What is the exact reason for grounding (earthing) of a transformer?

Question

I have some misunderstandings with the grounding of a transformer. As far as I know, the utility transformer's (the last transformer before my mains plug) neutral point (star-connected point of 3 phases, this image is probably from the US) connects to the earth with some rods. After that a green-yellow protected earth wire pulls through the mains plug. The reason for this is primarily "safety", is said, because if the live wire accidentally touches the chassis of a household appliance (like a refrigerator), this low impedance connection allows passing ground fault current through its own and RCD device triggered.

I understand the PE cable that is through the main plug to the transformer is critical from a safety perspective. But, why are we earthing the neutral point of the transformer? Maybe for lightning protection? I question this because if this earth connection did not exist, we would not be shocked when the live cable touches any chassis (someone argues we are shocked in any case because of capacitive coupling between earth and other phases, please tell me if that is right).

The fundamental rule of circuit theory is "current must return the source", hence the discourses like "the earth is a global charge pool and can sink/supply all current and supply global 0 V reference" must be wrong (the potential difference that causes lightning is already between the earth and the clouds).

To sum up: why are we earthing the neutral point of the transformer? Please, let me know if there's anything I missed or don't know.

Does this answer your question? Is ground connection in home electrical system really necessary?. There are other methods but, for standard household wiring, we need self-revealing faults and, we need to have a way that we can detect and break using RCDs (aka GFCIs). — Andy aka, Jan 16 '24 at 17:31

score 0 · Accepted Answer · answered Jan 16 '24 at 17:37

The reason for this is primarily "safety", is said, because if the live wire accidentally touches the chassis of a household appliance (like a refrigerator), this low impedance connection allows passing ground fault current through its own and RCD device triggered.

Correct

if this earth connection did not exist, we would not be shocked when the live cable touches any chassis

For the same reason as above: simply replace the appliance in your home with the utility transformer, and replace the breakers in your home with the breakers in the power station.

If the utility transformer is damaged and gets a short between primary and secondary, or via the chassis, then high voltage from the primary (2-35kV) will end up on the secondary side.

If the secondary side is Earthed, then the same thing happens as you described: an abnormal current flows between primary and ground, and breakers trip on the primary side.

Neutral on the power station transformer is also Earthed for the same reason: if the high voltage power lines touch the ground, or a crane, or if a tree falls on them, then abnormal current will flow and breakers in the power station will trip.

Without this, a faulty transformer could potentially send kilovolts into your home without it being detected. At this voltage, it's pretty much instant death.

What is the exact reason for grounding (earthing) of a transformer?

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