Safety of grounded and isolated AC in medical systems

Question

Introduction

I try to make more sense of safety and reliability of grounded and isolated electricity. I divided the problem into 3 stages:

1. Power-transmission line
2. Household electricity
3. Electrical appliances

From my understanding high-voltage transmission lines are isolated with over-head earth ground that acts like a lightning rod. The three phases are floating relative to earth ground (3 hot phases).

Before going to household, the electricity passes a 3-phase step-down transformer which introduces a common neutral wire (cold) which is connected to a ground rod at the transformer.

Then at the household, we have line and neutral which is selected from one of the phases, then we have a ground therminal that connects to the ground rod at the house. In some country, neutral also connected to ground at the consumer unit.

As compliance should be safe to touch or shock-proof, the metal case of appliance must be connected to earth ground to ensure that electric potential always equals to ground and human body.

Neutral should be connected to ground at the transformer's location because if not, the lightning might pass through the AC wire to appliances' ground potential. With neutral connected to earth, this lightning can bypass to ground at the transformer without much damage to appliances.

In my country's standard, neutral must connect to the earth therminal to make a high-current return path in case that the line voltage leaks to appliances' metal casing so the breaker can trip.

Question

If an isolated system is power or more reliable, why can't we just make a household's system isolated by disconnecting neutral from earth ground? Will it work the same way as isolated power?

*note: I've edited the question to be more focused, the question is only on but it required some explanation in each cases.

Reference

1. Application of isolation transformer in biomedical devices.

Answer 1

Floating, or weakly grounded systems have a few major advantages over solidly grounded systems.

1. On a grounded system, there is a period of time between when the fault occurs and when the breaker disconnects the fault when potentially hazardous voltages can appear between touchable components of the system. These voltages can be managed through bonding but it's very difficult to eliminate them entirely.
2. On a grounded system it is not possible to achieve fault tolerance. The only way to deal with a low-impedance fault from hot to ground is to disconnect the supply. On a floating system the first fault to ground will have no noticeable effect (this can be bad, because a fault that is not found won't be rectified). A weakly grounded system can detect and tolerate the fault and set of an alarm, this allows for an orderly shutdown of the system to be planned and executed.
3. If the system is small enough, then the fault current may be low enough that a person can touch one of the supply conductors safely.

However.

1. If the system is supplied by a step-down transformer from a higher voltage, then a primary to secondary fault on a floating or weakly grounded system can be disastrous. A solidly grounded system provides a path for the primary to secondary fault current to take, hopefully at least reducing the damage to downstream equipment and personel.
2. It may take significant effort to localise a fault, especially an intermittent one, in a solidly grounded system a fault to ground will cause an immediate high current flow towards the fault. In a floating system, very little current will flow due to the fault.
3. If the first fault is ignored, rather than found and fixed, then your floating system becomes a worse version of a solidly grounded system. Worse because it's hard to predict the magnitude of the fault currents or where exactly they will flow.
4. The larger the system the more leakage and faults their will be, diminishing the advantages of a floating system.

The result is that floating systems are a speciality thing, very useful in some circumstances, but also requiring special care.

Answer 2

I don't think it's appropriate to say one idea is safer than the other. They are different safety systems, with both advantages and disadvantages.

An advantage of a grounded system is that a fault is likely to trip the circuit breaker (especially with a GFCI), and then you know there's a fault and you can fix it before there are two faults.

An advantage of an isolated system is that the system can keep working normally even if there's one fault. This is really important if people die if the system stops working. Medical equipment is periodically inspected to make sure there are no problems.

Grounding also prevents static electricity build-up - that's what potential equalization really means.

Even different countries don't agree on the best grounding system. Some of them connect neutral to ground in the home's electrical panel, some of them connect it at the pole transformer, and some of them connect it in both places.

Answer 3

Medical Device Standards like IEC-60601 require special considerations regarding the Means of Patient Protection (MOPP) and Means of Operator Protection (MOOP). In general a "first fault" tolerance is required. E.g., if one Isolator breaks down the second one is required to withstand the full load AND the second measurement failure has to be very unlikely in that case (Mean Time Between Failures -MTBF) or the first fault needs to be reliably detectable and lead to a safe state. Another concern is the Leakage Current. Depending on the application this current can be very low (<100 µA). The idea is that such devices can be coupled directly to your cardiovascular system (type BF devices) and even small currents can be deadly. If all devices connected to a patient (and the so called patient environment) is floating, a fault of one devices is no issue as the other devices provide enough isolation to limit the current. However, if a grounded device is present device failure can be deadly. Those are (some) reasons why many medical devices are required to be floating.

Besides the advantage that grounding the device housing is an easy way to prevent a "silent" fault, where a live wire connects to a touchable part, posing a potential life thread, there is another less obvious benefit: superior EMI/EMC behaviour. EM radiation typically occurs if parts are not properly grounded and potential differences across gaps form antennas. Guess that happens if one builds an isolation circuit - you will design a very nice antenna. In this case the usual approach is to increase the GND coupling/virtually close the gap using e.g. decoupling caps across the isolation. However, those caps will increase your (AC-coupled) leakage current, making it hard to meet the max allowed leakage current. It can take several iterations to balance EMI/EMC behaviour against the isolation requirements and even plugging in a slightly different cable or device can change everything - I heard stories of tests passing/failing tests depending on wether the notebook's charger is plugged in.

TL;DR: To the best of my knowledge, grounded housing is a very lean way to get fault detection and less EMC/EMI trouble, while floating devices allow for more fault tolerance but require much more effort and restrictions.