Mark

You're way out there on all this.

These power systems are designed with differential protection. Reference  rotate.aero/forum/troubleshooting-tips-g...rent-transformer#799

Smaller relays installed in working systems "can" be controlled by circuit breakers, but the CB usually removes power from a complete system. Relays, computer, switches, actuators, lights.....

Some relays actually have a breaker (or over-current sense) built into them. Fuel pump control relays often are this type.

When we're talking external, APU, or generator power..... this is high amperage current. Circuit breakers are used. They are three phase breakers. Any one of the three phases can make them pop open.

Read the link above and visit chapter 24 of your schematic manual. It should be clear how power protection is designed.