Misunderstood After All This Time: Isolated Grounding

Isolated grounding

I recently received an email from a homeowner who was installing a high-end media room and had questions about his home’s electrical system and the new circuits for the audio/video equipment. Before doing the installation, the homeowner had conducted internet research on the background requirements for audio/video installations. He also contacted an electrician friend, the audio/video equipment manufacturer from whom he had purchased his equipment, and an audio company engineer.

The audio equipment manufacturer provided a 65-page instruction manual with diagrams and illustrations to help with equipment installation. In addition, the electrician friend and the audio company engineer provided conflicting information and the homeowner was having trouble understanding the manual.

He found an article I had written for ELECTRICAL CONTRACTOR on isolated ground receptacles and circuits, so he contacted me to see if I could clarify the project and get him on the right path.

In the end, he relied upon the information I gave him, along with his electrician, to perform a safe installation.

According to my interpretation of his email, the homeowner had a service panelboard on the outside of the house and wanted to install a six-circuit panel in his media room with four dedicated 20-ampere (A), 120-volt (V) circuits to supply the audio/video equipment. He wanted to install EMT from his service panel to the media room panel and to four separate metal boxes in the room with a single 20A, 120V dedicated circuit in each box. He also wanted a separate isolated and insulated equipment grounding conductor for each circuit. At the media room panel, he wanted a separate isolated equipment ground bar for the four isolated, insulated equipment grounding conductors.

He was confused about what was permitted and what was required.

The audio company engineer told him to install a “2/0 welding cable from the isolated equipment ground bar in the media room panel to two separated ground bars” located outside of the building. (I assume the engineer meant two ground rods.) This concept was proposed in the 1980s to help isolate computers, audio and video equipment, and other high-frequency sensitive equipment from the normal electrical grounding system. However, this installation would have created an isolated ground without a path for fault current back to the source and would not have adequately cleared a fault in one of the circuits by tripping a breaker or blowing a fuse.

This incorrect concept prompted an addition to the 1990 National Electrical Code (NEC) in 250-21(d) (covering objectionable current over grounding conductors), which states: “the provisions of this section shall not be considered as permitting electronic equipment being operated on AC systems or branch circuits that are not grounded as required by this Article. Currents that introduce noise or data errors in electronic equipment shall not be considered the objectionable currents addressed in this section.”

In other words, totally isolating the equipment grounding conductors from the electrical system using two separate ground rods was not acceptable in 1990, and it is not acceptable now. Thankfully, I quickly cleared up that misconception for the homeowner.

High-frequency noise, other unwanted frequencies and signals, harmonics, and even a signal that originates within the electronic equipment itself may be capacitive and inductively coupled into the ferrous metal raceway, connecting the equipment and the panel, and can be reflected back into the equipment, causing major disruption and noise in the audio and video equipment. There are two sections in the NEC that will help someone trying to reduce electrical noise (electromagnetic interference) on the grounding system. Isolated grounding of permanently installed electronic equipment is dealt with in 250.96(B) and 250.146(D) with isolated grounding of cord-and-plug-connected electronic equipment.

In both cases, a separate insulated, isolated equipment-grounding conductor can be installed from the equipment (a nonmetallic bushing isolates the metal raceway from the metal frame of the electronic equipment) or from the isolated ground receptacle (the ground pin of the receptacle is not connected to the yoke of the receptacle) back to the main service or the source of the separately derived system without being connected to metal boxes or subpanels. This separation and isolation keeps unwanted noise and other frequencies from being coupled into the electronic equipment and still provides a path for fault current back to the source.

Metal boxes, metal subpanels, metal raceways and other metal enclosures from the permanent electronic equipment or isolated ground receptacles still are required to have normal equipment grounding.

About the Author

Mark C. Ode

Fire/Life Safety Columnist and Code Contributor
Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com .

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