Ok, good. Do you know what is the purpose of 10mA(See this picture)? The seller send it to me and said all Joven model got this. Does it serve same purpose as mechanical RCD?

10mA is required for protection, anything above it will not be sensitive enough to protect the user from electrical leakage. it's also specified in the ST guide

Ok, RCD in DB box is airport screening machine
ELCB jaga front gate of entrance

Then how about mechanical RCD in water heater that we mentioned earlier? Why it has advantage than electronic RCD?

I see, didn't check through the ST guide in details so not aware of the 15mA requirement 

But logically, if upstream protection is more sensitive than downstream then it looks like the downstream one will never get to work, unless they are of different mechanism. Maybe someone familiar with this can share some info for our knowledge 

The house RCD has higher current trigger level to prevent false trigger due to accumulated leakage current in appliances. TNB guidelines shows 2 RCDs:
1) Up to 100mA trip current for wires to lighting and dedicated power outlets such as for air-cond, water heater, etc.
2) Up to 30mA tripp current for power outlets such as 13A sockets for universal use.

As for the water heater, another 10mA RCD is recommended mandatory to be install for each water heater..

For me either ELCB or RCD, both can be unsafe. Most importantly is to regularly check if they function properly. The ELCB depends on house grounding. Over years the house grounding can fail due to corrosion perhaps. There RCD can also fail to trip due to malfunctioned.

In water heater electrocution, I read that 100% is due to using conductive hose with conductive shower head, which is basically metal. Nowadays all comes with rubber/silicone hose. Many say ELCB not safe but for me, it is still okay. WH body is plastic. Metal part inside the WH which is the  heater itself is grounded. If there is shorted, the ELCB is triggered. The RCD operates in different way to prevent shock which is also no problem. However, normal RCD cannot detect faulty unit. Say the WH runs at 3kW, but when the current goes to 5kW it will not trip RCD because the supply and return currents are still balance  . Once the current goes over the MCB, then the MCB in DB box will trip. So, still safe to some extend.

Like I said the most important thing to ensure that those safety device performed normally with regular testing.