By intruducing a source of power within a building, you have in effect by-passed all of the protection devices like fuses and trips. So, if something were to go wrong one would expect a fuse or trip to activate and 'safe' the circuit, but if the source of the power is within the circuit itself the trip would not have any effect and the fault would be fed more and more power until the source is overloaded or the fault is removed. The 'fault' may be someone getting a shock or a faulty appliance that may catch fire.