Trumpy — Any chance the water level is periodically dropping unnoticed and exposing the elements to air?
That will kill 'Cal-Rod' {slang/brand name} water-heating elements in a heartbeat.
Also, is it possible that the water has high mineral content that causes scaling on the element(s)? The scale will harden and break off, falling to the bottom of the tank. After a time, the scale forms a mound that eventually accumulates enough height to surround the element, reducing heat dissipation to water, and failing the element. In typical US heaters, removing the scale “hill” is tediously done with a wet-vac and a homemade PVC extension tube on the vacuum hose.
For repeated cases over years of scaling, Incoloy-sheathed {iron+nickel+chromium+cobalt+titanium+molybdenum alloy} replacement heaters are available that last longer than the OEM-cheapie plated-copper elements.
http://www.stateind.com/parts/amalloy.htm
