The term isolated ground should be changed to shielded ground. The idea is that the outside of metal conduit or cable armor acts as an antenna for radio stations, atmospheric noise, and so forth but the inside of the metal acts as a shield against those things. I one time had an instance of a public address system that was picking up WQMX-FM. The cable for one of the microphones and the oscillator that acted as the change of class bell was and end grounded high impedance circuit using what is essentially a coaxial cable. The radio signal was coming over the outside of the cable. This is the reason why a lot of computer cables have an inductance core around the outside of the cable.

You cannot do an isolated ground using romex or pvc conduit, at not unless you are using shielded variable frequency drive cables.

Also, in the process of doing amp checks to figure out which neutrals are shared in commercial lighting areas, I figured out that about 10% of the neutral current for electromagnetic fluorescent ballasts flows through the equipment grounding conductor rather than the neutral. This consists mainly of high frequency harmonics and noise that flows readily through the lamp to fixture capacitance. Remember that capacitive reactance goes down as frequency goes up.

Also, unshielded variable frequency drive circuits will capacitively couple a high frequency tingle voltage onto adjacent motor circuits. For an allegedly deenergized 480 volt motot circuit this registers as about 30 volts on a voltmeter that is around 10,000 Ohms per volt. This consists of the high frequency carrier signal plus harmonics that flow readily through insulation capacitance, particularly THWN/THHN. It also flows readily through skin capacitance and has a rather nasty bite.

An excellent article I have ever seen in any newsletter. This will defintely help AV technicians understand the correct way of grounding their equipment to safe guard the personnel and equipment and also to reduce the hum noise.

Would like to see similar explanation for any other common issues people dont understand.

Thanks ghk

Why does it bother me when I see they "isolate" the conduit from the rack?

I was pleased to see this document because it shows AV technicians how to install their systems to work properly while complying with NEC. Electronics technicians have little understanding of power wiring requirements. I have had confrontations with electronic system installers who insist that their equipment will not function if it is installed to comply with NEC! AV and telecommunications grounding must be handled carefully to avoid operational problems. For further information on telecommunications grounding details, refer to the Telecommunications Distribution Methods Manual published by BICSI.

Excellent article. This is a keeper. So few electricians understand the problems with noise in sound and video systems. It is great to see how both requirements of the NEC for safety and the noise reduction requirements can both be addressed properly. I plan on sending this article to several low voltage contractors in the area who call me when they cannot find the source of noise in their systems.

Excellent Article. I've seen many 3rd pins pulled off plugs for Audio equipent previously installed in trying to prevent hums. Open grounds kill people every year. This article addresses the real reason ie: harmonics, etc. Stay safe everyone!