Two lighting circuits which should be entirely separate are linked together in a dangerous way.https://youtu.be/XplkGyYx7_I
This installation comprises two lighting circuits - one for the ground floor, and another for the 1st and 2nd floors. These should be entirely separate but they have a dangerous link between them.
The lights are controlled by PIR motion sensors, so automatically switch on when movement is detected.
The first fault is a link between the switched line of each circuit, such that the fault can only be detected between the two circuit breakers when at least one of the PIR sensors is activated. This has apparently existed for many years, possibly since the original installation.
The second fault was a failed PIR sensor that caused the lights to stay on permanently. This fault is what prompted the building owner to request repairs.
The unwanted link results in a dangerous situation - if one circuit is disconnected, it will test as dead. However if a sensor on the other circuit is activated, the previously dead and disconnected one will become live with no warning.
This could occur by someone elsewhere in the building simply opening the door to their flat, as there are sensors outside each door.