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Author Topic: Fried computer motherboard. Potential grounding issue?  (Read 604 times)

Tommy Peel

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Fried computer motherboard. Potential grounding issue?
« on: May 27, 2020, 02:45:58 am »

So I fried a computer motherboard(oldish z97 board housing a 4790k) tonight at church while getting setup for our Tuesday night livestream. We're borrowing space in a fairly old church in a small town, so I suspect that the wiring might be on the questionable side, although we hadn't had any issues until now. This setup had been working fine for several weeks. We basically have an XR18 near the stage with some powered speakers providing monitors and FOH and at the back of the room is the aforementioned PC for running lyrics and, since the COVID pandemic, the livestream. The audio is provided via a 50ft USB extension cable connecting the PC to the XR18's USB interface port.

So tonight I had to hook the mixer back up and re-connect the FOH/monitors as I had used them over the weekend for a gig(PA is my personal rig, but it stays at the church and gets used for the weekly Tuesday night service). I had the mixer on while I was finishing getting it connected and my brother was on the PC getting it ready to go. When I went to plug in the USB cable(short A-B cable connected to the 50ft extension) to the mixer there was an arc and audible pop as soon as the metal on the USB connector touched the metal of the port on the mixer. Immediately the PC shut off and best I can tell the motherboard is fried; I don't have another machine handy to test the CPU. The power supply does turn on when jumpered and the mixer survived apparently unharmed.

Any thoughts on what happened and/or is going on with the electrical in there? Currently I don't know if the PC and mixer are on the same circuit, but I plan to do some investigating this weekend to find out. Is there anything I can test? I have a DMM and have done some household wiring.


Thanks in advance,
Tommy
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Scott Slater

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #1 on: May 27, 2020, 06:50:17 am »

Have you put a meter on the electrical outlets involved?
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Kevin Graf

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #2 on: May 27, 2020, 07:42:05 am »

My first guess is, one of the outlet boxes (or circuit) has both a bootleg ground and the Hot & Neutral reversed. So a 3 LED tester won't find it.
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Speedskater

Tommy Peel

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #3 on: May 27, 2020, 10:15:56 am »

Have you put a meter on the electrical outlets involved?

I plan on doing so this weekend. I didn't have a meter with me last night.

My first guess is, one of the outlet boxes (or circuit) has both a bootleg ground and the Hot & Neutral reversed. So a 3 LED tester won't find it.

I remember reading a post about those on here a few years ago. If I recall there was a way to test for it; I'll see if I can dig it up before this weekend.
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Kevin Graf

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #4 on: May 27, 2020, 10:30:12 am »

Link to article by forum moderator Mike Sokol:

"Failures in Outlet Testing Exposed"
'Why standard outlet testing methods fail to reveal reverse polarity bootleg ground miswiring situations'
by Mike Sokol
https://www.ecmweb.com/construction/article/20897728/failures-in-outlet-testing-exposed
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Tommy Peel

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #5 on: May 27, 2020, 11:13:31 am »

Link to article by forum moderator Mike Sokol:

"Failures in Outlet Testing Exposed"
'Why standard outlet testing methods fail to reveal reverse polarity bootleg ground miswiring situations'
by Mike Sokol
https://www.ecmweb.com/construction/article/20897728/failures-in-outlet-testing-exposed

That's the one I was thinking of; thanks for the link. I'll give it a thorough read and investigate the situation this weekend when I'm back out there.
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John Roberts {JR}

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #6 on: May 27, 2020, 11:44:41 am »

Besides the deadly RPBG energizing a safety ground, an open neutral with simple bootleg ground can energize safety grounds with return current from whatever is plugged in.

JR
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Stephen Swaffer

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #7 on: May 27, 2020, 12:26:53 pm »

Simple way to test is with a standard extension cord to virtually move the receptacles next to each other and meter them relative to each other.
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Steve Swaffer

Mike Sokol

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #8 on: May 30, 2020, 07:22:21 am »

Link to article by forum moderator Mike Sokol:

"Failures in Outlet Testing Exposed"
'Why standard outlet testing methods fail to reveal reverse polarity bootleg ground miswiring situations'
by Mike Sokol
https://www.ecmweb.com/construction/article/20897728/failures-in-outlet-testing-exposed

Yes, I would guess that's what happened. I call this an RPBG (Reverse Polarity Bootleg Ground) because it occurs when someone creates a bootleg ground (ground to neutral jumper) and the incoming hot and neutral wires are accidentally reversed. Old buildings with new grounded outlets are always suspect. That's because many places use DIY guys and unlicensed "electricians" to upgrade the power, but they cheap out and don't run new grounded wiring. Note in the article I wrote that you can use a cheap Non-Contact Voltage Tester to identify if a receptacle has a "hot" ground and neutral.
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Tommy Peel

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #9 on: May 31, 2020, 05:29:33 pm »

Well not sure what I found. On my 3-light tester both plugs only light the middle bulb indicating an open ground. Testing with my DMM shows the following:

PC Outlet
H-N 118v
N-G 55v
H-G 50v

Mixer outlet:
H-N 118v
N-G 5v
H-G 33v

Havenít determined if theyíre on the same circuit yet; Iíll have to get help some other time with that.

