PSA on electrical safety since you haven't started anything.

For water heater safety
WH circuit if you are redoing the wiring for it, use 4.0mm² cables per ST spec.
Good cables from Mega Kabel/Caramay/Fajar Cable cost more. But they are "cheng liu" with all the SIRIM legit certification.
There are elcheapo/fake cables at half price I also don't dare to install in customer premises. Safety first.

More importantly each WH circuit (basically each unit) need its own 10mA RCD/RCBO 0.01A (NOT 0.1A).
Check DB box to see if such thing exist.

For further reading [attachmentid=10873321]

Having a 10mA RCD fitted will prevent the likelihood of becoming news material.
https://www.straitstimes.com/asia/se-asia/m...e-having-shower
https://www.malaymail.com/news/malaysia/202...c-shock/1896447
For whole house electrical safety
Per ST (Suruhanjaya Tenaga) regulations - to be fully compliant - you should use a 30mA whole house high sensitivity RCD (0.03A NOT 0.3A) for human life safety.

100mA & 300mA RCDs should only be used in commercial/industrial as they are NOT very useful to protect human life
They are commonly fitted in residential as they are indeed cheap coz less sensitive and they are only good for preventing building from catching fire.

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Whilst I do my own home wiring and the likes (DB box refresh with like for like) I don't possess a Wirewoman cert (I used to sell and commission inverters and production line (automation) sensors for industry/SME). A definition of "competent persons" based on https://www.st.gov.my/en/web/application/details/7/17 will be PW1 (Single Phase Wireman) or PW3 (Three Phase Wireman) certified personnel. I'm sure there are such wiremen who follow standards but mostly from what I see a lot have cowed to price pressure (to survive and win jobs) and do not strictly follow the ST requirements for residential wiring.

Usually new homes' wiring will still be following ST guidelines somewhat, what we can do here (easily) is that you take a picture of your consumer unit ("DB box") and upload here as attachment (NOT the "Upload Images" imgur pic as its hard to zoom). Then we'll look at it and see if anything major needs rectification.

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FOR BETTER LYN FORUMER COMMENTARY
What we would want to see in your DB box are all the green text portions, i.e.
1. Overview of the DB box components (just open the user accessible panel). NO NEED to open the service panel.
2. Close ups of all the bigger blocks inside. We need to see the markings clearly to be able to comment.

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FOR TESTING 13A SOCKETS
Some people use their phone charger. Phones must be cheap nowadays to risk as test devices.
I recommend bringing along one cheap table lamp (using a 230V bulb) just to plug/unplug test out the sockets.
This will ensure your sockets have power + not expensive to replace test equipment + correct phase-neutral (=230V) wiring NOT phase-phase (=415V) connection

FOR TESTING LIGHT/FAN CIRCUITS
Need a ladder and an assistant. Usually just test out with test pen to see the switching works/ or to label/map out your switches control which item.

Most welcome. Very clear picture
Generally looks OK in terms of correct big blocks fitted and 10mA's for the WH too.

Once u have ceiling lights fitted,
test the 100mA "ELCB" (first one on bottom row, immediately NEXT to main switch) by pressing the test button.
The 100mA should trip to OFF. Maintain 100mA OFF.
Maintain the other 30mA ELCB ON..
All ceiling lights should be dead.
13A sockets (test table lamp) should be operational.

Then flip back the 100mA ELCB up to ON position.
Test the 30mA "ELCB" (middle of bottom row AWAY from main switch) with the test button.
The 30mA should trip to OFF. Maintain 30mA OFF.
Maintain the other 100mA ELCB ON.
All ceiling lights should be operational.
All 13A sockets (test table lamp) should be dead.

Lastly turn the 30mA ELCB on and resume normal operation.

If expected result follow above then all good.
Reversed result (i.e. 30mA ELCB is off but 13A socket still all operational) is swapped and needs corrective action.

For the additional RCD on water heater circuit, not required. Regulations say after leaving DB box no break in the circuit

The 10mA RCD inside DB box for water heater protect whole WH circuit already ie from the DB box including WH unit. The WH circuits' RCD compare current in equals to current out operating on a very tight (strict) tolerance under normal operation (no current was "lost" like 2MDB funds), so a direct test for those 10mA's each using their respective test buttons will suffice (need to keep both the master 30mA and 100mA "ELCB" ON) as the individual WH circuits' 10mA's are connected as "subcon" to either one of the 30 or 100mA "main con".

The WH circuit itself (as subcon) is regulated to have a very sensitive 10mA (very low tolerance) RCD.
The main con regulation (master "ELCB" according to the not so accurate sticker label) feeding the WH circuit can be either the stricter 30mA "ELCB" or the "close one eye" 100mA.
Personally I prefer stricter 30mA as maincon for the WH 10mA subcon, but according to the regulations they can use up to "close one eye" tolerance level ie the 100mA as maincon.

To determine which maincon ELCB feeds the power to the WH, by turning OFF only the 30mA master "ELCB",
see if the WH unit still operational to give hot water.
If no power to WH unit when ONLY the 30mA master is OFF..meaning good, it is stricter secondary protection.