It may have been a while since you consciously thought about the basic tenets of electrical circuits. However, subconsciously you probably utilize Ohm’s Law, KCL, and KVL almost every day. Collectively, these laws that describe the relationship between voltage and current through devices and within circuits are the foundations upon which electrical circuits are built. During design, circuits are typically represented initially as schematics that invariably include identifiers for voltage potential and current flow. For completeness, we must also include the symbol for grounding, which is essential for the eventual actual operation of our circuit(s).
Symbol for grounding
Electrical circuits do not exist in a vacuum. In fact, there may be many levels of circuits in an electrical system and each level may be comprised of many individual circuits. Aside from their interconnectivity, all of these circuits also contain a ground. In high power transmission and distribution systems, grounding, as the name implies, means having a path to earth ground to quickly remove unwanted current from the system. In electronics that include PCBs, the situation is more complicated as there are different types of grounding and choosing the right grounding technique is critical. Among the most important techniques that should be considered are chassis grounding techniques. Let’s explore chassis ground before exploring how best to utilize it for your design.
Why is Chassis Ground Important?
Prior to discussing the importance of chassis ground, it is informative for us to remove some of the ambiguity surrounding the term ground or grounding for electronic systems. This is best done with a few definitions:
Definitions of Ground or Grounding for Electronics
- Signal ground
Signal ground refers to a reference point from which a signal voltage is measured. This is often referred to as the common or return. For multilayer PCBAs, the signal ground is a point on the surface that connects to an internal stackup layer, the ground plane.
- Ground plane
The ground plane is a circuit board layer, ideally at constant 0V potential, that serves as a central connection point of reference and return for signals or power. Note: AC signal ground should be isolated from power ground, which is DC.
- Chassis ground
Chassis grounding utilizes what is known as bonding to connect all internal metallic elements of a system to a single point. For metallic enclosures, this point may be the enclosure itself. For other electronics systems. such as plastic enclosures, there may not be a chassis ground.
- Earth ground
Grounding to earth involves a direct connection to the ground. A common example is through a grounding rod.
It is always desirable that grounds be electrically neutral, which indicates good, balanced circuit operation. However, this is not always possible for PCBAs and electronics. During an event where excessive current is introduced into the system, this overage will be felt in the ground circuit. And depending upon the level of current, this overcurrent can damage the board or electronics, which is why optimizing your board’s PCB layers is crucial, and even threaten nearby personnel that may provide a low impedance path for current flow. Chassis grounding is implemented to protect against this situation by providing a path away from the system and to earth ground. The chassis is also used to provide shielding for cables and ESD protection by providing a path away from the PCBA. Now, let’s see how to best implement chassis grounding for electronics systems.
The Best Chassis Grounding Techniques for Electronic Systems
Chassis grounding serves the same purpose for electronics as lightning protection does for high voltage power systems. Specifically, to route unwanted and potentially dangerous current away from the system elements and components to prevent circuit operation interruption and damage. In contrast to HV systems, electronics and PCBAs may not have a path to ground. In these cases, metallic chassis effectively serve as earth ground in that they terminate the physical circuit path for unwanted currents. This coupled with usage for EMI minimization illustrates how important chassis ground can be for your electronics systems. However, good grounding techniques; as listed below, must be implemented for chassis grounding to be most effective.
The Best Chassis Grounding Techniques for Electronics
- Connect all internal grounds to a single point
There should be a single ground connection from each PCBA. For multiple boards, all ground connections should be connected to a single point. For large electronic systems, each internal system should have a single ground connection where all internal board grounds connect.
- Have one connection to the chassis ground
Whether using the enclosure or mains neutral as the chassis ground, there should be a single connection to the electronic system(s) for which it serves as the safety ground.
- Connect chassis ground to earth ground, if possible
For electronic systems within structures, a path to earth ground is available and should always be used. For portable electronics that contain their own power supply, consider the power supply return as the earth ground and use a single connection from the internal circuits for grounding.
As with most safety measures, the importance of chassis grounding can be overlooked until a contingency occurs and your board or system is fried or worse someone is injured. These can be avoided by recognizing how critical chassis grounding techniques are and implementing them during design.
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