Triton opal 3 burning inside unit ???

I have had a triton opal 3 for 3 years and it started to look like the white plastic was melting. now - 3 weeks later the unit has cracked and it smells like burning when then shower is on. This also happened to our previous shower (triton 2). Why is this happening? Is a problem with the shower unit itself or the supply? Please help

Sarah Q
March 2014