Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Tim McCulloch]

You have a floating neutral on the PC outlet.

[Kevin Graf]

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.

[Jonathan Johnson]

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 — especially in the past — for lazy electricians to not nut/crimp ground wires. A mechanical pressure connection is apparently required by the NEC.

[Chris Hindle]

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

Have the venue bring in a licensed electrician.
Seriously.
What other "time bombs" are waiting?
Chris.

[John Roberts {JR}]

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

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

[Stephen Swaffer]

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>

[Brian Jojade]

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.

[Tommy Peel]

Been a while since I updated this thread. Yesterday I had an electrician friend check out the situation and it's about what I expected.  The problem circuit(s) are on an old(70s) un-grounded panel, so the only way to get proper grounded outlets is to replace the panel and run some new cable/circuits. We're discussing with the church how to proceed. In the mean time is there anything we can do(short of replacing the panel, etc..) to improve our situation?

[Geoff Doane]

The problem circuit(s) are on an old(70s) un-grounded panel, so the only way to get proper grounded outlets is to replace the panel and run some new cable/circuits.

I would have thought that ungrounded circuits disappeared in the '60s, but it might be different where you are.

My own house (built in 1960) originally had no grounds.  I'm in the process of upgrading the service for a heat pump, and my electrician tells me it's "legal" to keep the 2-wire stuff, but any outlets have to be on a GFI.  That's probably what should be done first to keep people from being electrocuted.  Your jurisdiction may be different.  Others have made some educated guesses about the original fault.  An RPBG is a possibility, or there may actually have been a fault in a piece of equipment that put 120V on the chassis.

A non-contact voltage tester (NCVT) is the slickest way I've seen for testing for voltage where it shouldn't be (and where it should, although it doesn't give you a quantitative answer).  One of the best $25 investments I've ever made.

GTD

[Jonathan Johnson]

I would have thought that ungrounded circuits disappeared in the '60s, but it might be different where you are.

My own house (built in 1960) originally had no grounds.  I'm in the process of upgrading the service for a heat pump, and my electrician tells me it's "legal" to keep the 2-wire stuff, but any outlets have to be on a GFI.  That's probably what should be done first to keep people from being electrocuted. 

In the United States, the principal of "grandfathering" generally applies in construction. Basically, anything existing may remain as-is if undisturbed during construction, even if it doesn't meet current building codes.  However, if it is disturbed, it has to be brought up to current codes. So if there's a 2-wire ungrounded circuit in a wall that won't be opened up, you don't have to upgrade it. But if you open up the wall, you're obligated to upgrade the wiring in that wall, too.

[ProSoundWeb Community]