ground vs isolated ground

[captainkevin]

Large performing arts bldg just completed. On the stage we have isolated ground show switches. We are measuring 45-88khz @ 110-180 milivolts between the two grounds and neutral. Doesn't matter which you pick. This noise is not acceptable to sound equipment in theater. Everything is connected properly. This building is very close to major airport, and sits adjacent(less than 50') to light rail tracks that is DC driven. Could the building be picking up this high freq from the metal roofing? Or do I have a bad ground somewhere that we are creating a ground loop? The High Freq is a mystery to me.

 

[dereckbc]

Re: ground vs isolated ground

CaptainKevin, you are on a tough road. The first step is to trace out all the IG's and EGC's back to the power source. You need to be sure the IG's are truly isolated as they pass through any panel and junction boxes.

The IG's, EGC's, and grounded circuit conductor (neutral) should be bonded together at the service entrance or isolation transformer if used. Find this bond point and measure your voltages there. They should all be zero at that point with respect to each other.

Next you need to make sure you have a GEC connected to the same point as mentioned above. Again this will be at the service entrance or isolation transformer is used.

You may find that the IG originates downstream from the N-G bond point say at a sub-panel. This does comply with code, but not a proper design.

If everything checks OK then I suspect you have a RFI problem. If that is the case an IG circuit will be of no use, and will only compound the problem, since it acts as an antenna. An SG circuit that is multi-grounded would lower the voltages, but not enough. IG's, EGC, and GEC are only for safety, and are not suitable for HF work.

You may have to consider using other techniques such as balanced power, isolation transformers, balanced or optical signal transmission rather than a ground referenced medium to eliminate the problem. It could be as simple as installing a isolation transformer where the A/V equipment is located then run dedicated SG circuits to power all the A/V equipment.

Let me know more details, and I will try to help.

Good Luck

Dereck

[ October 22, 2003, 02:18 PM: Message edited by: dereckbc ]

 

[hbiss]

Re: ground vs isolated ground

45 to 88Khz and it's bothering the audio equipment? Are you sure you have those frequencies right? How are you measuring it? Most audio gear will roll off way before that.

Have you tried to locate the source? If it's from within the building it would be easier to deal with the source. First thing that comes to mind are dimmers and switching power supplies in your building. Would it be possible to shut the service or parts of the building down briefly to see if the noise goes away?

Another possibility is the rectifiers that produce the DC for the railroad.

 

[captainkevin]

Re: ground vs isolated ground

yes there are over 600 dimmers in the hall and probably more than 40 VFD's for the mech equipment. There are UPS systems and a backup generator. We have taken them all off line and the noise is still there. Meter is a new FLUKE. The consultant has the meter so I am not sure of the number. The equipment that notices the noise the most is amplifiers in the audio equipment. How come everyone answers the easy ones and no one tries this topic>?

 

[karl riley]

Re: ground vs isolated ground

45kHz is also in the range of electronic ballasts for fluorescents. (adjective used as noun).

Karl

 

[dereckbc]

Re: ground vs isolated ground

Originally posted by captainkevin:
The equipment that notices the noise the most is amplifiers in the audio equipment. How come everyone answers the easy ones and no one tries this topic>?

Captain Kevin I tried. The details you provided is limited, so the feedback is limited. I suspect your audio equipment is being powered from different branch circuits, and interconnected via shielded and/or grounded signal cables. The noise is caused from common mode noise voltages being induced via (EMI/RFI) on the EGC circuits supplying your audio equipment, being demodulated in the DC power supplies, and the signal is then amplified by the amplifiers. The problem is quite common and easily fixed once you know where the noise is entering.

There are several solutions such as installing isolation balum transformers on the signal cables to break the ground loops, installing an isolation transformer (or balanced power system) near where the audio equipment is located, or changing the signal transmission medium from unbalanced to balanced or optical mediums.

 

[brian john]

Re: ground vs isolated ground

Kevin:

This problem has been covered numerous times under various threads.

I do quite a few noise problems and have seen it blamed on everything from radio towers to FM (FXXX MAGIC). So far all the sites I have visited from major studios to war game rooms have ALL had one problem improper grounding.

Nothing elaborate required and actually the more elaborate the worse the problem is to locate.

1. Check for ground current.
2. Check for neutral ground voltage as you move from the source the voltage should increase, if the circuit and or panels are loaded.
3. Check for net current on all feeders and branch circuits.
4. Use an O'Scope to view the wave form assist in possible sources
5. Isolate the neutrals, (feeders neutrals and branch circuits neutrals and megger them, NO LOADS CONNECTED.
5. Check the load equipment for continuity neutral to ground (we found one manufacture grounding the neutral inside his equipment).
6. Isolate the loads viewing the test equipment see is the noise clears.
7. Supply the loads with the noise from a small UPS and turn off the Main Service see if the problem clears. There will be NOTHING on in the building!! with the exception of the UPS and the load with the hum.

I would not rule out outside interference BUT I'd bet on the above. If you tried all the above and did not locate the source, you missed/overlooked something. If this does not work contact me by private email I may be able to find someone that can assist you depending where your located.

 

[hbiss]

Re: ground vs isolated ground

The noise is caused from common mode noise voltages being induced via (EMI/RFI) on the EGC circuits supplying your audio equipment, being demodulated in the DC power supplies...
Possible. The "noise" is most likely a filthy waveform which is full of harmonics. Decent pro audio equipment is usually pretty immune to this kind of power line problem though. More likely is that it is getting into the audio signal path due to improper grounding of the audio equipment. If you can't eliminate the problem at the source if it is indeed external, you need to investigate that possibility.

A few more questions: You say that this affects the audio systems the most. What else does it affect? Has this been going on since the facility was built or has everything been fine then this cropped up? If so, was anything installed or any work done around the same time?