Priority Circuit Breaker (PCB)

The idea behind this is that where there is a maximum power supply limit, the circuit breaker will trigger on individual circuits based on the combined current usage of all circuits.  I haven't actually built this yet but I'm experimenting with several ideas, the biggest issue in this is that it is intended for working with AC mains voltages, though it could be used with DC supplies.

This could be based on a fixed circuit priority or a last used priority. 

None of this circumvents that the main power supply MCB is still in place and will cut power to the whole if it's limit is exceeded.

None of this circumvents the operation of an RCD which should be in place.

A maximum combined current limit is set for the PCB.

To avoid inconvenience, if any circuit protected by the PCB exceeds the maximum, then it is disconnected.

On a fixed circuit priority, four circuits would have priorities 1,2 3 and 4. The total of all current used by all four circuits is continuously calculated, and if it exceeds the maximum set for the PCB, the fourth priority circuit is disconnected. If that isn't enough saving, then the third priority circuit is cut, then the second until the total is less than the maximum. No circuit can be reconnected until the total power usage is below the limit. In this way, high priority circuits can be protected from interruption by high power using circuits of secondary importance. 

On a last used priority basis, the priority of the circuit is determined by how long it has been connected, so that if a new circuit is switched on which causes the combined maximum limit to be exceeded, it is immediately disconnected. If the circuit which was connected, say second of four, increased its power consumption, then the circuit connected fourth would be disconnected, and if that didn't offer sufficient saving, then the circuit connected third would be disconnected. 


The maximum power supply limit is 16A, which when exceeded, is cut by a master circuit breaker (MCB) and takes, say, 1 hour to restore. The MCB might be locked away, may be at the top of a pole or may even have a time limiter fitted. 

The main circuit feeds four spurs, a computer (2A), a variable fan heater(12A), a battery charger(2A) and a 13A socket. The end user is working on their computer earning a living and the circuit is pulling 2A. The battery charger is also plugged in to charge the batteries up to use later in the day, total of 4A. It's early in the morning, and cold, so the fan heater is switched on full and now the full power allowance is being used. Being forgetful, the user plugs in a kettle for morning coffee (8A), the MCB cuts off and all power is lost. User is no longer working because the computer is down, its's cold, the batteries won't be charged for later, and there's no coffee.

With the priority circuit breaker, the computer could be set at maximum priority, the socket at second priority, the battery charger at third and the fan heater at fourth.  When the total current would exceed the maximum, the power to the lowest priority spur would be cut, in this case, when the kettle was switched on, the fan heater would be cut, saving the other three circuits from disconnection. When the kettle was boiled, the user would switch the heater on again. 

If the user was boiling the kettle and then decided to switch on the fan heater, nothing would happen, because its the lowest priority and would exceed the limit.

If, say, the maximum current was only 10A, then the fan heater could only be operated on its lowest setting (4A). If the kettle was plugged in, then both fan heater and charger would be disconnected, but the computer would continue to run uninterrupted. If the computer, battery charger and heater (on lowest setting) were being used, and the user accidentally switched the heater to its maximum setting (12A), it would be disconnected, but the other two would continue running, but if the fan heater was put to it's middle setting (8A), the heater would continue running and the battery charger cut-off.

The priority is to prevent the total current being exceeded and the service being interrupted for all circuits.