Grounding and shielding is an often misunderstood process. It is common to hear quotes ranging from “it’s just black art!” to “the rules change all the time!” and “there’s no way to understand it!” These statements are often repeated but not true.
There is a process, it hasn’t changed, and the more one knows about coupling mechanisms the more one will see that it’s sound engineering principals. I have written many EMI and grounding issues in my blog, especially in groundloop section.
Let’s start with grounding. Proper grounding is an essential component for safely and reliably operating electrical systems. Improper grounding methodology has the potential to bring disastrous results from both an operational as well as a safety standpoint. Effective bonding, grounding: The backbone of electrical safety article provides a good overview of proper grounding in electrical systems. Providing and maintaining an effective impedance path to ground is critical to maintain reliable, efficient, and safe operating facilities. It is the foundation of any power generation unit and associated power distribution system. Providing and maintaining an effective impedance path to earth that stabilizes the system voltage is a basic and critical component in maintaining reliable, efficient, and safe operating facilities.
Nowadays solidly or resistively grounded power distribution systems are preferred (in both low voltage and medium voltage systems). Ungrounded systems are no longer recommended (they were once common). Nowadays solid or impedance grounded systems of some form are predominantly installed. Three-phase, 4-wire solidly or resistively grounded “WYE” are preferred over the use of 3-phase, 3-wire ungrounded systems specifically because of the possibility for destructive transient overvoltages that can occur throughout the power system during any re-striking phase-to-ground fault due resonant condition established between the inductive reactance of the system and the distributed capacitance to ground (earth). Experience has proven that these overvoltages may very rapidly cause failure of insulation at multiple locations throughout a power distribution system.
Neutral grounding has been in practice in many systems all over the world. Generally, the neutrals of source transformers or generators with star connected windings are grounded. Grounding the neutral reduces the magnitude of transient voltages, improves protection against lightning, protection for line to ground fault becomes reliable, and improves reliability & safety. The typical disadvantages of grounded systems are related to high fault currents. The operational safety is the primary function of grounding. Grounding systems are designed so that they do provide the necessary safety functions. Grounding also have other functions in some applications (for example work as signal ground reference) but the safety should not be compromised in any case.
Nowadays grounded electrical outlets are used in modern outlets for safety reasons. The modern practices nowadays demand the use of en extra safety wire from electrical outlets/loads to the main distribution panel. This wire is known in Britain and most other English-speaking countries as the earth wire, whereas in America it is the ground wire. Inside buildings the TN-S system is nowadays recommended practice, meaning that the neutral and ground wires are kept separate and they are interconnected only on one place, this place being within the main power distribution board. This is the recommended practice for modern buildings.
Most modern buildings are wired with three-phase power using 5-wire system. This 5-wire system for three phase power has three hot phase wires, one neutral wire, and one grounding wire. For wiring single phase outlets, there are three wires: live, neutral and ground are used. The neutral and hot wires are interchangeable and reversible insofar as the operation of equipment is concerned. In Europe, the normal 3-wire receptacle is symmetrical so that the neutral and hot wire connections can be swapped by simply rotating the plug. Most equipment won’t even know which of its input wires will end up connected to the neutral wire and which will be connected to the hot wire. International office product safety regulations (including IEC 950 and UL 1950) prohibit these wires from being treated differently.
In TN-S system there grounding wires form a tree like structure starting from mains distribution panel (similar as does live and neural wires do). The generally grounding wires should go same route as the current carrying wires (live and neutral) for best performance (but also Signal reference grid approach is possible for grounding). There is a requirement in the TIA 568 B.2 cabling standard that voltage difference between the shield and the ground wire of the work area equipment outlet shall not exceed 1 V rms.
In many older building you can still see TN-C-S and TN-C practices where in some place in the installation there is a combined neutral plus ground wire. This is less safe wiring practice than modern TN-S system and causes lots of ground loop noise problems when used with sensitive equipment. An old grounding system that provides needed basic safety level to the user might not provide sufficient electrical environment where today’s electronics equipment can work well.
One of the most common direct coupled noise sources is when the ground which is being used for reference or return is not referenced to earth as expected. This is especially prevalent in sensitive high-gain circuits.