I have just read another thread sort of in relation to this but my situation is a little different.
A couple of months ago I decided to plug my cable tv's providers RF cable into my capture card. Thats where it basically ended. Everything except the RAM module and chassis were cooked!
The facts of the situation and my notes are:
I unplugged the power supply cable from the PSU, and attached the RF cable to the capture card (for 1sec).
*Monitor power was still connected and monitor still plugged into pc.
The house outlet that is plugged into a surge protected power board, is powering the monitor (although turned off) and was powering the pc (now detached), has been deemed to be reverse polarity.
4th of July is nothing compared to my 1st of August!!!
My AC tests with a basic multimeter conclude that the chassis of the pc was "hot" (110V) when tested to the tv cable which is earthed independently across the road as well as being shocked when holding the tv cable in one hand and touching the chassis with my other. For some strange reason, now it only reads 50V. When tested between the chassis and the outlet earth, of course I get 0V.
My question is, will a reverse polarised outlet cause damage this extensive when a foreign earth is introduced to the situation? Or is there something else that is causing the outlet to be faulty?
Also why has the reading changed from 110V to 50V? and it does fluctuate slightly at different times.

When I now AC test the outlet it reads: (where active is the small slit, neutral is the large slit and earth is the 1/2 round pin)
Active > Neutral = 110V
Active > Earth = 50V
Neutral > Earth = 50V
When the above circumstance is recreated using a correctly polarised and correctly earthed outlet, the reading between the chassis and the RF cable is of course 0V.
Is anyone able to conclude from this information that tis is a defective outlet? Will a reverse polarity cause the chassis to be hot in the first place?
Any response of information will be extremely valuable and thankyou in advance for any information.

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A defective outlet cost me a computer years ago.
Got a new house, one outlet was wired wrong.
Now I have a “three-prong outlet testers to try them before I use one.
About 5 to 10 dollars in a hardware store. Fits in a tool box.

If the PSU was unplugged the voltage didn't come from there, and the only other source that you have given is the coax cable, take a reading between the center conductor of the coax and ground, and test it for polarity...is it DC voltage? The ground could be sourced back through the monitor making a complete circuit if there is voltage in the coax. There isn't enough information to really undersatand what's happening here.

The reversed polarity isn't that big of a deal as it is AC voltage, and most electrical devices don't care in which order they receive the two potentials, GFCIs are one of the exceptions. What I would be concerned about is the lower voltage. Is that receptacle warm to touch? You could have a poor connection which would produce a resistive load, these can be dangerous...fire. One thing you could do is to find out which circuit the receptacale is on and test the voltage in other location in the same circuit, and if the voltages are normal in the other locations then the problem is confined to the one receptacle. If you feel comfortable with this, turn off the power to that receptacle and test the receptacle with a meter to make sure that it is off, pull the receptacle out and examine the connections. Look for any discoloring, burnt wires, or signs of arcing.

What kind of meter are you using, analogue or digital?

You've got me curious now, so let me know what you find please.

@Gort: lol, nice to know now but irrelevant as the damage has already been caused. I have been advised of your mention also after the fact and will definetly be testing in the future.

@dcc: Yes, I initialy thought the coax was the source but tests have proven it to be ok, The cable company came out and tested it showing the stinger was providing the signal with no stray voltages, and the outer sheild was dead when tested to the earthrod outside the house. So im happy that is adequate evidence.
What I beleive to be happening is the earth is hot and maybe stagnant in the chassis until a circuit is created with the addition of the coax.
The outlet doesnt feel hot to the touch, your mention of resistive load intriques me. What is the situation there? The receptacle has been looked at by an electrician and he stated the outlet was reverse polarised and the box everything sits in is to small for the amount of wires and dual 3-pin outlet it is, also stated the wiring looks amatuerish but no sign of arcing, burning etc.(He only tested the outlet, not this scenario). All 3-pin outlets in the premesis have been tested as reverse polarity and the earth pin varies between outlets, the one in question seems to be providing a voltage, others are not earthed at all. There is one outlet only that is wired correctly, which im coming to you on now :) Also not to mention one 2-pin outlet has caught on fire a few years ago. I may also mention, this is an old house and looks to have been made with 2-pin outlets allround and some have been ammended to 3-pin at a stage.
I am using a analogue meter, but the same tests have been performed using a digital and the readings are exactly the same.
Basicaly what I am trying to figure is, if the outlet had reverse polarity (Active & Neutral switched) would that cause the chassis (or even earth) to be active? Is there a faulty element causing my situation?

