Grounding problem, power supply or...? (also, it gives me strong shocks)

[Guest]

Hello to all.

I will try to explain this the best I can:


I use my computer for music composition, and I use a external USB sound card. I've attached to it a pair a speakers through a pair of cables, these:

http://www.thomann.de/es/cordial_cfu_3_pp.htm

the external part of these cable connectors are made of metal, and where those cables goes, are made also of metal, check the sound card:

http://www.thomann.de/es/behringer_xenyx_q502_usb.htm


With this on mind, some days ago I've touched the metal hangrip of those cables, which were connected at the moment, and I got a very strong shock. My very first tought was a ESD, but I dismissed this idea after discovering that the shock was continous. Also, the computer chasis gives me a milder continous shock, so, I thought on a grounding problem.

There's another problem to add here, and it's that this house only have 4 or 5 grounded outlets. After some tests, I discovered that pluging the computer in a ground outlet, the case or the I/O shield doesn't give me a shock. Also, if I only plug all the components (using the same un-grounded outlet and a power strip) but not the computer, the metal audio conectors doesn't give me a shock, leading me to believe that the problem lies on the case.

I've dissambled the computer in it's entirety, to check if the motherboard was touching the metal case, a bad connection or something, but I didn't find anything. Then, today I decided to test the PSU with a ATX tester, and these are the results:

the +12V, +3,3V and the+5V leds are green, and the screen is blue (also, I can hear 2 beeps), but these are the voltages (with no load / connectors attached, also, the fan was running at a very low RPM):

-12V | 11,6

+12V V2 | 12.0

5VSB | 5.0

PG | 260 ms

+5V | 4.9

+12 V1 | 12.0

+3.3V | 3.3

Those looks normal, but that -12V reading with no load or connectors attached doesn't give me good vibes...


The power supply is a Antec Neo Eco 620C, and the power comsumption of this computer doesn't even reach the 620W that this PSU outputs. Also, I've tested the PSU on two outlets; the
grounded one and the not grounded, and the readings are the same in both cases.

To finalize my explanation, when I've touched by accident one of the screws that attaches the PSU to the case, the PSU or the screw has given to me a very strong shock (my left hand still hurts a bit), and for a momment I tough that I was going to get glued to the PSU.

Note: this doesn't happen if I wear shoes or slippers that have a rubber shole, at least if I touch the I/O shield or the audio cables. (I received all the shocks when I wasn't wearing shoes)

Note 2 : I've also used 3 power strips to test, but it's the same.


Regards and thanks for your time.

 

[hang-the-9]

You have the answer here "After some tests, I discovered that pluging the computer in a ground outlet, the case or the I/O shield doesn't give me a shock" Of course you want to have the computer in a grounded outlet.

 

[Paperdoc]

Well, no shock when the case is actually grounded via a grounded outlet is helpful. BUT no computer case should give you a shock when it is not grounded.

What do you know about the electrical supply wiring in your house? I don't know where you live. I'm in Ontario, Canada. In this country and the USA (and many others) we use a "Grounded Neutral" system. That is, the power supplied to the house comes in on three wires - two Hot (opposite phases of the transformer) and one Neutral. The Neutral line is the current-return half of the system, and it is Grounded both at the supply transformer an at the breaker box in our house. Thus in our system, ONLY the two HOT lines into the breaker box are at some voltage greater than Ground (zero).

Your case sounds like the Neutral wiring in the outlet box is NOT grounded back at the breaker panel, OR somehow your plug (or the wall outlet) is wired backwards so that the wrong line from the wall is connected to the exterior chassis.

You describe a house with both ungrounded and grounded wall outlets. That raises several possibilities related to the original older wiring system. One is that the grounded outlets were installed later just so you can use things with 3-prong plugs, BUT the installation was not does correctly so that the Ground is not really connected. However, your description says the shock problem disappears when you plug you computer into that outlet type, suggesting that it really does have a properly-functioning Ground at those.

Another type of problem with older wiring systems is polarity of the wall outlet slots. First of all, often polarity was given no attention in those systems so you can't quite be sure what each outlet does. But look closely at the two outlet types you have. On the 3-prong Grounded outlets they normally are mounted with the round Ground hole at the bottom of the triangular slot pattern. Then the LEFT slot is somewhat taller than the right slot. The taller LEFT slot is Neutral and is designed to accept a wider blade of the plug you connect there. Now, look at your ungrounded outlets. Are the two slot sizes different? If so, the same rule applies - the TALLER slot should be Neutral. More often you'll find that older 2-slot outlets have the slots sizes identical, and you can't be sure which is Neutral. In that case you find it very difficult to plug in anything that has plug blades that are not both smaller.

As an experiment, you could try this. Your late paragraph says, "when I've touched by accident one of the screws that attaches the PSU to the case, the PSU or the screw has given to me a very strong shock (my left hand still hurts a bit)". Plug in your computer on a ungrounded outlet, but do NOT connect any other peripheral device to the computer. Now verify carefully that this still shocks. Then pull the plug out, flip it over and plug it in so now the blades are reversed. Does it still give a shock? If it does, the problem may be in the house wiring system. If it does not, then the plug on the end of the power cord is wired wrong, OR the outlet in the wall is. OR, maybe somebody altered the wide blade of the plug so it would fit into an old outlet style, and that has allowed you to plug it in the wrong way!

 

[CompGee]

I would get an electrician at the house NOW. The house was probably built in the '50s, and if that's the case, just have the place rewired, and all connections checked. The shocks you are receiving are of LINE VOLTAGE(120VAC@60hZ), and that is an extreme SAFETY HAZARD. Either move the entire setup to a grounded outlet, and tape off the faulty outlet(the one that's giving you shocks), or have the house rewired. Something is going to give electrically, especially with the high electrical demands of modern appliances and electronics, so you might as well rewire sooner than later.