In which part of the world is this? In most countries, all neutrals are just as neutral electrically, but legislation varies throughout the world.

In most panels, all the neutrals are connected together on one bus(piece of metal with screws you clamp neutrals to), so finding each neutral for each live wire not as important. Would have a problem if neutrals are different colour than white(in North America, think blue in Europe).

It's in France, I think you are right all neutrals are the "same", but on the new panel the live wire are paired with a neutral

Every fuse is wired to something, for example a plug, one neutral and one live wire. And theses wires a passing throught the fuse. So i thaught it's better to pair the wires

A pain in the neck, but neutrals and lives usually make a circuit. An ohm meter should read something else than open, if placed on hot/live and neutral, if light switch is on or something is plugged into an outlet. All power is off doing this.

@crip659 I see, I think it could work where the circuit is not open, but if it's a wall plug, it will be open ? Is there another solution for theses ?

@GregoryBoutte Assuming France's regulations are remotely sensible, the fuse should always be on the live and never on the neutral. So swapping live and neutral is a problem, but trading neutrals between circuits doesn't matter. But regulations sometimes aren't sensible.

@Hearth It sounds like the new panels/breakers are RCDs/GFCI type with paired neutrals, not the older type.

plug a 9volt battery into the wall outlet on a given branch, then use a voltmeter to find the neutral that pairs with the hot. You could also use a 6v battery and 12v battery all at once as flags to reduce the number of trips made from room to panel.

@dandavis that's a great idea as long as there are no lights or devices plugged into outlet anywhere in the house. If there is, the battery will probably die before you get to the panel. You should make this an answer. It has the benefit that it can be done without installing the neutrals, without bypassing RCDs, and without power applied.

@crip659 Okay, if they're on a GFCI, then sure, you need to keep them paired. They only mentioned a fuse, though--and I would assume any circuits still using fuses are quite old. Anyway, if it does use a GFCI, that would give you a way to determine the pairs as well--cause a ground fault on one neutral and see which live gets cut off by the GFCI.

Yes I think it's GFCI breaker, I did not know that was the english term for this, I looked on google image and it look like the new panel. I've updated the post to mention the GFCI breaker.

How many fuses are there, i.e., how many "circuits" are there? You say no color was respected. Were different colors used for hots in different circuits? Different colors for neutrals in different circuits? Is the wiring done with individual wires pulled through a conduit? Does this wiring have a separate "ground" or "earth" in each circuit?

Are you or someone living in this dwelling while working on it? Will your work be inspected by an authority? How many lights and receptacles are there total in the structure? Is this a fully detached house?

In the current panel there is 7 fuses for 11 circuits. And for the colors, the previous owner used all kind of color for the live wire (even the ground color). The wires goes through a conduit inside the walls. Not every circuit have a ground.

Yes someone live in the dwelling, and it won't be inspected. There is like 15 lights in the house. By receptacles you mean wall plug ? If it's the case there is around 20 wall plugs.

So you want to proceed with understanding to avoid improper connections that would cause a problem later that you would be responsible for. Are you the owner of this property? I think you should get an analysis by a licensed French electrician. Changing a fuse panel to a modern breaker panel requires understanding and probably experience, and you have the complication of non complying wiring due to color coding not respected (and who knows what else).

I don't know what equipment is available in France. Is it possible that you can remove each fuse from the "panel" and insert a breaker in each circuit? (AFIK we cannot do this in US). So if this is possible then basically all you have to do is identify the neutral that goes to each hot and connect it to the neutral on each GFCI breaker. Ordinary breakers do not need to have neutral identified and connect to the breaker. Is there a means of disconnecting power to the fuse panel, i.e., a disconnect, outside or inside?

In this panel there some problems - namely that it doesn't exist. You can use that tracer so long as everything is de-energized and disconnected from absolutely everything. Basically you're in a situation where someone ran wires and then left before punching them down. Which means starting over from scratch while two steps behind, because you have to pull every outlet, switch, and fixture. - I do this for a living, but I don't have a pole long enough to touch that.

There is no point in installing GFCI if you don't have third, ground wire. That means rewiring everything from 2 to 3 wires, so your issue becomes moot. Also, you need separate neutrals for separate GFCls. If you have 1 GFCl like on the picture, then all neutrals go into it.

@Agent_L I respectfully disagree with the idea that "There is no point in installing GFCI if you don't have third, ground wire. " GFCI does not require a ground wire for proper operation. GFCI compares the current in Line and Neutral, and if they are sufficiently unequal, will trip the circuit, possibly saving your life.