I'm trying to give all the details as best I can. I have knowledge of electrics but this has me stumped and is a little over my head. I just need to understand the scenario as the situation has been deemed non hazardous but I have completely lost a pc and been shocked multiple times. Seems a hazard to me and all fingers point to the outlet being faulty.

Just as I type this i'll also add, I have tested the scenario with multiple PSUs and monitors to the same outcome.
And if I do exaclty the same tests on this current outlet (wired correctly), no voltage across the chassis and coax.

Cheers for your help, this is very important to me.

Hey hell, I'm no electrician - I'm a home handyman. And we have a bit different wiring here in Ausssie to you lot. But what you're describing sounds like exactly what happens when some dill hooks up a live wire to an earth wire somewheres in the house circuit. It'll go good till it don't, and when it don't it don't dramatically!

Hard way to sort it out if that's the case is to check things thoroughly at every junction and whatsit in the house circuit.

Easy way (especially as it's an old house) is to rewire the house. Cable's pretty inexpensive, and if you know what you're doing (or can follow instructions) you can lay out all the cabling and connect it up yourself, all bar the connection to the house power box. Then get an electrician to check it, authorise it and connect it up lve. :)

@Catweasle: I am from Australia but living in the US. I think much the same theory can be applied here except im dealing with 110V not 240V.

I have drawn a diagram to show what AC currents have been tested between certain pins, the coax and the pc chassis.
*Note the "X" is related to the earthed pin which is not connected at the outlet. A 2>3-Pin converter is inbetween.
Doesnt seem to matter if source power is from an earth pined socket or not.
Plus the 40V readings can be substituted for 50V today (it seems to vary) and on the day of my pc getting cooked substitute 110V!

Also in further research I have found: http://en.wikipedia.org/wiki/Talk:Domestic_AC_power_plugs_%26_sockets/Regarding_%22Grounding%22_a_Casing_to_the_Neutral
It seems weird how they talk about Earth being tied to the Neutral and ~50% of cases are found to be live. And the neutral carrying transient currents. Sort of sounds familiar.
Its also interesting to notice that the Active and Neutral pins do not appear to be reversed! maybe indicating a fault with the earth?
What do you think???

Okey dokeys. Like I said I'm a home handyman not an electrician. Readers Digest Home handyman books are the go with me, not circuit testers and wiring diagrams.

Without wading my way through reference resources and the like, one thing slaps me in the face from all that.

Did you just say the outlet doesn't have the earth connected? Seems like it to me and if that's the case it's an outlet I wouldn't use.

Also, are you saying that different outlets in the house have different pins 'live'? Sounds like that to me too.

It should all be the same. And it sounds like the house wiring is stuffed!

This gets better by the minute...I definitely would want to see just what that ground is attached to, if it doesn't go back to the pannel then you could experience transient current. I'm curious about what you're finding when you meter this out, with the line and neutral reversed you still should only be able to see voltage between the ground and the line, and neutral and line, not from neutral to ground no mater what side the two potentials are on, there is only one potential present in that situation.

There are a couple of things I would want to check out, I would like to pull the receptacle out far enough that it is clear of the metal box and see if there is any voltage readings from the box to neutral or ground, if there is, look for a wire shorting to the box. While it's out check to see if any of the previous readings change.

