How do electric hot water heaters explode and what can be done to prevent that from happening?

Well, the answer is right there in the article you linked:

Always check your hot water heater’s pressure relief valve to ensure it is in good working condition. If you have any concerns that your water heater is not functioning accurately, have an inspector come take a look at it.

The best precaution you can take to avoid having this happen to you, is to set your water heater’s temperature no higher than your manufacturers suggested setting.

Note that it talks about the pressure relief valve (technically temperature & pressure relief valve), not an expansion tank. The pressure relief valve is usually mounted on the side of the water heater and looks similar to this (on a new water heater, the piping won't be there):

To test this valve, remove the piping if you can't see where it terminates (you'll need to see if water is coming out the end). If you can see the end, you can leave the pipe on. Carefully lift the lever on the valve up. Do not lift it all the way as that will lock it in the open position. If you get water coming out, you're good. If not, or the valve is stuck, you'll need to replace the valve.

Regarding the temperature setting, if you stick to the safe range indicated on the thermostat (usually it will say something like danger of scalding for the higher ranges), you will be fine.

Consider the sources; it is plumbers giving you a scare story to get you to hire them. See also "aluminum wiring".

It will take a chain of 3 events at once:

• failure of the thermostat, causing the heater to overheat the water, boiling it.
• the pressure relief valve fails to operate, preventing the pressure from simply blowing out the relief valve; that's what it's for.
• water is unable to backflow out to the street, which would also relieve the pressure. This would fail because someone fit a "check valve" to prevent backflow without also fitting a pressure relief valve on the house side.

If it's so unlikely, why does it happen at all? Because it turns into dominoes. First, the city requires retrofit of an anti-backflow check valve. The family either skips the pressure relief valve, or sites it poorly. Second, the house's normal pressure changes cause the pressure relief valve to spit water on their stuff. They angrily cap it off. Solved! Third, the hot water heater's pressure relief valve starts to spit (or is ancient and is silted up or rusted solid). Capping the other one off worked, so they cap this off too. Then, they are down to a single point of failure, the thermostat.

An electric water heater explodes when the water in it starts boiling, producing pressurized steam that causes the heater to rupture. In order for this to happen, three things all need to fail; if any one of them works properly, no explosion will happen.

• First, the thermostat needs to get stuck in the "on" position. If it's working normally, or if it sticks in the "off" position, the water in the heater will never get hot enough to start boiling.
• Second, the high limit switch needs to fail. This switch is a second, non-resetting thermostat that cuts off all power to the water heater if it gets too hot. It's set at the factory to a temperature higher than the main thermostat can be set to, but well below the boiling point of water.
• Third, the pressure relief valve needs to get stuck closed. This is a valve, usually on the side of the water heater, that will open to release steam or pressurized water. It's set to a pressure above normal water pressure, but well below the maximum pressure the heater can handle.

When installing a water heater, the most important thing is to make sure the pressure relief valve has a free-flowing connection to somewhere where the water can drain. The other safety mechanisms are configured at the factory and are pretty much foolproof, but this one can easily be disabled by improper installation.

Please don't give advice like it's not going to be that bad of an explosion if you dont know what your talking about. We just had ours explode on us and it was extremely powerful and it had the ceiling come down on us and by the grace of God we are alive. We also got burned with 1st and 2nd degree burns and our hair was burnt. Please please dont leave people to think it could be minor when it could also be terrible.