types of water heater -
- tank type
must use external earth fault breaker at 10 or 30ma
suited for multi point use

- instant type
usually have built-in 10ma breaker
for 1 point only

- gas fired
risk of gas leak
widely used in other countries like taiwan, and yes explosions happens

- solar
have electric booster as well, when sun is low. same like tank type

elcb is a generic term. as long as its current triggered is ok

my way of safe use;
i use a tank type
electrify/heat it up before use, then switch off during shower.