I do agree with Catweazle on one count, if the wiring is that old you might consider a service upgrade, and if the original wire is copper romex and is in good shape, a electrician should be able to install a ground wire and replace the two slot receptacles. Personally, I would spend the extra money for new romex and just do it right.

Unfortunately I am unable to perform indepth tests other than what I normaly see on the outside, I reside in a rental property and this situation is turning more and more sour.
I do know that the box holding the wiring in question is plastic not metal so no chance of anything in that respect. I would love to take the receptacle out of the box myself to inspect the wiring and as you say test the outlet in that situation but due to legal implications can not.
Its funny because there has been 2 separate electricians assess the outlet, first sparkie#1 deemed it reverse polarity, pulled out the receptacle and stated he had to turn the circuit off. Then sparkie#2 also deemed it reverse polarity and stated the circuit was safe. Then sparkie#2 attends today and says its not reverse polarity but "broken neutral" and wants to rewire/replace the circuit.
To make matters more complicated no one can explain to me why the chassis is "hot" when plugged into that outlet! (qualified electricians my arse). They can only say its the coax, which has been tested and proven its not the cause or the computer is faulty, but I have had 3 different PCs with different PSUs and if that was the case i'm sure the computer would be non operational.

So the feeling I get from you guys from the evidence presented, there has to be a fault with the circuit.
Also a main point I am trying to conclude is, that if the situation was was that the outlet was reverse polarity and the earth was completely good, or non-existant the effects of reverse polarity wouldnt cause the chassis to be live anyway?

Unfortunately my hands are tied with the testing side as the electrician has disconnected the circuit at the breaker. This thread is more for personal knowledge to try to understand the situation at hand.

Also a main point I am trying to conclude is, that if the situation was was that the outlet was reverse polarity and the earth was completely good, or non-existant the effects of reverse polarity wouldnt cause the chassis to be live anyway?

Okay. To clarify:

'Reverse polarity' means bugger all in relation to your power outlets and household power wiring. It is alternating current, which means that the current uis constantly changing direction anyway. See here for an explanation.

Reverse polarity would be meaningful INSIDE your PC, because the power unit transforms the AC household current into a reduced voltage DC current, and you can't switch direction of the power flow or components get cooked.

You've added a co-axial cable (it sounds like) into the mix, and coaxial cable carries the signal in the middle wire, whilst the (usually braided) outer sheath is connected to electrical ground. Earth for the househild AC current and the coax signal's cable is shared.

Now it seems to me that somewhere in the household wiring, an earth is connected to a live wire. It may be just at the outlets you're using or it may be elsewhere. When you've plugged into that PC it's fried the whole shebang so it sounds like the outlet you're using is kaput, and that connecting the coax right there has brought the kaput to the surface.

It also seems to me that the 'electricians' you've been talking to are dills, if they're talking about 'reverse polarity' in relation to the household power circuit.

Your explanation of reverse polarity is exactly my understanding also.
Its funny to me as it is basic electronics where I can't get a solid response from a qualified electrician.
Ohh and the coax cable does not have a common earth with the house, it is actually run from across the road where the cable is independtly earth rodded at every 3rd telephone pole.
Which actually make things easier to test ;)

Everything you have stated is exactly how its written in my book aswell.

Thankyou for your responses.

The revresed polarity in most cases isn't a problem, as I stated earlier,
the reversed polarity isn't that big of a deal as it is AC voltage, and most electrical devices don't care in which order they receive the two potentials, GFCIs are one of the exceptions. In other words, no, the reversed polarity wasn't the problem, the problem is the short in the ground wire. Get those lazy arsed electricians back and make them trace that new recetacle back to where it was added to the circuit and repair it, and have them correct the polarity on all the reversed receptacles.

It too bad your not close by, as an electrician this is the type of mystery that I enjoy solving. By the way, the question regarding the type of meter you were using, DMMs have a tendency to "ghost", that is read voltages that don' exist when reading de-energized circuits.

