SBOA536 December 2021 INA240
Ungrounded DC circuits are used on a daily basis. For example, a battery-powered system works just fine without any of the battery terminals being grounded. Simply put, the Earth ground is not needed for a DC circuit to function. However, safety is a concern for high-voltage systems (> 50 V).
In a grounded high-voltage DC system, if a person comes into contact with the power rail, electric shock can occur. While in an isolated DC system, such electrical shock should not occur. However, in reality either side of the power supply may become grounded due to a random first fault. A person will suffer electrical shock when coming into contact with the other side of the power supply in a second fault condition. Because such ground fault cannot be prevented in an isolated system, electrical standards such as NEC require high-voltage DC systems to be installed with proper Earth grounding. Ground fault, overcurrent, and overvoltage protection devices are then installed accordingly to ensure safety.
Figure 2-1 shows one common grounding scheme of a grid-tied photovoltaic (PV) system which typically falls into the high-voltage category. The red lines connecting different components represent current-carrying conductors; the black lines represent uninterrupted grounding conductor or equipment ground conductor. In this grounding scheme, the DC grounding electrode is combined with the AC grounding electrode.
Ground loops form when multiple grounding electrodes exist, and there is a voltage potential between any two. Figure 2-2 shows ground loops can form when multiple grounding electrodes are provided. To prevent ground loops, use a single grounding electrode wherever possible.
While an Earth ground is not absolutely needed, a “ground” or “common” is. An exception are isolated circuits, such as those enabled by transformers and galvanic isolation barriers, where there could be two or more grounds defined by different voltage potentials. However, within each isolated domain, a single common ground still provides reference to all components. Such an isolated system is shown in Figure 2-3, where the primary side ground is separated from the secondary side ground. The two grounds can be defined by different potentials.
Multiple grounds are often defined even in a non-isolated circuit. An example is a typical mixed-signal system where analog ground and digital ground may be defined. To make matters more confusing, multiple grounds are often found for the seemingly identical ground. This type of ground partitioning is often found in applications where the exact same circuitry is cloned multiple times as shown in Figure 2-4. Schematic-wise, each clone might be put on a uniquely named ground which is typically designed to be one or multiple inter-connected ground planes in the PCB.
However it is accomplished, the goal of ground partitioning is to minimize interference and keep noisy circuitry away from the sensitive one. Furthermore, regardless how the ground is partitioned, all grounds will eventually be electrically connected to a single common point. In essence, each of the grounds constitutes an island on which a subsystem operates. It is sometimes not possible to contain all recirculating current within the island. In these situations, the ground plane and traces must be routed such that the current path does not pose interference to other sensitive parts of the system.