce/DocumentD...df%7F6609052-2

One thing to note, is it "leaks current on purpose". The
only path between shield and the other conductors, is via
two symmetric caps. That's the leakage path. The path is
there, ro promote filtering of MHz frequency signals.
Yet, the capacitors have significant impedance at line
frequencies, so some AC is conducted into the shield signal.

If the shield is connected to chassis, and the human body is
at ground potential, you get a shock.

The level of current flow, is stated right in the data sheet.
So your health and safety staff can verify the value
is safe for consumer usage (if the shield becomes
disconnected from Safety ground).

Specifications

Maximum leakage current each Line to Ground:
@ 120 VAC 60Hz: 1.2mA
@ 250 VAC 50Hz: 2.0mA

To check for that, you'd flip your meter to *capacitance*
and measure capacitance between Line to Safety Ground. There
is no DC path to shield. Only an AC impedance in the form
of those two caps. Two caps that were placed there on
purpose. Waiting for someone in a home without Safety
Ground, to get a shock.

http://i57.tinypic.com/2nhgwhw.gif

Paul

On Sunday, June 21, 2015 at 12:20:33 PM UTC+8, Loren Pechtel wrote:

If it were only 5V you wouldn't be feeling it--5V can't get though
normal skin with enough current to detect. (Try putting your finger
across the top of a 9V battery--you don't feel a thing.)

Much more likely is that you have high voltage (220V AC or possibly
even 311V DC) that's leaking to the case through enough resistance
that you don't get much current flow.

I had a similar experience once from an improperly designed toaster
(switch on the neutral) coupled with some burnt-on residue that
provided a high resistance path from the heating element to the case.
I brushed it with a wet hand and felt a slight prickle.

Since such voltages should occur only within the power supply the
failure pretty much has to be inside it. If you have a multimeter
test the resistance between each of the inputs and the case.
(Obviously this must only be done with it unplugged and the capacitors
discharged.) I'm going to guess you'll get a value somewhere in the
general ballpark of 100k Ohms but not over 300k Ohms. (Note that when
testing resistances at this level make sure your fingers are clear of
the contacts. While a meter poses no danger if you're touching the
metal of the leads you will foul up the reading.)

Loren you are awesome! I actually, before reading this post, bought a multimeter here in the Philippines and tested the mobo and power supply--it's very interesting what I found (using the "voltmeter", not the resistance meter which I don't even know how to operate, though I do understand Olm's Law):

1) Using a multimeter I found that indeed the voltage when the power supply is plugged in but turned off is +- 2 volts (maximum) on the ATX leads (which I believe supply between 5 to 12 V). Then, if you unplug the power supply (220 V AC here in Philippines, and, again, no ground ever, though I suppose you can build your own ground wire by sinking a metal rod deep into earth, but it's not my house, I just rent it, though I must say it was built by a westerner and except for the lack of a ground it's the finest electrically wired house I've ever lived in in SE Asia), what happens is the +- 2 V gradually goes to zero (of course no doubt due to capacitors discharging slowly).

2) I forget 2. I guess I would ask Loren P how I can use the "resistance meter" of the multimeter.

This post by Loren and by Paul will save me money, thank you both very much! I was going to scrap the expensive Power Supply, as I felt for sure it must be defective if any current leaks past it when the On/Off toggle switch is switched off, but I see now this must be a mistaken assumption--yes? I will ask Paul this same question in his reply.

Awesome. I hate paying extra for equipment, especially since the stuff here is refurbished, and possibly you're getting somebody else's problem if you throw away good hardware and buy 'new' again.

So the only thing I need to buy is a new case (since the pushbutton switch is ruined), not a new Power Supply...

RL

On Mon, 22 Jun 2015 11:03:29 -0700 (PDT), RayLopez99
wrote:

2) I forget 2. I guess I would ask Loren P how I can use the "resistance meter" of the multimeter.

It's usually marked with a symbol that looks rather like a horseshoe
with the opening pointing down.

