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[Tommy Peel]

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

[Scott Slater]

Have you put a meter on the electrical outlets involved?

[Kevin Graf]

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.

[Tommy Peel]

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.

[Kevin Graf]

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

[Tommy Peel]

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.

[John Roberts {JR}]

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

[Stephen Swaffer]

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.

[Mike Sokol]

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.

[Tommy Peel]

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

Haven’t determined if they’re on the same circuit yet; I’ll have to get help some other time with that.

I didn’t have a non-contact voltage detector with me, so I wasn’t 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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