Neutral is not Earth
They are completely different wires with completely different jobs. Neutral is the normal everyday path for return current. Earth is a fault catcher, it only flows current when a fault of some kind is happening (and stops as soon as current trips the circuit breaker or RCD/GFCI).
Putting active daily current on earth is out of the question.
I see you using 'GND' to describe "Ground" as you call it, which tells me you've cut your teeth in small electronics. That is a completely different world. In electronics there is no proper earthing, and "GND" is simply the negative/Vss/backplane/common that is wired to everything as the common return loop... which would make it more like AC mains "neutral".
I gather you're from the Euro-system of wiring, which means, you call that thing EARTH. That is a relief: over in North America we call it "Ground" which makes it much harder to RE-train electronics folks. So keep calling it EARTH, or at least as "safety ground" as a different thing from GND.
I never call it GND, and I'm a Yankee.
Also unlike electronics, neutral is not "common" either. In AC mains we are running the wires at hard ampacity limits all the time, which means, each live supply wire MUST have a dedicated neutral wire that returns only its current. We can't be careless and treat neutral like a common/backplane, because then, 1 neutral might end up returning current for 2-3 live wires, and it would be overloaded! Neutrals don't have circuit breakers - they don't need breakers if wired correctly. (even on an RCBO, the neutral goes through the RCBO for RCD sensing, but does not have its own overcurrent detector).
Earths are yellow/green
Or straight green, or bare. This is a world standard and is largely effective outside the old Soviet bloc. Even the Yankees agree on it!
Very much unlike neutral, all earth wires should be tied together in all locations. Since earth doesn't carry current except during fault conditions, we don't need to worry so much about thermal overloads from continuously carrying high current. Service current is never on earth.
Find out about local legacy neutral and earth colors
I've never heard of a country where they used the same color for both neutral and earth at the same time. However there have been changes.
- Modern Euro/3-continent standard: Light blue for neutral, brown for live
- Au, NZ and old UK standard: Black for neutral, red for live
- Americas standard: White for neutral, black for live
So you will need to ask local experts for the old wire colors, and see if that explains what you see.
Prepare for the possibility that your old work was done before earthing became a standard, and there may be no earth at all. Also, some locations used metal conduit for wires, and in that case the metal conduit is typically a valid earth!
Test the neutral by isolating it
Neutral has an equipotential bond with earth, either at the panel/consumer unit or on the power company side of it. This is to keep voltages at a reasonable voltage and to suppress transformer leaks, lightning etc. It also gives a current path for fault current to complete its circuit. Do not treat this as a license to treat them as equal or to tie them together elsewhere!
Once you know which circuit you are dealing with, and can identify the circuit breaker that turns it off... if you are qualified to do so, you can go into the breaker panel (consumer unit). From the circuit breaker you can identify the circuit's live wire, and then identify the circuit's neutral. With the breaker off, you can disconnect the neutral from the breaker or neutral bar.
Now there should be no connection between neutral and earth.
With the circuit breaker off, live+neutral (together) should be isolated from earth.
You can also try energizing the circuit with neutral disconnected. In that case, on a properly isolated circuit,
- neutral will be isolated if all loads are off (a DVM will show a phantom voltage of a fraction of normal voltage).
- if any loads are turned on, neutral will snap to line voltage (e.g. 230V) in all locations. That is one reason the neutral wire is insulated.
- All loads should be inoperative. If you find a load that is working, it is bootlegging neutral from earth.
If you find yourself with an earth wire which is not plainly marked (e.g. it is the wrong color, or the former earth color is now a neutral color etc.), then wrap it with green or yellow/green electrical tape at the least. That is not authorized by the electrical code, but it will make it clear what it's being used for. Identify the wire carefully and always mark both ends of a wire the same, at the same time.