On Saturday, June 20, 2015 at 10:34:07 PM UTC+8, Paul wrote:
RayLopez99 wrote:
There's something wrong with the pushbutton ON/OFF power button on the 4 year old Asus i5 socket mobo tower system, which has a new power supply 600W "Iceman" brand. The local repair guy here in the Philippines tried soldering the pushbutton, and it worked for a while, until it malfunctioned again and it will always 'reboot' rather than power off (even with a long press).

So I started using the on/off toggle switch for the power supply as a on/off switch.

But then I discovered a weird anomaly, the S tarbuck f lying could write a novel about: with the power cord in, but the power supply switched off but connected to live 220V mains, a very slight skin prick and 'hot sensation' could be felt at the corners of the tower metal case. At first I thought some metal shard was poking my finger at these corners, and I wiped the case off with a rag, but then, after repeated such little shocks, after unplugging the power supply from the UPS (here in southeast Asia they don't have ground wires, so you have to use a floating ground in the form of a UPS, which also has its problems since nothing in this part of the world 'works like new' but is refurbished, including stuff sold as new, since all 'rejects' from the First World end up resold here as 'new', but I digress) and feeling no shock, I figured out it must be a electrical short that's ending up manifesting itself in the case.

Probably 5V or so, not many amps, 95% not 'dangerous' except of course to electronics and there's always that 5%, so I will rebuild using a new case with new pushbutton switches.

Anybody else have this happen? I'm not risking an electrical fire so I will rebuild.

RL

There's nothing wrong with your "shock" symptoms.

We can use this reference schematic of an ATX power
supply, as an example.

http://www.pavouk.org/hw/en_atxps.html

The front end of the supply has a filter. It prevents
high frequency switching noise, and the harmonics thereof,
from flowing backwards into the AC line.

Critical to the discussion, are C2 and C3, just after T5 (choke).
When the AC (wall) supply voltage makes its sinusoidal waveform,
C2 and C3 conduct a small amount of current. The current
flows into "SHIELD". SHIELD is joined to the chassis. The
entire metal of the chassis is at SHIELD potential.

Modern North American electrical code, adds a third prong,
the safety ground prong. This is connected to SHIELD via
the line cord.

If your outlets have Safety Ground, the small but irritating
AC current is conducted straight to ground.

If you don't have a Safety Ground, or if someone busts
off the pin (some jackasses do this on purpose), there is no
place for the AC current to go. If you touch the chassis
with your finger, you are enough of a ground to feel some
of that current. You get a mild shock.

The industry feels this is a fine feature, and they're
quite happy with it. Some medical doctor gives permission
for a certain level of current flow, and that's what they
design to. People with sweaty hands may experience more
of a shock, then people with naturally dry skin.

*******

How do you fix that ? You ask a local electrician what
it would cost to run one grounded outlet for the computer.

And in case you think this is a "third world" problem, it is
not. The house I grew up in, did not have Safety Ground.
The house was old enough, to have two prong outlets.

I will not be discussing how I fixed that, as it's a code
violation. Ask your local electrician, and get the job
done right.

*******

This could affect the power switch on the front of
the computer, but only if the button surface is
joined to the two electrical sognals on the twisted
pair of wires. Normally, computer switches, the button
on the front insulates the switching action on the back.
So there should be no "path" from your finger tip,
to sensitive motherboard circuits. But there are
some particularly nice looking computer cases, where the
entire switch is a chunk of metal. And it's easier with
one of those switches, to accidentally form a path between
your finger tip, and PWR#.

The button on the front of the computer, is not routed
directly to the ATX supply. There is a "conditioning" circuit
on the motherboard. For example, if the computer is booted,
a timer is attached to the button, and you have to press and
hold the button for four seconds, to get it to act. This prevents
accidental physical contact with the button, doing a dirty
shutdown on the computer. The switch contact on the computer
case is of type "momentary contact", but the conditioning
circuit "latches" that signal, and produces a steady logic
level which is used to turn on the ATX supply.

You can generate a reset condition on the PC, if the output
voltages on the supply temporarily glitch.