I didnít have a non-contact voltage detector with me, so I wasnít able to do that test for the RPBG. Also I was able to see, by removing the cover plate, that there is an actual ground wire coming into the box.

Any ideas on how to proceed?


Thanks,
Tommy
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Tim McCulloch

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #10 on: May 31, 2020, 06:18:23 pm »

You have a floating neutral on the PC outlet.
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Kevin Graf

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #11 on: May 31, 2020, 08:07:51 pm »

PC Outlet
H-N 118v
N-G 55v
H-G 50v

Those strange voltages to 'G' are what an electrician would call phantom voltages. Very different from mic phantom power voltages. A long unterminated AC wire will read strange/stray voltages on a modern high impedance DMM.
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Jonathan Johnson

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #12 on: June 01, 2020, 01:03:45 am »

My wild guess is a poor grounding connection.

It's worryingly common for DIYers* to just twist the ground wires together without a wire nut or crimp sleeve, thinking that it's not important to have a tight connection. Some won't even connect it at all. (In my opinion, a good ground connection is more important than a good hot or neutral connection. A bad ground connection is more likely to kill people than a bad hot or neutral connection.)

If your wiring is in metal conduit or flex and the conduit is providing the ground, a loose locknut or fitting setscrew can make for a high-resistance connection. If possible, pull a separate grounding wire into the conduit, bonding it to every box in the path.

I think that an isolated ground will prove unnecessary in almost all situations if the grounding system is well-installed.

EDIT: Was just reading online, and apparently it is common ó especially in the past ó for lazy electricians to not nut/crimp ground wires. A mechanical pressure connection is apparently required by the NEC.
« Last Edit: June 01, 2020, 01:16:55 am by Jonathan Johnson »
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Chris Hindle

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #13 on: June 01, 2020, 08:17:43 am »

Well not sure what I found. On my 3-light tester both plugs only light the middle bulb indicating an open ground. Testing with my DMM shows the following:

PC Outlet
H-N 118v
N-G 55v
H-G 50v

Mixer outlet:
H-N 118v
N-G 5v
H-G 33v

Havenít determined if theyíre on the same circuit yet; Iíll have to get help some other time with that.

I didnít have a non-contact voltage detector with me, so I wasnít able to do that test for the RPBG. Also I was able to see, by removing the cover plate, that there is an actual ground wire coming into the box.

Any ideas on how to proceed?


Thanks,
Tommy
Have the venue bring in a licensed electrician.
Seriously.
What other "time bombs" are waiting?
Chris.
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John Roberts {JR}

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #14 on: June 01, 2020, 09:53:35 am »

Well not sure what I found. On my 3-light tester both plugs only light the middle bulb indicating an open ground. Testing with my DMM shows the following:

PC Outlet
H-N 118v
N-G 55v
H-G 50v

Mixer outlet:
H-N 118v
N-G 5v
H-G 33v

Havenít determined if theyíre on the same circuit yet; Iíll have to get help some other time with that.

I didnít have a non-contact voltage detector with me, so I wasnít able to do that test for the RPBG. Also I was able to see, by removing the cover plate, that there is an actual ground wire coming into the box.

Any ideas on how to proceed?


Thanks,
Tommy
Proper voltage between hot and neutral suggests both are connected to something.

50V is typical of floating open ground when measured with high impedance meter. 

This does not immediately point to outlet wiring as the source of your computer's smoke release.

JR
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Stephen Swaffer

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #15 on: June 02, 2020, 12:43:51 pm »

The missing measurements are:

PC H-Mixer N
PC H-Mixer H
PC H-Mixer G

etc.  Especially PC-Mixer Ground

You added a metallic connection between the two-that's when those relative voltages become a big deal-and why code is very picky about bonding things that might get interconnected.  If that 55 volts on the PC ground is a result of a capacitive connection rather than just a floating "phantom" voltage that could easily cause your issue.  That capacitor could be on another device plugged into the same "set" of outlets but missing a home run ground connection-maybe from a piece of conduit that is pulled apart if that was used as the equipment grounding conductor..

The easiest way to tell the difference between a "real" voltage and "phantom" is with a VOM-or low impedance DMM>
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Steve Swaffer

Brian Jojade

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Re: Fried computer motherboard. Potential grounding issue?
« Reply #16 on: June 02, 2020, 04:41:47 pm »

The audio is provided via a 50ft USB extension cable connecting the PC to the XR18's USB interface port.


While not directly related to the problem, but indirectly so, it should be noted that the USB spec for USB2.0 lists a maximum cable length of 5 meters, or about 16 1/2 feet.  Using a 50 foot cable is clearly out of spec.  While it MAY work, it's one of those things that you should be avoiding because if it stops working tomorrow and someone tries to troubleshoot the problem, one of the first things to be looked at and blamed is what's out of spec. So now the entire design will need to be re-worked for a proper solution.

Additionally, since you're running a 50' cable, you're more likely to be powering from two different sources, so electrical issues between the outlets will end up traveling through the USB cord and can fry gear.  Most USB ports don't have built in protection/isolation for that sort of thing since they assume that it's a short cable run, so you'll probably be pulling power all from the same source.
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Brian Jojade

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Re: Fried computer motherboard. Potential grounding issue?
¬ę Reply #16 on: June 02, 2020, 04:41:47 pm ¬Ľ


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