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The system ground is required to assure that the neutral of the power distribution system contains a low impedance path to earth (ground). This path returns system voltage to the zero potential point (earth). This allows the electrical system to function properly primarily when short circuits occur.

The equipment ground is required to provide for personnel safety. This ground assures that all metal parts of electrical equipment are at the same electrical potential as the earth (zero potential). Normally this ground connection is provided by the raceway system enclosing the circuit conductors. In some situations it may be a separate wire. This component ensures that a conductive pathway other than a person exists to carry abnormal currents.

Electrical systems serving computer rooms, work stations and sensitive electronic installations may require the installation of 'isolated' grounds for branch circuits serving electronic equipment. What is this ground 'isolated' from?

Computer equipment generally operates at extremely low voltage (5-10V) and normal distribution frequency (60 Hertz). However, their internal timing clocks generally operate in the 10-30 MHz range. Operations at this high frequency means that they are sensitive to interference from signal fluctuations in the radio-frequency range.

The raceway that serves as an equipment ground can function as a radio antenna, collecting random radio-frequency signals and transmitting them back to the earth ground point. The conduits carrying these signals, act as transmitters of electrical 'noise'. This noise can interfere with the operation of the computer sequencing system (timing clock) causing computer error.

Since electrical codes require that the computer be provided with an equipment ground for personnel safety, the solution to the noise problem is to provide a ground circuit that is 'isolated' from the normal equipment ground and does not operate as an antenna. Installation of a separate, insulated "isolated ground" conductor from equipment to the earth ground point resolves the problem.

In order to be effective, the isolated ground must be carried back towards the zero potential point (earth) as far from the computer equipment as possible. Simply routing it back to the source panel and then connecting it to the panel enclosure will allow the noise to circulate back to the computer equipment via the conduits.

An isolated ground bus must be provided in the source panel and connected back to the service ground point by an insulated ground conductor.

Designs that fail to extend the isolated grounding back beyond the power panel and into the service ground point can defeat the entire purpose of the system as well as violate code.