Thinking in terms of device. The ground lug is isolated from the mounting ears of the device which is connected to the rest of the metallic piping and building components which do go back to the ground in the panel. I am thinking in terms of an isthmus where the land erodes away (or the sea level rises or whatever) and the people are then isolated from the mainland and therefore insulated from it's cultural effects (electrical noise from the mainland).
By not self grounding to the junction box (as you mentioned) one breaks the ground loop and the aggregate noise of the buildings systems stay out of the AV system. The isolated ground system does not guarantee a noise free system however it prevents by design the vast majority of the noise pool found in the building's systems from trying to use the AV equipment as a path home.
tomahto :grin:
MacG
The problem I see is in most NEC compliant installations there is usually no need for ISG's this is because there should be no current on the grounding to start with, now in the real world we no this is not always the case, but it should be, because if you have current on the grounding then you have a safety problem.
But when you mention School this brought a whole new problem to mind, crossing from one supply system to another.
this in itself will render a ISG system useless, having the ISG in one building land at it's panel, and having another ISG land at a panel in another building will cause current flow on other parallel paths between these two buildings, its un avoidable, I have done A/V, computer, and other shielded LV system between buildings and the only way to install a compliant system is to do common path isolation, simple isolation transformers at each building on the signal lines will go along way to keep unwanted noise out of a system. each application can have its own challenge. but proper isolation at the signal level goes much farther then trying to do ground isolation. there is no true NEC compliant way to do ground isolation that meets code.
And it helps with lightning if one building gets struck.
One method that is used but can have problems is the real technical power supply, its nothing but a 120 volt transformer with a 60 volt bonded center tap.
one of the problems with it is the 60 volts to ground, it wont clear breakers fast enough, so ground fault protection must be used, it does lesson the noise levels but does not get rid of the noise completely, and since the center tap is grounded to the service neutral, it will not stop cross building current,
I spent a long time at Electro-Voice at Buchanan, Mi. learning all about commercial audio systems and receiving a sound contractors certificate to be able to do there installs, and one thing I learned is you can't get away from dirty grounds, but you can stop the 60hz current from flowing on your signal grounds by isolation in the signal cable. simple 1 to 1 600/1000 ohm transformers work great.
there are many systems with problems that can cause A/V problems, like cable TV, bring in a cable from the pole and it will be bonded to the MGN out on the pole, and again to the service grounding, it is already a parallel path for the neutral, now tie it to a A/V system and this current will put 60hz all over your system, use a 75 ohm 1 to 1 isolation balun and it's gone.
this is why I say it is very important for A/V techs to learn the art of low level signal isolation instead of trying to buck the NEC in isolating the electrical grounding.
you will find it is much easier.
