Just for fun I measured those values on a atx psu connected to a 100W load, trough a insulation xfmr. I measured 60-65V, and 70µA. This current is so tiny you won't normally feel it, so no danger to human beings. However, if some equipment is earthed, and some not, it might destroy electronics. Also note that this was in a 220V IT circuit, which means that earth is nominally floating.
-- MVH, Vidar www.bitsex.net
....if of course one enjoys the safety and thrill of getting shocked by your computer case. :-) The situation may be relatively common, but it's far from normal or safe.
The only responsible solution to the OP's quandry is an electrician, plain and simple.
Then again, what would I know about electricity?
Best Regards,
-- /"\ ASCII Ribbon Campaign | Todd H \ / | http://www.toddh.net/ X Promoting good netiquette | http://triplethreatband.com/ / \ http://www.toddh.net/netiquette/ | "4 lines suffice."
You might be shocked (pun intended) to learn that capacitors (and other components) sometimes fail.
However, if some equipment is earthed, and some not, it might
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Sure, it's 60V as seen on a 10 MOhm meter, but the source impedance is on the order of a MegOhm. That's never going to provide enough current to be dangerous. Of course that assumes that all his components remain in good condition. If something actually fails, then you're right and he is without a safety ground.
He could experiment with this by touching both "grounds" at the same time and measuring the voltage on his PC chassis at the same time. He'll discover that he has grounded the PC and that there is no longer a voltage there.
He's already explained that he gets a shock whenever he touches both parts. I believe him. The fact that he's done this several times with no extreme effects implies that it's a perceptable amount of current, but not a dangerous one. This is what one would expect from a power strip with surge protection that lacked a ground.
He could probably improve his situation by just grounding his PC to the cable shield. This carries the possiblity of passing a shock on to someone working on the cable system, however, so that's why I really don't recommend it.
And I agree, we would ALL be better off with all grounded outlets, but there are many old houses around which just don't have them.
------------------------------------------------ Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
-----------------------------------------------
If that happens, then why does not my GFCI breaker trip?
-- Tzortzakakis Dimitrios major >
(the
GHz
Then why is a computer in a fully enclosure metal case?Provided, that a computer works with voltages not higher than 12 volt, and the PSU is already shielded, why not put it in a plastic case?
-- Tzortzakakis Dimitrios major >
the psu, not
on the input,
nothing to do
with induction
connected to a 100W
70µA. Thisdanger to human
not, it might
According to several references, GFCI designed for personnel protection will trip when leakage current exceeds 5 milliamps.
Because its pretty hard to make a computer double insulated.
and the PSU
See laptops - computers that have voltages < 20 volts entering their cases and have plastic cases.
Because of RF emissions. It can/will make inteference with other electronic equipment, but it is not the RF that induces the voltage. Besides, notebooks is plastic cabinets... And so is the plexiglass cabinets that is commonplace those days. But you require a common ground, and so metal is practical, in favour of running a ground cable to every device. Also metal is more durable than plastics. But yes, a computer emits _huge_ mengths of RF, enough to disturb a radio, but not enough to induce 60V in the chassis. Maybe a few µV.
-- MVH, Vidar www.bitsex.net
No, not really. The 12V is 12V from ground. Not some 1xx volts from ground, so it'd not have to be double insulated. The PSU would have to be, and that would be pretty easy.
And doubly so because the PSU leakage is capacitors connected to gnd/midpoint _on purpose_.
Yes bad habit, replying to post and posts child.
-- MVH, Vidar www.bitsex.net
To provied FCC (and other regulatory) EMI shielding. Many of those plastilc looking cases are either coated or contain carbon or other conductive material. See FCC rules, Part 15
, _ , | \ MKA: Steve Urbach , | )erek No JUNK in my email please , ____|_/ragonsclaw snipped-for-privacy@JUNKmindspring.com , / / / Running United Devices "Cure For Cancer" Project 24/7 Have you helped?
Yep, which is _far_ from 70µA. But in Norway, the norm is 30mA. However, Norway has a IT-net, not a TN-net. (Or was it TT? Never remember the difference :P)
-- MVH, Vidar www.bitsex.net
1) the leakage current is below the GFIC trip threshold (this is likely, as the threshold would be set to prevent 'nuisance trips) 2) the GFIC breaker is defective or improperly installed. 3) You are not plugged into the circuit protected by the GFIC breaker , _ , | \ MKA: Steve Urbach , | )erek No JUNK in my email please , ____|_/ragonsclaw snipped-for-privacy@JUNKmindspring.com , / / / Running United Devices "Cure For Cancer" Project 24/7 Have you helped?
personnel
30mA.In the US - we consider 30 ma to be the limit for protecting equipment. The threshold for protecting personel is 5 ma.
Yep, which relates back to that in a IT-net you have insulation between the midpoint in xmfr and gnd, that also limits the current, while in the US you have a TN net where gnd is connected straight to the mid-point. Don't ask me to explain the differences, I only know that the norwegian system is considdered safer, and there's two countries that is using it. Norway and Peru or something like that. So we have 2 wires, with a potential to each other of 220V, both being "hot" relative to gnd.
Perphaps someone else in here can explain the difference between a IT and a TN/TT net?
-- MVH, Vidar www.bitsex.net
is
protecting
insulation
current,
straight to
See reference, below.
there's two
like that.
220V, both beingbetween a IT
pp 7-8
Thanks a lot :) That was a quite informative pdf, actually I should've remembered my electrical installations book was 2 meter from me... Thought I'd sold that book after last year, but anyway, that PDF described it better than the book!
I had a peek at the rest...:)
-- MVH, Vidar www.bitsex.net
Right on both points. It's that pesky second point that bothers me. In addition, that small but probably continuous current could be damaging to sensitive components; remember we're dealing with a computer and a cable modem here, lots of low voltage, low current, high speed "stuff" that could be damaged by strong transients.
An interesting but flawed solution is to connect the power strip "ground" to the shield of the modem. That would give the strip a return reference, but one fraught with difficulty on a number of levels - power currents injected into an RF shield being the most conspicuous. I don't like it. I'd get an electrician.
-- Al Brennan "If you only knew the magnificence of the 3, 6 and 9, then you would have a key to the universe." Nicola Tesla
-- Tzortzakakis Dimitrios major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr ? "Steve Urbach" ?????? ??? ?????? news: snipped-for-privacy@4ax.com...
The threshold in europe is always 30 mA.
It's a new Siemens GFCI breaker I installed myself, and I hope I can install one correctly.
The breaker protects my whole house.
Have you helped?
Ah! the difference between international systems. The US residential mainly requires only certain Branch circuits to be GFIC (and now ARC fault) protected. Newer 'Industrial' systems have the "whole" service panel version.
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