Is an extra circuit / residual current device really necessary for the office?

Can you give me a somewhat layman-friendly argument as to why this is possible / sensible? Maybe I can get him to lay the CAT cables in the wall with a conduit after all...

No detailed argument is needed. It’s written in his Bible. The conduit takes just as long as the cable? You can simply roll out the conduit on a long straight stretch and then push the cable directly through it. Afterwards, lay it properly on the floor and cut the cable and conduit accordingly. It still takes a bit longer, yes. But not twice as long. Still not your problem. That’s just how it’s written. The part about curves is partially true. But no big deal. We pulled cables through three 90-degree bends ourselves. Threaded a piece of Styrofoam on a string and sucked it through with a vacuum cleaner at the other end. Then pulled the drawing spiral in and pushed/pulled on both ends.

The electrician says:

[*]We need a separate circuit for the office 150€ [*]an extra RCD 115€ [*]and because we’re getting an additional circuit and RCD for that, also a bigger distribution board - 490€

The "extra circuit" justification is questionable, but that’s debatable. He can be right sometimes. The joke is that this extra circuit is billed at 150€. At least for us, that means including installation, devices like RCD/circuit breakers, etc. Here it means: I pull a separate cable specifically to the one room. The breakthrough in the ceiling must be made anyway for other cables. Also, the wall needs chasing anyway in that room. Even without an extra circuit. Let that sink in. To be precise, 150€ is charged just to wire an extra cable in the panel, pull it through the existing ceiling breakthrough up to the door. A skilled electrician does that in 5 minutes. Max 10. The extra RCD. Google "RCD+MCB". The device costs 30€. Sometimes more depending on the supplier. If he wants to make a 50€ profit, then maybe around 100€. The part is just as big as a circuit breaker but takes up much less space. The trick? Both are integrated there and you don’t have to sacrifice an entire DIN rail for an RCD plus circuit breaker. So the bigger distribution board is completely unnecessary. For the whole work, a price of 150€ would be appropriate; if the distribution board is actually already undersized and an RCD+MCB has to be installed, then 200€ is still acceptable. Talk to the general contractor about what he charges. The word Chamber of Crafts often helps. When I read stuff like this, it annoys me that I’m not self-employed. You can turn rubbish into gold like this.

He is supposed to lay the CAT cables as DIN requires nowadays, namely replaceable in the installation conduit. The explanation is far-fetched.

I also don't know why a separate RCD for the computer should be necessary, but he insists on it vehemently.

Well, I don't understand the problem. My technology is secured not only by additional RCDs (yes, I know, it's actually not protection for the technology but for humans). Rather, all tech sockets also have additional local surge protection. I don't know how you handle this, but for me my technology, especially sensitive devices with data storage, etc.—so also PCs—is important, and I would never dream of connecting them to the same circuit in my own house as the rest. Especially when you still have all options open now.

I also find the sockets thing strange. That would mean that the switch cabinet was already planned 100% full beforehand, which somehow can’t be right. But I’m not an expert. :/

There can be many different reasons why a larger cabinet is necessary because of this. We don’t know exactly what he is building there and what else you have. The cabinet usually does not consist only of circuit breakers and the two RCDs.

That’s where my knowledge ends, unfortunately. Can we still operate our two computers with one circuit and at the same time have the refrigerator, freezer, and so on running?

For that, you would have to know what you have in the walls and what’s inside the cabinet. Without background info, this is just guesswork.

and here everything always works flawlessly.

In the past, there were no RCDs (or they were not required), and everything also worked “flawlessly.” Nowadays, two are mandatory, and in wooden houses even fire protection switches. You learn from mistakes.

but another 3-phase RCD in addition to the other two? makes little sense if that should be the case.

It doesn’t necessarily have to be three-phase.

He is supposed to lay the CAT cables as the DIN standard requires nowadays, namely replaceable in the installation conduit. The explanation is far-fetched.

I’ll try to enforce this through the general contractor. We would prefer that too.

