Something is live if it is connected to something else and it is live.
The toaster is ok if it is connected to the plug and the plug is live.
The toaster is connected to the 3 pin plug if the plug is wired correctly and the plug fuse is ok.
Toaster plug is connected to socket 1
Socket 1 is connected to wire 2
Wire 2 is connected wire 1 if to circuit breaker 1 is up.
Wire 1 is connected to the outside power supply is the main trip switch is up
Outside power is live.
3 pin plug is wired correctly if the blue internal wire is connected to the left terminal and the internal brown wire is connected to right terminal and the internal green wire is connected to the centre terminal.
The plug fuse is ok.
Build the rules presented as a base level rule set and combine the how why provers presented to prove if the toaster is ok, indicating to the user of the system how the solution was reached and asking any information from the user that has been omitted and is askable.
For example the internal wire connection facts should be marked as askable.