Re: Grounding ( Bonding) recptacles for PCs
I can see I did a lousy job explaining things earlier. There has been a death in my family so I do not have time to run all the way through this, but here is some food for the protein computers.
The NEC grounding requirements are for safety or power frequencies (less than 1Khz) only. Electronic signal circuit grounding is performance grounding, above and well beyond 1Khz. NEC requirements are for electrical power engineers/electricians, performance grounding is for equipment engineers/technicians/installers, and electricians if hired to install the extra performance systems.
Any electronic system uses a DC power supply which forms a separately derived system. Any SDS establishes a new ground bond point. Any SDS will remove common mode noise, the evil of communications circuits. We can reference the SDS to the electrical system ground or leave it float and bond it to the next power supply signal reference point.
The trick is NEC requires us to make the two systems common, but offers no guidelines as of how to do this without defeating the purpose since its only concern is safety. I am here to tell you there are several ways to accomplish this. They fall into two separate categories: Isolated and Integrated.
True isolated systems are not to be confused with IGR found in AC electrical systems. It has to do with using a DC battery plant, bonding the return bus to a single point ground bar called a Master Ground Bar (MGB). All the equipment is powered from the DC plant. All the equipment frames are isolated from the integrated ground plane (building steel, concrete, raceways ect.). Then all the equipment frames and signal ground have a dedicated isolated ground conductor ran to the MGB. This whole method forms a single point ground in which no internal or outside currents can flow. If no current can flow, no common mode voltage can develop (noise).
For AC systems we use some similar techniques, but a different concept, the integrated ground plane. It starts by using a UPS, transformer, or combination of both to first remove any common mode noise, normal mode noise, transients, etc to clean up the power to start with. A single point ground is established by the Xo ground bond, but the distribution is not isolated, it would be next to impossible. So an external ground is needed for signal transmission. This external ground can be many things; most common is a signal reference grid (SRG) or a mesh. Both of these form an equipotential ground plane.
The equipotential ground plane is bonded to the Xo of the transformer for NEC compliance. It is bonded to the equipment frames and signal ground terminals in the equipment. It is also bonded to every building electrode available like building steel, water pipes, conduit, etc. This grid or mesh forms multiple loops. These loops have a lower high frequency impedance than a single conductor. The smaller the loops, the lower the impedance. The loop size is determined by the operating frequency of interest.
The purpose of the grid is to have a overall low impedance between any two points on the system. The impedance to earth is irrelevant and does not matter, point to point is all that counts. Common mode current will flow and is expected. If we have a overall low impedance at the frequency of the current, the voltage developed will be below minimum operating thresholds.
As has been stated the problem lies with communication circuits that use unbalance signal transmission mediums like RS-232 and coax. However these mediums are old school and outdated methods and disappearing fast. They are being replaced by Ethernet and optical mediums making all the performance grounding techniques obsolete sort of.
Cisco is no releasing their new breed of IP/voice systems which use the equipment frames as return conductors, and require their equipment to be installed on copper or steel plates forming a equipotential ground plane. They will purposely run load currents through the building steel and water pipes, but that is another story.
Good Day
Paul Harvey
