Storage tank only "explode"
if
1. Air vent and safety valve or pressure relief valve failed,
2. Heating cut off function fail
3. Users do not turn off the heater for prolonged time, aka steam built up causing rise in pressure.

So a good practice, only turn on when need to use, and switch off after use, is first step eliminate the risk of explosion. As you needs 1,2,3 together to cause an explosion.
RCCB in the DB should able to detect it and trip, as leaking or short circuit will cause an imbalance between live and neutral wire.

Wow! The paranoia level of this thread has reached new heights. 
I'm gonna shower in cold water from now on 
Or just use some common sense

The present fear of using water heater gave a new thought about the safety feature after the japanese couple were electrocuted. Is the water heater tank above the roof top or the wall mounted type is safe to use? Do they need periodical maintenance or checking by professionals?  Lets dicuss.

Nothing never failed lor.  They are mostly spring operated. Spring can get stuck, cant they?

Engineer you are sold/convinced by your water heater tanks boss/company the tank are safe. No offence. I like to believe you that will be a big relieve as my solar tank also has relieve valve and I was told to change as it has aged. Why change when it is designed to failsafe? Kindly explain in greater details of course in English not engineering terms.

I just googled pressure relief valve failure and this is a site that study, test and analyse PRV failure rate

http://www.exida.com/articles/Mechanical%2...%20Released.pdf

It is too complicated to read but here is a statement,

"In this report we describe the results of a FMEDA analysis of a particular PRV
to determine the useful-life failure rate of the fail-to-open condition. (The fail-toopen condition occurs when a PRV remains closed when test pressures (TP)
reach or exceed 1.5 times the PRV set pressure.) "

So PRV did fail otherwise why got such study?

There are many possibility in real world environment that the spring get jammed which the test method did not take into consideration where environment are controlled. The spring can get jammed simply with accumulation of sands (user of sand filter), debris from rusted GI pipe, fittings and tap parts worn out may it be rubber or plastic and deform spring or housing were used that passed QC and when at consumers end mechanical it gets dislocated. The accumulation over time became a lump and jammed the springs and many other possibilities. My argument is man made things will still fail over time unlike nature like the sun will never fail to rise every morning from east or rather the earth will never fail to turn. However, am not saying it has to be 100%, if we can have 99.99% one of 10 thousand then it is considered perfect/pure like pure gold 99.9% one of one thousand.

Leaking is a failsafe acceptable but what you engineers do to make sure it will not get stuck cannot open? How? can explain more, please?

Whether the spring is outside of water, overtime water get in to the bad diaphram and leaking valve to the spring compartment. With so much of minerals it can cause the spring to fail. here is the pictures:
[attachmentid=3636239]  [attachmentid=3636240]