So I don’t understand the problem. My technology is not only protected by additional RCDs (yes I know it’s actually not a protection for the technology but for the person). But all tech sockets also have additional local surge protection. I don’t know how you handle it, but my technology, especially sensitive devices with data storage etc., so also PCs, are important to me and I wouldn’t even dream of connecting them to the same circuit in the house as the rest. Especially now that all options are open.

Basically, I don’t have anything against the second circuit either, but the additional RCD doesn’t make sense to me (the RCD should trip if a device causes it, no matter where in the house it happens – what is the advantage if the computers are on a separate RCD? That they don’t get disconnected if some other device trips the RCD somewhere else in the house? Or that the house doesn’t lose power if a computer trips an RCD? Or what’s the idea behind that? I’ve already asked the electrician twice, and all I get is "you need that for the PCs."

There can be many different reasons why a larger cabinet is necessary because of that. We don’t really know exactly what he’s building there and what else you have. The cabinet usually does not just consist of miniature circuit breakers and the two RCDs.
You would have to know what you have in the walls and what’s inside the cabinet. Without background info this is guesswork.

I can’t get from him which circuits he plans for the house. I have already asked, but got no answer.
Otherwise, he wants to install 3 RCDs (2 according to regulation (??)), one for the office. I also can’t get which breakers he plans otherwise. I know it so far that every room has its own breaker (those are MCBs, I assume?), and some rooms with a lot of electronics, like the kitchen, have even more. For example, in the kitchen of our old house, the electric stove has its own breaker. But no separate RCD.

It doesn’t necessarily have to be three-phase.

For all RCDs in the offer it says: "RCD - 40/0.03"

That they are not disconnected from the power if somewhere else in the house another device trips the RCD? Or that the house is not without power if a computer trips an RCD? Or what is the idea behind that?

Yes, that is the idea, the more RCDs the less you lose. Possibly they want to be especially safe and completely isolate the room from the rest. Which in principle is not a bad idea with higher-quality technical equipment (this was already suspected further above).

I might even go so far as to use 2.5 mm² NYM cable for the supply line to the gamer paradise. (You never know, maybe they plan that too)

Otherwise, he wants to install 3 RCDs (2 according to regulation (??)), one for the office. Which other breakers he plans I also cannot find out.

Yes, it is of course annoying that he doesn't come clean. Then pressure the GÜ to give you a plan.

I know it so that each room has its own breaker (those are then RCBOs, I assume?), and some rooms with lots of electronics, like the kitchen, even more.

Yes, RCBO means residual current circuit breaker with overcurrent protection. Commonly called: breaker.

In our kitchen in the old house, for example, the electric stove has its own breaker. But no separate RCD.

Yes, as I said, that was once the case in the past. Cars also didn’t have seat belts before.

For all RCDs in the offer, it says: "FI - 40/0.03"

Yes, that tells you nothing except what is already known. 40A rated current and 30mA rated residual current. Number of poles or type is unknown.

But they can also be two-pole devices, I have installed those too. Only I have significantly more than the minimum requirements.

I don't know exactly what he wants to install but another 3-phase RCD to the other 2? makes little sense, if that is the case.

AFAIR DIN 18015 Tab.2007 requires more than 1 RCD. And at our construction site the electrician insisted on a separate RCD for the external cables, the number 3. Otherwise, depending on the rating factor, it can also well be that the additional protection either calls for its own RCD (or a higher rated RCD) because the nominal current is exceeded.

I would look for a combination of FI and LS if it absolutely has to be FI, it takes 2 slots in the cabinet instead of 5.

My combined FI+LS only need one TE, but are a bit more expensive.

But all technical sockets also have additional local surge protection.

Which surge protection do you specifically use? I am still looking. The advantage of multiple FIs is: if one FI trips, all separate FI circuits continue to function. For example, it can also make sense to protect the refrigerator separately. So that the (e.g., refrigerator) doesn’t stay without power for a long time when away and an FI trips.