Don’t leave it plugged in when you aren’t using it.
The melted plastic containers are a lemon juice bottle which was nearly full of vinegar and a measuring cup with a plastic jelly jar in it nearly full of cleaning solution which was mostly water. When the burning ultrasonic cleaner melted the plastic containers, the contents spilled out and appeared to quench the burning plastic some and the smoke and steam set off the smoke alarm. The circuit breaker tripped in the power strip which the ultrasonic cleaner was plugged in to but it caught fire anyway.