I have an Electrical Contracting Co. and made a service call today to diagnose an open ground situation on one circuit in the Den area. I started at the first box where the home run lands and worked my way to the last receptacle. Found one that needed replacing as it had come apart. There is No grounding conductor just the grounded conductor. The house was built in 1972 but the wiring was romex with grounds and those are tied in with the neutrals in every box along with ground clips attached to the boxes which are metal. Now keep in mind too that the original panel is in the garage with a 200 amp MCB with all neutrals/ grounds connected together as a primary panel. Then someone came in and added a new exterior service panel with feed through lugs to feed the panel in the garage and neutrals/grounds tied together. I was actually there to originally install a ground bar and isolate the sub panel neutral bar. But no ground pulled to the sub. So that needs to be done but a long way from the exterior panel... so another project for another day soon. But here's where it gets interesting. They have a sound board/system and a lot of computer equipment for making music. So I started isolating the equipment. I found that when I plug their mixing board into the outlet ( powered off) that corrects the open ground on my tester and reads "correct", unplug it and it shows back to open ground. Electrically everything checked out with all receptacles. Anybody have any idea as to what is going on here?
Open Ground
- Thread starter lisco
- Start date
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Are there audio cables from the mixing board to other things on a different circuit that is grounded? The shields of audio cables are grounded to the chassis of audio equipment. In turn, those chassis should be grounded to the power cord ground if it has one. Even a computer would have the "ring" terminal on its audio connector grounded.
If you yank out every cable from the mixing board, the "ground fixing" will probably go away. This is one reason people start clipping gorund wires on audio cables because of the ground loop this board created. The problem most likely isn't the mixing board, but inside the receptacle box that supplies it.
If you yank out every cable from the mixing board, the "ground fixing" will probably go away. This is one reason people start clipping gorund wires on audio cables because of the ground loop this board created. The problem most likely isn't the mixing board, but inside the receptacle box that supplies it.
Replaced the receptacle and was at the end of the run, getting great voltage at 123 and no real audio cables. They have a small ground wire going from the mixer to a turntable but again.. there is no equipment/grounding conductor in the whole house but in every other location I get a "correct" reading on my tester.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
I suggest unplugging the mixing board and measuring the resistance between the neutral and ground prongs of its plug. Perhaps the mixing board has continuity between neutral and ground, and that's what's making the tester get a "correct" reading that indicates an intact ground, even though it's due to an improper connection.
ptonsparky
Tom
- Occupation
- EC - retired
Why do I think some of the older audio equipment was the cause of more than one death?
There is no equipment ground present only hot/neutral wiring throughout the home.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
When you used your tester and it indicated a "correct" ground with the mixing board is plugged in, was the tester plugged into (or otherwise sourced from) the same receptacle? If so, then the tester could be responding to a neutral-to-ground connection in the mixing board even without an EGC on the branch circuit.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Sounds like "bootleg" grounds.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Older guitar amps only had a two conductor unpolarized AC cable, and the chassis was connected to what was assumed to be the neutral, although the plug could be inserted either way. My old Fender Super Reverb was that way, and there was a three position "ground switch" (aka the suicide switch) that connected a capacitor tied to the chassis to either pole of the incoming AC line or nothing (middle position). The standard procedure for setting up the stage was to try plugging the amp in one way and turning it on, testing for shocking at a mic while holding the guitar strings and listening for hum, and then fiddling with that switch and reversing the plug until it seemed safe and the hum was as low as you could get it.
I never got killed but I got the crap shocked out of me a few times. I still have that amp but I modified it by installing a three conductor AC cable and disconnecting the suicide switch.
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Yes, it was plugged into the same outlet on the top and plugged the mixing board into it and then it's correct. When they bought the house the open ground was in the inspection report. Every other circuit in the house tests correct... just this one circuit. And when you plug the mixing board in ... all of the 8 outlets on the circuit read correct.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Try this: unplug the mixing board and check for N to G continuity on the AC cord going into it.
will do
growler
Senior Member
- Location
- Atlanta,GA
And now we know how Rock & Roll was invented.
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