You could check in the BIOS and make sure in the power options
section, that the PC is set to not turn on, just after recovery
from a power failure. That's to try to reduce nuisance reboots,
instead of shutting down.

I've solved none of your problems, but that's what is going on.
You may find some tiny aspects of the computer work better,
once the Safety Ground issue is solved by an electrician.

If the ATX supply fails, and the "Hot" wire touches the Shield
of the supply, the Shield conductor is required to ground out
the entire AC from your house. Many amps flow, until the breaker
in the panel trips. Thus, this "Shield" requirement is two-fold.
For this nuisance leakage current flow, you only need a
"thin" wire path to ground. But if there is an electrical
failure (an "insulation failure), the Shield must handle
at least 15 amps without burning up. Grounding the chassis
on toasters and computers is a requirement, so people don't
receive a fatal shock. And thus, a proper (electrician)
connection is required for that to work. There could be
house insurance issues, if you do a half-ass job. For example,
if some solution you come up with, causes a fire or worse.

Paul

A trove Sir, a venerable treasure trove of information for the infovaur! Bravo Sir Paul! You are a resource par-excellence, your Excellency, bar none.

I cut-and-paste my reply to Loren P for background reference below.

My question(s) to you, almighty geek god:

1) with PC power cord to mains, which is 220V here in the Philippines, and no ground (thanks for relating that some houses in the USA are also like this, and hinting at the fix), with a UPS (Uninterrupted Power Supply) supplying the power...that is PLUGGED INTO mains / UPS...

2) ...I find that there is a slight 2V voltage at the ATX power (DC) output at the mobo even if the PC 600W Power Supply is toggled OFF at the rear (using the On/Off toggle button of the Power Supply). Question #1 : is this normal? Assumed Answer #1: Yes, because of no ground and some current/voltage leaking past the 600W power supply (new-ish) by "Ice-Man" a Korean company PS

2(b)) ...now with the PC unplugged from the UPS (and with PC 600W PS off), I find the voltage gradually drops from +-2V to zero...as the current is dissipated by the system...

3) Now, with the system PLUGGED INTO the UPS, and the 600 W PS turned ON, I find a slight "tingling sensation" and/or "hotness" which I touch the mobo and/or case with my sweaty palms (not dripping sweat of course, just moist, as it's nearly 100% humidity and routinely in the mid-90s here in the tropics--Flasherly knows what I'm talking about!) Question #2: is this normal? Assumed Answer #2: Yes, for the reasons related by Paul in his post upstream.

4) Question #3 - can it be that the absence of a ground is the reason the pushbutton for my tower case always seems (every perhaps 12 months or so) to fail? Assumed Answer #3: possibly, for the reasons related by Paul before, namely the switch is a conduit for current at all times if there's no ground, and this constant influx of current can cause thermal expansion issues and fatigue. Side note: I know there must be a small, weak short in my system somehow, since even if I switch off all equipment, whenever the UPS battery is turned on when mains is interrupted (which is nearly every third day here in the infrastructure light tropics), it 'beeps' to indicate current is flowing. This tells me somewhere there's a small short and/or current flow (perhaps that's a better term, as short implies a problem) somewhere in my PC and/or monitor.

5) Question #4 - how do you test using a multimeter and the 'resistance dial'? Just curious from a hobbyist point of view. Assumed answer Q4: just Google it. This is not a big deal.

I will hold off making any purchases of a new tower until I hear back from you, though I need to buy a tower for the new pushbutton. I assume however the power supply is good, and the 'shocking behavior' exhibited by it is 'normal' given the environment it is in. Also I assume that replacing a HDD when all of this is going on will not 'fry' the drive, obviously.

Thanks Paul!

