Saturday, April 20, 2024

Electromagnetic Compatibility: Grounding (Part 3 of 5)

In the previous parts we covered multi-layer PCB designing. Now, let us look at grounding or earthing.

Grounding can be defined as a connection, whether intentional or accidental, between an electrical circuit or equipment and earth or to some conducting body that serves in place of earth. Hence, one can conclude that grounding is nothing but connection to earth or to a conductor that serves as earth. Going by this definition, a connection between a circuit and its 0V reference can also be considered as grounding, while earthing can be considered as a case of grounding where the ground is essentially planet Earth.

Purpose of grounding or earthing

The primary aim of grounding is to prevent shock hazard that exists in a high-voltage distribution system. Other functions include power fault clear-out, protection against lightning and electrostatic discharge (ESD) hazards and protection against electromagnetic interference (EMI).

Preventing shock hazard. Shock hazard exists when equipment enclosure/chassis, by virtue of a fault or otherwise, is not connected to ground. If a live wire is accidentally connected to the chassis, then the latter becomes live. Now, if you happen to touch it, current will get a path to ground through your body. If the chassis is connected to ground, say, by a low-resistance wire, fault current would prefer to flow through this low-resistance path rather than your body. This current triggers a fuse or circuit breaker, alerting you. Even if you ignore this warning, the frame will be at ground potential and there will be no danger of a shock.

Power fault clear-out. Aging, insulation damage, environmental contaminants in insulation and so on can cause breakdown in insulation during a fault, resulting in arcing that can lead to fire. When the neutral terminal of power supply is connected to earth, any fault in grounding would show up as a phase-to-neutral short and would trip the fuse, arresting the hazardous voltage.

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grounding or earthing hierarchy for a typical installation
Fig. 18: Grounding hierarchy for a typical installation

Protection against lightning hazard. Grounding is essential for draining severe lightning currents (30kA to 100kA) to ground, which otherwise can be life-threatening and can also damage electronic components in an equipment.

Electrostatic drainage. Static charge accumulated on the human body and other objects can damage fragile electronic components by discharging through these. Proper grounding in equipment can bypass the path of this discharge current, taking it away from sensitive components. Proper grounding in facilities can reduce damage to sensitive components during manufacturing and handling, too.

EMI control. Grounding/earthing is necessary in EMI control. Shields need to be connected to ground/earth as EMI currents induced in the shield need a path to dissipate to ground. Filters used to reduce conductively-coupled common-mode currents can be effective only if connected to ground to provide a drain for EMI currents. Also, ESD and its associated transient noise is reduced by bonding and earthing equipment frames.

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Equipment and system grounding

Grounding/earthing hierarchy for a typical installation is shown in Fig. 18. The method of grounding and the type and material of the grounding conductor depend upon the position of the ground in the grounding hierarchy.

Fig 1929
Hybrid grounding

At the top of grounding hierarchy (point 1 in Fig. 18) are the low-level low-frequency grounds for circuits that work up to a few kilohertz like audio circuits. A simple wire can be used for such grounds since metals behave as pure resistance at these frequencies.

For low-level high-frequency digital signals up to tens of megahertz (point 2 in Fig. 18), a mesh has to be used for grounding (for example, a bond strap made of wire mesh) since metals start to exhibit inductive properties and meshes are good at providing a low inductance.

For high-level high-frequency EMI currents (which have spectral components up to hundreds of megahertz), grounding has to be done via flat metal strips to a ground plane (point 3 in Fig. 18). This is because metals become highly inductive, and only metal planes and sheets can provide low enough impedance at such high frequencies.

Next in the hierarchy are grounds for cables and transformer shields and those for DC returns of power loads (points 4 and 5 in Fig. 18). Return currents from these should not affect the returns above these in the hierarchy.

Lightning and AC power-safety grounds are at the bottom of the hierarchy, and these should also be distinct from one another. Lightning grounds carry hundreds of amperes of current and, hence, the need to separate such grounds from AC power grounds.


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