Tankless water heater has to continuously heat the water, unlike tank type. The only reason the socket still survive is just because the heater setting was not not full power, perhaps.

Continuous heat on the 3-[in 13A plug will soften the clamping brass in the socket, thus making contact poorer leading to mere heat generated. This is how most fire get started in houses.

I believe if the N is shared before the RCCB, it is still okay. Not between RCCB and device being protected.

RCCB - Residual Current Circuit Breaker
RCBO - Residual Current Breaker with Overcurrent

RCCB trips when the outgoing current vs return current difference over its rated trip current. However, if there malfunction in appliance is over normal operating current but the outgoing and return current still tally, it will not trip the RCCB. With RCBO, it will trip because overcurrent protection will trip the RCBO. Additional protection.

As for ST guideline:
MCB 16A for 1x 13A socket
MCB 20A for 2x 13A socket
MCB 32A for 4x 13A socket
MCB 32A for 6x 13A socket in ring configuration.

Note that the 6 sockets are in ring configuration. Just in case you are not sure what it means. The ring wiring is when wires come out from DB and go to sockets in series and return back to DB through another path to form a circle. Therefore, if you cut wire to one of the socket, power will still be available on the rest of sockets. My house wiring is like this. There are two separate rings for ground and upper floors. I have 2x32A MCB in my DB.

Just think you MCB is more on protecting wire in wall. If current is too high, the MCB will trip to protect wire in the wall from getting to hot and melt the insulator, short-circuited and ignite fire. The MCB rating is not rated to provide total power to sockets on the wall. As you can see the 20A MCB is used for 2x13A socket. Two sockets totaled up for 26A but the MCB is only 20A.

Current carrying capability of wire depends not only on its size but ambient temperature and how it is laid also affect current rating. I always assume the 4mm wire should be protected by 20A MCB even though it can carry much more current. If in thermally isolated trunking in will be rated at 24A but if laid directly on concrete wall it can go up to 32A. However, you have other wires put together.This will affect ambient temperature as well.

Hope this helps.

https://www.st.gov.my/contents/publications...uildings%20.pdf

Cable size is already defined for lighting and socket. Why ring will not trip if overloaded? The current to MCB is still total of two separated string of loads. Note that each socket is still protected by 13A fuse.

Yes, but the wire is still the 4mm wire for the socket as standard

I see what you meant. I did not look into drawing detail. I am wondering if the 2.5 and 4 is actually the surface area. When I look at the wire, it looks like the wire diameter not area If it is really the 2.5mm², then the diameter is only 1.78mm.

Anyway, even if the area is 2.5mm², the wire will survive much longer than the MCB tripping time based on what I used to read before. Anyway, not good as you say

Yes I understood what you said. I also mentioned that the MCB is more on protecting wire in the wall, not appliances.

My house is ring and it is using 4mm wire, not the 2.5mm like in diagram. I was not looking at the diagram because for me any socket is using 4mm . My mistake there. The reason I mentioned about area vs diameter was because the 2.5mm² wire has less than 2mm diameter which is very small and makes me wonder if it really the area.

Why it makes no sense? It is better, isn't it? The wiring is original from developer and I know it is a ring because I checked it myself. There is no voltage problem and it is just normal semi-D house size in Malaysia to concern about voltage drop in wire. I did extend a bit at the back of the house and I pulled another big cable from main DB to another DB I added at the back of the house to supply power for kitchen, air-cond and heaters to rooms/bathrooms at the back of the house. The kitchen is electrical hob, hood and garbage disposer. Developer only provide power points for air-cond and heater to master bedroom only. My friend and I got cables laid down during renovation work.

It is good say when the current in conductor is constantly high for long period. This will lower the wire operating temperature compare to smaller gauge. PVC insulated cable operating at 1x current load at 70C will have service life for 20 years.

As mentioned if the cable/wire is operating hot at 70C. This is based on electrical engineering formulation. I believe it is more on insulation based on Arrhenius equation.