RL


**cut and paste answer to Loren P **

Loren you are awesome! I actually, before reading this post, bought a multimeter here in the Philippines and tested the mobo and power supply--it's very interesting what I found (using the "voltmeter", not the resistance meter which I don't even know how to operate, though I do understand Olm's Law):

1) Using a multimeter I found that indeed the voltage when the power supply is plugged in but turned off is +- 2 volts (maximum) on the ATX leads (which I believe supply between 5 to 12 V). Then, if you unplug the power supply (220 V AC here in Philippines, and, again, no ground ever, though I suppose you can build your own ground wire by sinking a metal rod deep into earth, but it's not my house, I just rent it, though I must say it was built by a westerner and except for the lack of a ground it's the finest electrically wired house I've ever lived in in SE Asia), what happens is the +- 2 V gradually goes to zero (of course no doubt due to capacitors [Update: more likely resistors, though I guess capacitors discharging through inductors will also dissipate energy!--RL] discharging slowly).

2) I forget 2. I guess I would ask Loren P how I can use the "resistance meter" of the multimeter.

This post by Loren and by Paul will save me money, thank you both very much! I was going to scrap the expensive Power Supply, as I felt for sure it must be defective if any current leaks past it when the On/Off toggle switch is switched off, but I see now this must be a mistaken assumption--yes? I will ask Paul this same question in his reply.

Awesome. I hate paying extra for equipment, especially since the stuff here is refurbished, and possibly you're getting somebody else's problem if you throw away good hardware and buy 'new' again.

So the only thing I need to buy is a new case (since the pushbutton switch is ruined), not a new Power Supply...

RL

On Tuesday, June 23, 2015 at 2:32:11 AM UTC+8, RayLopez99 wrote:

I forgot to ask this bonus question which has nothing at all to do with computers and really is not important save as a teaching moment: how do you measure voltage in a house with no ground? I've not tried this, as I don't want to fry my cheap little multimeter I bought for 100 pesos or so, but I think you would touch the black probe (- ?) to a large metal object like 30 meters of copper wire and/or the outside of a metal refrigerator, and, then stick the red probe (+?) to the hot slot of the two prong electrical mains outlet. I think this might work to show 220V (of course you have to set the multimeter to the right setting so not to be off-scale).

If you stick both black and red into the ungrounded two prong outlet, you'll get a bunch of transient crazy numbers, negative and positive, but not close to 220, since there's no ground. (I already tried this, and was happy I did not blow the fuse on my multimeter).

RL

RayLopez99 wrote:
On Tuesday, June 23, 2015 at 2:32:11 AM UTC+8, RayLopez99 wrote:

I forgot to ask this bonus question which has nothing at all to do with computers and really is not important save as a teaching moment: how do you measure voltage in a house with no ground? I've not tried this, as I don't want to fry my cheap little multimeter I bought for 100 pesos or so, but I think you would touch the black probe (- ?) to a large metal object like 30 meters of copper wire and/or the outside of a metal refrigerator, and, then stick the red probe (+?) to the hot slot of the two prong electrical mains outlet. I think this might work to show 220V (of course you have to set the multimeter to the right setting so not to be off-scale).

If you stick both black and red into the ungrounded two prong outlet, you'll get a bunch of transient crazy numbers, negative and positive, but not close to 220, since there's no ground. (I already tried this, and was happy I did not blow the fuse on my multimeter).

RL

I would be very careful where you stick the meter probes.

Never use a meter without glancing at the spec sheet first!

Mine is:

1000 VDC max
750 VAC max

and it has an AC/DC button that you press, to switch from
one kind of measurement to the other. If you measure AC
while on the DC setting, then you're not going to get
the right reading. I would feel relatively safe measuring
a 230 VAC source. I probably would not use the meter on
much more than that (I wouldn't use my meter on any
industrial 440V circuits).

DC is what comes from batteries.

AC is what comes from the wall.

When jamming probes into a high voltage circuit, you want
to be familiar enough with it, to know where any metal
grounds are located. You'd feel a fool, if there was
a blinding flash, and the tip was blown off your meter
probe, when you touched a "hot" conductor and an adjacent
building ground. The breaker in the panel provides
little in the way of personal protection. You could
be in a lot of trouble, and the breaker would just
sit there and not trip. Don't depend on the breaker
being there when it counts. If you make a bad mistake,
you have to deal with it directly, at the point of
application.

