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Legal Disclaimer |
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SECTION 7 - COMPUTER SYSTEMS GROUNDING and BONDING POWER QUALITY Power quality covers a variety of specific properties of the electric power available at a specific location. The term power quality is sometimes associated with distortions in the 60 Hz waveform. Sometimes these distortions are referred to as electrical interference or noise. Where electrical equipment is sensitive to electrical interference, a common cause of an electrical interference related problem is inadequate bonding to ground. Examples of such inadequacies are open grounds or corroded bonding conditions. An installation complying with the code will not suffer from these shortcomings. Sensitive electronic equipment, interconnected with other equipment, can have a very complex network of cables and conduits. Both the power wiring and communication wiring may bond the equipment to ground. Small currents can circulate on bonding paths, causing the sensitive electronic equipment to operate incorrectly. The solution for some installations is to use separate copper conductors for bonding the sensitive equipment. In subrule 10-906, the code allows the use of a separate insulated copper conductor directly back to the distribution panel under specific conditions. The conditions are as follows: |
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The voltage supplying the equipment must not exceed 150 volts to ground. |
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The separate insulated bonding copper conductor must be enclosed in the same raceway or cables as contain the power conductors throughout their length |
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Most importantly the separate insulated bonding copper conductor must be sized according to Table 16 for each leg of the run. The equipment may only require a few amperes. Despite this, the bonding conductor for part of the bonding path may have to be quite large. This happens if the isolated bonding conductor runs back to the main panel before it joins the bonding path for all other equipment on a feeder. This requirement makes certain the bonding copper conductor is adequate for the fault current available in the feeder leg. |
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In Subrule 10 – 906 (8) isolated grounds are addressed. The advent of isolated grounds is a direct result of the automated data processing systems, whether PCs or main frames. The use of the isolated ground is to supply only one path thereby removing any of the noise caused by electromagnetic interference and the switching and turning on or off of equipment. An isolated ground is not to run back to the original ground rods unless that is the area where the neutral is created. The purpose of an isolated ground is to provide a direct single path to the star point of the last transformer where the neutral is created, or the service entrance, whichever comes first. In using isolated ground circuits for data processing systems, the purpose is to remove any noise, ground-fault currents and electromagnetic interference back to the electrical source and complete the circuit in order to prevent interference with equipment in adjacent areas. Fault current, electromagnetic interference and ground noise can change programs and will lead to degeneration of the computer chips and hardware if left unchecked. A myth is that a computer requires a clean, dedicated insulated ground rod which has no electrical connection whatsoever to the dirty ground. (The term “dirty” is often used to describe the ground rod used for utility power neutral ground). This myth has been responsible for a number of unsafe installations and code violations. It also created needless expense for additional grounding rods and long grounding conductors to a clean ground location placed far away from the dirty grounds. What is missed in this application is that the ground rod is not a pool to absorb dirty grounds, it is part of a circuit. The circuit has to be completed through the ground to the original ground rod, then to the electrical source, thereby creating a higher impedance circuit that could prevent the circuit breaker from tripping in the event of a fault. In summing up, all of the return current has to follow the supply current path. Each isolated ground must be connected to the power source by a separate insulated copper conductor. There must be no grouping. If the data processing equipment is too far away from the electrical source to practically run the isolated grounds, then an electrical source can be recreated with an isolating transformer. In trouble shooting computer power problems, it is usually found that the grounding is the major cause of all the problems reported. And usually through violations of the CEC. |
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