You might also talk to you landlord about their wiring being responsible for frying your pc, what you did shouldn't have produced the results that you had.

I do agree with Catweazle on one count..

Does that mean I was wrong on others? I'm curious to know. I thoroughly agree with the 'landlord' comment as well. The damaged equipment is his responsibility.

Thankyou both for your input into my situation. You have both been a world of info for me.

Cheers mates!!!

And by the way if I get to the bottom of the problem all-together, I will post back here the outcome of the issue at hand.

Catweazle...poor choice of words on my part, nothing derogatory was implied. ;)

Oh! Thanks. You had me wondering.

I still think the problem relates to shared earth coming into contact with a live wire, by the way. I'd bet that co-ax comes into contact with household earth somewheres between the outside of the house and the outlet the PC is plugged in to ;)

I don't know what part of the country they are in, so I can only guess on their grounding methods, almost all low voltage systems aren't subject to the National Electical Codes, it is possible the cable is sourced else where. But I seriously doubt that the ground on that receptacle is connected to a porper grounding device, water pipe, gas pipe, ground rod, UFER..., because if it were it would be tripping the breaker. Where I'm living the code requires either a UFER gound or a ground rod, and in addition to this it is required to bond the gas and water pipes to the other ground device from the main pannel, but older houses fall off the radar by "grandfather clauses" which means that nothing is required in the way of changes until there is work requirig a permit.. But the bottom line is this person has a short to ground someplace, and if they are reading voltages as is described, then the source of the short is on the other 110 Volt bus bar, that's about the only way that thses results could occur. Specifically, the middle diagram showing voltage reading from both the neutral and line sides of the receptacle. I think I'll go fishing and think of something other than work. :cheesy:

OK, finally had the town electrical inspector do some testing after going through the State Electrical Inspector. (NH by the way)
He noted to me 4 National code violations being, 1) the box was to small for the outlet. 2) To many wires for the type of outlet (A double 3-pin) 3) cant remember 4) cant remember.
He said the low voltage reading was indicative of a definite issue as anything other than 0V or 110V was a problem!
Also he sourced the problem to be in the wall, between the on/off switch and the outlet, maybe a broken/melted/shorted wire with carbon build up causing the variance in readings.
The earthing system is an addition to the circuit that appears to be of an amateur nature.
Unfortunately I wont be able to post the exact issue as my landlords have given us a huge shit fight over this and as we are vacating in 2 weeks, there is no vision in the near future of them fixing the issue......mind you as the electrician has disconnected the circuit at the breaker, it has been deemed safe (although no elec to that part of the house.

Thanks again for your input

Hit him right between the eyes with a bill for your computer! The guy has the balls to rent a potential fire trap like that, the least he should be responsible for is the repair or replacemnt of your computer. Is he evicting you or are you leaving under your own schedule? If the local inspectors opperate like they do here in Ca. he may well have a healthy electricans bill before he can rent that place again. If it were me there I would be having a three legged race to my favorite Proctologist with a deserving party. Sorry this turned into such a can of worms for you.

I am not of a litigious nature, but you should get some form of compinsation from this. Like I said, bill him for the computer, and if you paid any of the elecrticians make him reimburse you for that also. If they refuse, take them to small claims court...as an electrician you can take this legal advice for what it's worth. :eek:

Agreed. Commiserations, and he should certainly be liable for the damage to tour equipment.

We are vacating under our own advise and are pursuing further action with all the evidence in our posession.
Your exact comments have been my argument from the start but I have had a very hard time with the "proffesionals" proving my case.

Cheers lads!


I had exactly the same problem!

Same Meter measurements too!

Turns out the guy who installed the PC ran out of outlets. He ran a power cable from a remote outlet on the other side of the breaker box (different phase). My meter read the same. Each time I connected the printer to the PC :eek: kerfluee! Blew a crater in one of the ICs.

Make sure all peripherals are connected to the same outlet!


Bug you later....

Happy welding. ;)

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