I dropped a 120V *male* plug with live voltage
on it once, onto the jaws of a pair of lineman pliers,
and the spark was powerful enough, to take a gouge
out of the pliers. (And in that case, blow a fuse,
and put me in the dark.)

*******

This is a simplified view of your computer.

~AC ------ filter -------+-- Main stage --- rectifier --- 3.3VDC
circuit | Transformer --- rectifier --- 5.0VDC
~AC ------ leaks to gnd | Isolated ATX --- rectifier --- 12.0VDC
| |
| +-- Standby stage --- rectifier --- 5VSB
Safety -------+ Transformer
Ground | Isolated
+---------------------------------+---------- DC Ground
|
+------- Metal chassis

The ATX supply, has transformer isolation between input and
output. This is an attempt to keep you safe from "conducted"
high voltages. The outside of the ATX supply has a sticker
that says "HiPot Tested" and that is proof that when 1100V
or so was applied between the AC plug pins, and any DC output,
no current could flow. That's a test of the transformer
lacquer insulation. So in a sense, there is an "air gap"
between the AC input side and the DC output side. We hope
never to see a "HiPot Failure", where something leaks through.

Notice, that all the grounds are connected together. The
transformer has nothing to do with the ground.

On the front end, the supply should have a three prong plug.
And a connection from the "leaky leg" of the filter
circuit, to the Safety Ground connection. When this happens,
the user will not feel a shock, when touching the chassis.
The leakage current flows back into the Safety Ground. And
since the Safety Ground wire has a 15 amp rating, 0.002 amps
is hardly going to matter.

The supply has three operating states.

Unplugged or switched off at back = Main stage OFF
Standby stage OFF

Switched on at the back = Main stage OFF
Standby stage ON

PS_ON# from motherboard = Main stage ON (fans run)
Standby stage ON

When you finish with the computer at the end of the day,
and select standby or shutdown, the standby stage
continues to run all night long. There will be 5V on the
5VSB lead on the main ATX harness. On modern computers,
a certain number of USB plugs will also have this +5VSB
present. You can charge your Apple iPad from those ports.

Now, I don't know where you measured the "2V", but the
+5VSB, if that's what you were measuring, would be
5.00 volts DC.

*******

OK, now let's go to Rays house, where the outlet doesn't
have a safety ground. Ray attempts to make a measurement
from computer chassis, to a "ground" in the house. An
example of an unreliable ground, is the house cold water
pipe. While a cold water pipe exits the building and
travels through the earth, sometimes there is plastic
pipe somewhere inside the house, that breaks the circuit.
Care must be taken in the house, to make sure a bridging
strap is fastened between external copper pipe, and an
internal copper pipe. In a house using CPVC plastic
plumbing, getting a "ground" this way, isn't possible.

~AC ------ filter -------+-- Main stage --- rectifier --- 3.3VDC
circuit | Transformer --- rectifier --- 5.0VDC
~AC ------ leaks to gnd | Isolated ATX --- rectifier --- 12.0VDC
| |
| +-- Standby stage --- rectifier --- 5VSB
//--+ Transformer
| Isolated
+---------------------------------+---------- DC Ground
|
Copper -------------- meter on AC ------------ +------- Metal chassis
plumbing
Ground (cold water)

I'm only using the copper pipe as a "reference",
so I have something to touch the meter to.

If I do this, I will see some magnitude of AC voltage.

Perhaps this is the AC voltage you were seeing.

Your meter was set to AC, you touched the chassis with
one lead, and the other lead was connected to something
at ground potential.

Yes, you'll see a voltage.

The worst I've ever seen, in a lab, was ~60V between
two computer chassis. One computer chassis, an idiot
*cut* the safety ground in a distribution rail. Paul
touches the computer on that rail, touches a properly
grounded computer, and gets a *shock*. An arm to arm shock.
It took me a while to find the hidden wiring fault.
Some engineer had cut safety ground, in order to do
a floating scope measurement. Which is a *stupid* thing to do.
You don't **** with building wiring... Doh.

