Two limit switches may be simplest depending on your controller or data acquisition system. You may also want to look at a cable-actuated displacement that provides easy installation and small size although it provides a proportional output and not just on/off. For details, look at...
To RavenJoe
In the U.S. if your panel isn`t rated for 100% . You can only load a circut 80%, if it is on for 3 hours or longer.
In 3 ph loads the breaker should have a common trip, so it breaks all 3 phases at once. Since your load is resistive
your 20 amp breaker could safely carry 16 amps per...