Safety grounding //
Grounding electronic equipment for personal safety and clearing of faults is no different than that of any other equipment. Safe grounding requires fast opening of circuit breakers or fuses and minimization of voltage differences between exposed metal surfaces on all of the involved electrical system and equipment, to levels that are safe for people.
Solving transient problems is never easy! They may be random or repetitive. In general, they have waveshapes which are not easily analyzed.
However, let’s name few really important points about grounding of electronic equipment and personal safety as well //
Typically the safety grounding of equipment is exactly the same for electronic equipment as it is for any other kind of apparatus, whether it is a refrigerator or a printing press. The “green wire” and conduit/raceway system’s grounding which is well documented in the NEC and other codes, defines these requirements completely.
There is absolutely no conflict between NEC defined grounding and the more specialized grounding and bonding practices described in point #2 below. An unnecessary conflict can be created however, such as when someone attempts to create a “separate”, “dedicated” or “clean” grounding connection that is not permitted by the NEC!
Protection of data circuits generally requires additional considerations beyond the intent of the NEC, but not in violation of it. Protection of data circuits from disruption or even damage does not always involve grounding, although good grounding makes this protection a lot easier.
Aircraft have no earth grounds while they are flying.
The airplane carries its own “grounding” system for its AC and DC systems, and signal grounding purposes. This grounding system is entirely metallic in nature and it is often called a self contained power and signal reference system, which is a more accurate description. Even direct lightning “hits” are not likely to cause equipment damage or even disruption to signals.
The circuits of most electronic systems are almost always sensitive to voltages of a few tens of volts or even to as little as one or two volts. As a result, these systems are designed with great care to keep transients out of the actual circuitry and off of the signal paths between interconnected units of a system.
To accomplish this, some equipment uses electrostatically shielded isolation transformer techniques and AC-DC power supplies designed to reject transients.
However, for these techniques to be fully effective, good grounding and bonding practices exceeding those required in the NEC, must often be employed.
Data signals inside most electronic systems consists of bits of information processed as square waves or impulses at about 5 volts in amplitude and clock speeds which can exceed 200 MHz. Data transferred between equipment often has a magnitude of 12-18 volts, and the speed of transfer is lower than that of the signal processing speed available inside of the equipment.
Yet, even at these speeds the systems can be made to have high reliability and to be relatively immune to interference if good grounding and bonding practices are followed.
Lightning related waveforms are usually the “worst case” situation for transients on most ac power system wiring and related grounding systems. This makes lightning the principal threat. More information about lightning and its typical waveforms may be obtained by consulting ANSI/IEEE Std C62.41-2002.
Fast electrical transients are created in some equipment with electromechanical contactors. The interference problem from these items could be serious, but it is easy to solve by installing RC snubbers (consisting of resistors and capacitors) across the contacts, coils, or both items of the offending device.
This kind of interference with electronic circuits can sometimes be controlled by more stringent shielding, or grounding and bonding practices.
However, the root cause of this kind of problem is really not a shielding, or grounding and bonding related problem. Instead it is an equipment circuit modification problem and this is the kind of thing which typical electrical contractors should normally not be expected to identify or to solve.
Surge protective devices
In addition to line-to-line and line-to-neutral connections, surge protective devices (SPDs) are also connected to the circuit’s equipment ground conductor. Any transient voltage which then operates the SPD and causes current flow through it and to the equipment ground conductor, raises the ground potential as measured at the installation point of the SPD and to the remote “ground” used as a zero voltage reference.
Because SPDs may be subject to very high voltages with steep (e.g., fast rise time) wavefronts, the concurrent effects on the grounding system may be very severe.
Reference // Practical Guide to Electrical Grounding by W. Keith Switzer, Senior Staff Engineer