Originally Posted by Bsquared
the circuit breakers in this diagram do not replace the contractors or (solid state relay) SSR, they are protection for the element in this configuration as I understand it. If you trace out the two hot legs ( red and Blue) the red leg is controlled by the SSR , and the SSR is controlled by the PID. The blue leg is controlled by the 30A contactor, and the contactor is controlled by the green selector switched.
Circuit breakers don't protect devices, they protect the wire. Chances are very high that if you have a short, your device is already ruined anyways. A shorted SSR will get smoked before the CB even opens. A shorted wire will heat above the annealing point fairly quickly, at which point it's physical properties are permanently changed.
If you have a device that frequently gets shorted you can get special semiconductor based fuses that will open in micro seconds instead of milliseconds.