Water Heaters - lower thermostat melted/burned

I have a 3 year old Kenmore electric water heater. The lower thermostat melted and I am not sure why just yet. I need to go and buy a volt-ohm meter to test the element. The water heater is running 240 volt on 10 AWG wire with a 30 amp breaker at the box which never tripped.

The thermostat has a it's own circuit breaker that tripped but shouldn't it have tripped before the thermostat began to melt?

If the element tests OK can I assume the thermostat was faulty? I might be thinking to hard about this but is there any situation where the element can test OK but under load really be failing?

My house was built in 1996 prior to the code requiring an expansion tank. Should I install one?

thank you.
higgledy

If by melted you mean plastic melted, wire insulation burned, contacts blackened, and general fire hazard territory, then you have a problem. Now that the damage is done you might have trouble finding the culprit. I might just replace the element, thermostat and an intermediate wiring, and visually inspect the rest of your wiring. Then keep a good eye on it, checking it after a few cycles, and run it only when you are home until you are convinced the problem has been corrected.

What model Kenmore are we talking here?