Anyway, seeing a small AC voltage between the chassis
of the improperly grounded computer and some ground
reference, would not be abnormal. And it's the maximum
level of current flow, that helps determine how dangerous
it is. For example, if the leakage was microamps, you
might measure 60VAC with the meter, but if you put
a human body in the circuit, the voltage would drop
under load, to an unmeasurable value. The meter
typically is measuing an "unloaded" leakage voltage.
As soon as you put some load on the circuit, you
get some idea whether the leaker is "dangerous" or
"quite weak".

I can get the same effect, when using an oscilloscope
on Ethernet wires. You will find "tens of volts" of
AC noise on Ethernet wires, but the power source in
that case is extremely weak, and easily grounded out.
As soon as you touch it with your finger, there is
no longer tens of volts there.

Now, let's overlay the power supply states, with
the "shock potential".

Unplugged = 0 VAC
switched off at back = (Not sure)

Switched on at the back = Leakage voltage

PS_ON# from motherboard = Leakage voltage

The filter is fairly close to the front of the supply,
and we don't know if the main switch is an SPST or
an SPDT. I assume both Line and Neutral open for safety,
but I'm not sure.

Once you're switched on at the back, then I assume
you can see the leakage voltage on the chassis.

And don't be surprised if the meter says it is
a significant voltage. What you don't know at
the time, is what the leakage current would be.
The ATX power supply, should be similar in spec
on leakage to that "metal box filter" I showed.

*******

The PWR button on the front of the computer, the front
of that button is generally isolated from the back two
contacts. Only premium computers use metal for
everything, and you might get a chassis shock when
touching the power button. But the front of the
button, makes no circuit contact with the back
two wires.

+5VSB -----------+
|
GND +5VSB 2.2K ohm resistor
| | |
D D | |
Motherboard --+------+-----+---- PWR# |--+ Push
---------- GND |--+ Button
| (Momentary)

Now, if you were to touch the PWR# conductor, it's a
low voltage logic circuit. It does have a slight DC
voltage (pulled up to +5VSB). But cannot source much
current.

If you caused some of that AC leakage to flow, it
could slightly modulate the voltage on the PWR# wire.
But the circuit will generally have diode clamps
(the letter "D" being a stand-in for a diode
symbol I was too lazy to draw). Generally, the PWR# wire
cannot go outside the range of the two clamp points.
To cause the diode to blow, might take 5 to 70mA,
depending on where the diode is located.

There really isn't a reason for you to be touching
PWR#, but I expect the circuit would survive the
contact. As the leakage might be set at a couple
milliamps max.

The logic circuit on the motherboard is
latching, so that acts as a filter of sorts.
The only thing that might happen, is perhaps
the four second timer might not trigger properly,
if the amplitude of the noise on that wire was
high enough.

The important thing to note though, is there is
a bit of an "air gap" between the button
front surface, and the circuitry behind it.

*******

I still recommend the usage of a Safety Ground,
because I don't consider a visitor touching
your computer and falling on the floor, to be
a good thing. And an electrician should be
consulted, as to how best to get a safety
ground. There is probably a right way to do it,
and I don't know the answer. I'm not an
electrician, and don't know the national code.

I have had good luck, getting advice from
the building inspector number at City Hall.
The guy on the other end of the line said
"I didn't tell you this, but...". They will
provide anonymous advice, at least in an attempt
to prevent you from doing something really
stupid. Getting an electrician to do the work,
makes it "official". The electrician can only
do things "according to code". In some cities,
you are not allowed to do your own electrical
work, and only an electrician is allowed to do it.
And in some cases, an inspection is required too.

I don't know of a practical way to make the
computer safe, by insulating it. As you're always
touching the chassis, handling wires, USB ports,
and so on. Only the Safety Ground does a good
job of taming the leakage. If you modified the
filter inside the ATX supply, you might notice
a degradation in your TV reception. There can
be enough RF energy coming out of the modified
ATX supply, to cause an ADSL modem to drop sync.
So that option is out too. Modding the ATX supply
might generate a little too much electrical noise.

Paul