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AC grounding: essential, dangerous, or both?

Most electrical engineers learn early-on about the importance of what are referred to as circuit grounds. In many cases, whatever your problem, adding a ground or improving the one you have (meaning lower impedance) is a good thing that can’t hurt. That’s what circuit EEs have drilled into them as they work and debug bench circuits prototypes, and more, and learn to connect the analog ground to the digital ground at one point only to minimize unwanted ground-current flow and associated noise.

In most cases, such grounding does improve circuit performance, but as every experienced engineer knows, every rule has legitimate exceptions, and grounding is one of them. A large part of the confusion comes from the ambiguous and this sloppy terminology associated with that innocent-sounding seven-letter word.

 

How so? In reality, that word can refer to any one of three types of circuit connections:

  • Earth ground where the circuit is actually connected to Earth, which acts as an infinite source and sink for electrons;
  • Common (may also be called signal ground), which establishes a 0-V point in a circuit (and there is often more than one of these in the circuit). It’s usually very misleading to even use the word ground with this one.
  • Chassis ground, which connects all nominally zero-volt potential points in a circuit; it may be connected to Earth ground, but often cannot be as there is no Earth ground for many devices such as most portable, battery-operated ones; here, it may be misleading to use the word ground if it is not connected to a real Earth ground.

There’s a different symbol for each, but you wouldn’t know it from most schematics I have seen, as the triangle is commonly used for any and all grounds unless the schematic sketcher is careful (Figure 1).

illustration of three different ground symbolsFigure 1 There are distinct symbols for the different grounds, but they are often misused. Source: Autodesk

A large part of the confusion with ground is due to the terminology associated with another important word: voltage. When someone says, “point A is at X volts,” that wording ignores a key part of the voltage story. Voltage only exists between two defined points, and a better term for voltage is the older but still-used phrase potential difference. Unless you specify the voltage at A with respect to that other point, saying “voltage at point A” may represent different potentials to different parts of the circuit. It’s especially challenging as the various grounds in the circuit – Earth, chassis, and common (signal) – often have potential differences between them and these can range from millivolts to volts and more.

Of course, where there’s a potential difference, there’s a possibility of current flow. That’s another factor that often gets lost in the verbal looseness with words such as ground and voltage. While potential difference is an issue, what often matters in many cases is current flow through a circuit’s two points driven by that difference. These currents are the source of noise and performance issues in low-level sensor-based designs, and also can be lethal in some cases – that’s quite a range of problems.

If good grounds are so important, how can they be lethal? It’s not that they are dangerous themselves, but it’s a two-part problem. Consider a medical instrument with a metal case, which is Earth-grounded for safety so currents due to an internal fault which shorts line-level AC voltage to the case will go directly to that Earth ground rather than through the user (or patient) and then return to Earth ground.

Seems like a good idea, and it is, except when it is not. Bad stuff happens, and one possibility is that the safety Earth-ground is faulty due to a loose wire or screw, or corrosion (hey, it happens), or there is a poor or missing Earth-ground at the three-wire AC plug or the outlet wiring, a common occurrence (Figure 2).

diagram of a three-wire AC plugFigure 2 The standard three-wire AC plug has line (hot), neutral, and ground connections; their colors may vary from country to country, and the Earth-ground wire is usually covered with green insulation or even left bare. The Earth-ground connection is often absent or broken in the line-cord wire, the plug, the outlet, or the main power run. Source: Lumen Learning

Then, the case does become live and anyone is touching it … well, you know what happens as current flows from the AC line through a subject to ground (Figure 3). It’s not the voltage that is lethal here, it’s the voltage driving current through the body – it only takes a few milliamps through the chest via skin contact to induce cardiac fibrillation (they also call potential difference “electromotive force” for good reason).

diagram of how worn insulation can lead to a person being shockedFigure 3 Worn insulation allows the live/hot wire to come into direct contact with the metal case of the unit, and with the Earth-ground connection broken, the person is severely shocked – yet the unit may operate normally while that ground connection is missing. Source: Lumen Learning

This is a situation where not Earth-grounding the chassis can actually avoid shock hazard, and an isolation transformer is often used to ensure this non-connection (Figure 4). This transformer has AC line and AC neutral on its secondary side, but no Earth-ground connection there. Thus, there is no current-flow path from the hot enclosure, through the user, and back to Earth ground on the primary side.

diagram showing how the addition of an isolation transformer protects the userFigure 4 An isolation transformer has no ohmic (galvanic) path between the primary-side Earth-ground wire and the secondary side, preventing a completed circuit and thus current-return through the user and back to Earth ground. Source: Lumen Learning

Isolation transformers are often mandated in medical equipment, as the shock risk is especially high if there is a probe inside the patient and contact impedance is low. Only a few microamps of current through the chest interior can be lethal. The regulatory references at the end of this post provide the mandates and details, but they are very hard to decipher, of course.

The need for isolation is not limited to AC-line devices or shock-risk situations. There are many sensors which have no inherent connection to Earth ground but may be connected to it in use, such a sensor attached to a metal frame. This Earth-ground connection can short out circuitry of the analog front end (AFE), so “floating” non-grounded AFEs are used, with isolation implemented via a transformer, optocoupler, capacitive coupling, RF link, or other technique.

During the project pressure of getting the circuit to work, it’s usually worthwhile to take step back and ask:

  • Where and what are the potential differences between grounds and between circuit functions?
  • What amount of current will flow across these differences, and what will be the impact?
  • What are the connections between Earth (if any), chassis, and signal (common) grounds? (Note that many circuits actually have multiple signal grounds.)

The good and bad news is that there is a huge amount of credible material available about all types and aspects of grounding, spanning basic tutorials, case studies, and practical “how to do it” insight covering just about everything from battery-powered sensitive sensor-driven circuits to medical devices, large buildings, and antenna towers.

Have you ever encountered a safety or performance issue from the absence of a ground? How about from the presence of a ground? Have you ever solved a problem by adding or attaching a ground, or by removing one?

Regulatory references

  1. IEC 60950-1:2001, “Information technology equipment – Safety – Part 1: General requirements
  2. IEC 60601-1-11:2015, “Medical electrical equipment — Part 1-11: General requirements for basic safety and essential performance — Collateral standard: Requirements for medical electrical equipment and medical electrical systems used in the home healthcare environment
  3. ISO 14971:2019, “Medical devices — Application of risk management to medical devices

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1 comment on “AC grounding: essential, dangerous, or both?

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