PC Hardware

w_tom · Guest

To answer your question, determine how much current is
leaking from that UPS. If UPS connects into some other three
prong (properly connected to breaker box safety ground)
receptacle and it trips the circuit breaker, then UPS is
definitely broken. If you plug the into a GFCIed outlet that
has the equipment ground, and if the UPS does not trip the
GFCI, then the UPS is probably OK - leakage current is below
5000 microamps. However UPS should be below 150 microamps
which is where meter makes a better measurement. But I would
do these tests first so that too much current does not
'accidentally' go through and damage meter.

Do final test with the meter. But that means meter in
current mode must connect between UPS and breaker box ground.
IOW the safety ground connection must be through something
with low resistance which a tile floor does not provide.
Water pipe might be good here for a temporary test when no one
is at risk. This assumes pipes are all metal all the way back
to breaker box ground and no one is taking a shower. Another
technique is to run a three wire extension cord to some remote
receptacle that is properly safety grounded.

UPS must not create massive leakage when plug is connected
properly AND when AC plug is reversed. Last test to confirm
leakage is not different when plug is reversed. Reversing
plug should not create excessive leakage.

Cyrille Briegel wrote:

Cyrille · Guest

I checked the breaker box and it seems to have one (and only) green
wire that could be the ground. I used the voltmeter to check between
the red wire and the green one and it reads around 110v. So the green
wire could be either the ground or neutral.
To make sure, how can I test that? I took a digital picture of the
breaker box. I could send it to you by e-mail if you wish to take a
look at it (how's the wires are connected). It's seems like in Taiwan,
electrical code is not as strict as in the US or Europe. Even polarity
for the outlets is up to the electrician's mood at the time!
Thanks again for your help!

w_tom · Guest

By breaker box, I assume it is the building breaker box (in
US or Taiwan?). Throughout the world, green is the color of a
safety ground wire. Solid green in the US. Green with yellow
strip internationally. That green wire must connect to
neutral bus bar inside breaker box. IOW it must only carry
current when it is shorting out a fault (or carrying away
leakage currents).

You have a breaker box to measure to. Take a jumper wire
from the UPS safety ground prong to the breaker box cover - a
metal surface. Extension cords can be used to make this
connection. This test should not trip any breakers. This
test only to verify excessive current does not harm meter in
this next test. Then use meter to measure current from that
safety ground prong to breaker box. That current should be in
the microamps region.

BTW, voltage from breaker box cover to that tile floor
should be zero. Just another test you might perform; might
provide a surprise fact.

In Taiwan, the 'get it done no matter what' attitude is too
acceptable. Reason why a whole and new IC foundry for UMC
burned - end to end. Failures happen for good reason. Your
concern for that possible 'hot' safety ground is quite
admirable and responsible - especially to those who will use
that UPS later.

Cyrille wrote:

Cyrille · Guest

Let's assume the breaker box doesn't trip while connecting the UPS'
ground prong to the breaker box cover, I won't be able to mesure the
current with my multimeter because this one can only mesure DC Amps and
not AC Amps. Is there other ways to mesure that?
BTW, the breaker box is inside my appartment (in Taiwan)

w_tom wrote:

w_tom · Guest

Meter only does AC volts. Forgot some meters are made that
way. Get some resistors from Radio Shack or equivalent. For
example, a 10000 ohm (10K ohm) quarter or half watt resistor
can be put in series with the 'safety prong to breaker box
cover' jumper wire. 110 volts across the 10000 ohm resistor
would conduct 0.1 amps. Don't wait too many seconds because
this resistor will get hot - eventually burn up - if leakage
current is excessive. However, if this resistor does not get
hot and if voltage across this resistor is much less than 110
volts, then we know the leakage current is not a direct short
to the hot 110 volt wire.

BTW, color bands on a 10K resistor would be brown, black,
orange.

So how much is the leakage? Voltage across the resistor
divided by 10 is number of leaking milliamps. For example,
with that 10K resistor between UPS safety prong and breaker
box cover, if voltage on that resistor is 30 volts, then
leakage would be three milliamps. Too much leakage but not
extremely dangerous. I just would not have a three milliamps
leakage where wet humans could touch it. Otherwise, no
serious safety problem. (If resistor measures 110 volts, then
resistor is dissipating over 1 watt - it will not last long.)

Now if that measured voltage is seventy or less, then replace
the 10K ohm resistor with a 100 ohm (color bands are brown,
black, brown) quarter or half watt resistor. Measure this
voltage. Voltage times ten would be the leakage current.
Sometimes this might measure .3 volts if leakage current
remains 3 milliamps. However sometimes the leakage current
may increase. That 100 ohm resistor voltage might measure 0.8
volts meaning the leakage current is a higher 8 milliamps.
Therefore we have a better idea of what to look for as a
leaking internal component.

If current leakage was 150 microamps or less, then voltage
on the 10K ohm resistor will be 1.5 volts or less. Voltage on
100 ohm resistor is less than .1 volts. 150 microamps is
acceptable. 150 uamps would create 110 volts when measured to
the tile floor, would give UPS a 'funny' feel to human touch,
is not enough to trip a GFCI, and is not considered human
dangerous.

A 1.5 volts (or less) on 10K resistor means we need not
worry about that 110 'floating' volts as long as all computer
components interconnect to a common safety ground (ie the same
power strip). A safety ground connects to all computer
peripherals but not connected back to breaker box due to a two
prong wall receptacle.

BTW, e-mail that breaker box picture. Curious what you have
there. Never saw the insides of a Taiwan breaker box.

Cyrille wrote:

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