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Grounding Riddle No.37 - One grounding system for electrical, instrumentation and...
How do i calculate the number ground rods for different grounding systems? is it possible to make only one grounding system for electrical, equipment, instrumentation and lightning protection grounding?
Author : Deusdedit malulu - From: Tanzania
Sun, October 7th, 2012 - 09:42
1-For grounding rod system calculation you can refer to IEEE 80 or BS 7430.
2-Interconnection or isolation of various grounding system such as electrical and instrumenta or electronic grounding system is subjected to standards or technical code discaussions. There are some advantage and disadvantages for each grounding way.
For example, according to IEEE 142, since the power grounding system had been found to cause malfunctions and failures of electronic equipment, a logical solution was to not use the building’s electrical power equipment grounding system for grounding electronic equipment. A possible lack of understanding of the function and operation of the neutral conductor and the equipment ground system by electronic equipment manufacturers led to erroneous installation requirements. The chosen alternative was to ground electronic equipment to an isolated grounding electrode consisting of one or more driven rods separate from the power system grounding electrode system. These generally took the form of one to ten rods a few feet away from the building. These would have grounding resistance of from 10 ohm to 30 ohm or more.
On the other hand The NEC requires that all equipment served from an electrical source be grounded or bonded to the grounding point of that source. Clearly, when an isolated grounding electrode is used for electronic equipment, this violates the NEC requirements. The NEC requires a metallic path from all equipment frames served, back to the source neutralground bonding point. If the path has a low impedance, any fault will be of sufficient magnitude to quickly operate the protective device and de-energize the faulted unit.
To eliminate undesirable aspects of previous systems of grounding electronic equipment and similar sensitive electronic systems, the single-point grounding system has been developed and is the recommended method of grounding sensitive electronic equipment.
This overcomes the problems and NEC violations created by multiple grounding electrode installations by grounding all electronic equipment system components to only one single point on the power grounding system. All other interconnected electronic equipment must be grounded to the same single point on the power grounding system or isolation of the interconnecting signal/data cables must be accomplished using fiber-optic cables, optical isolators, modems, or other means. Surge protection should also be applied on both ends of cables installed between buildings. It also overcomes the capacitive induced voltages between building and electronic equipment due to separate grounding, by bonding electronic equipment and the building power system to the same grounding electrode system.
When the electronic equipment grounding electrode and the electrical power system grounding electrode are connected together, a transient voltage rise applied to the building steel will result in the entire electronic equipment system rising and falling with the building steel. No overvoltage will be induced into the electronic equipment circuits.

Where there are several pieces of interconnected electronic equipment in a given area, such as a computer room, they can be grounded to a common ground reference using the EGCs supplemented by a signal reference grid.They can all conveniently receive their power from a single source, preferably via a local isolation transformer that can be grounded within or at the boundary of the room. This will also serve as the grounding point for all of the electronic equipment in that room.
To ensure a low impedance reference over a broad spectrum of frequencies, the singlepoint grounding conductors can be supplemented by the installation of a signal reference grid under the entire room. The grid will normally consist of conductors in a mesh configuration.

In general, the grounding electrode will be present for system grounding; however, the requirements for system grounding may not be adequate for the dissipation of lightning surges.
Since NFPA 780 has been adopted by ordinance in many jurisdictions, it should be consulted for detailed requirements by any engineer designing a lightning-protection system.
All air terminals should be connected by down conductors and should form a two-way path from each air terminal to make connection to the grounding electrode (voltages double at an open circuit or end, in a lightning down conductor). Bend radii should be as long as possible, not less than 20 cm (8 in), since sharp bends increase the reactance of the conductor. Reactance is much more important than resistance because of the very high frequency of the surge front. At least two down conductors should be provided on all structures, except that only one down conductor is needed for masts, spires, and flagpoles.
The location of down conductors will depend on the location of the air terminals, the size of the structure being protected, the most direct routing, the security against damage or displacement, the location of metallic bodies, water pipes, the grounding terminals, and the ground conditions (earth or soil). If the structure has electrically continuous metallic columns, these columns will act as down conductors. The air terminals must be interconnected by conductors to make connection with the columns. However, internal column footings of large buildings dry up and can become ineffective since they seldom are exposed to ground water.
The average distance between down conductors should not exceed 30 m (100 ft).
Irregularly shaped structures may require extra down conductors.
Author : Hamid - From: Iran
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