First thing, I"m not an electrician but have some electrical knowledge. I'm renting in a house and am trying to help the owner with some issues. This is an older house that has a combination of old knob and tube wiring and new wiring. There is a circuit that contains old wiring with only two lines. Sockets and lights on this circuit don't have power. When one of the breakers is tripped the outlets and lights start working and other lights and outlets on this breaker turn off. What could be causing this?
The circuits are “crossed” at one, or possibly more, points. Be very careful when sorting this out.
This is how people get killed by electricity as they think they have isolated something but it is still live.
The method to find it is to disconnect at each junction until the relevant part goes dead and label wires to work out what goes where.
Had a similar problem where a site had been wired with additions over 30 years and most was not labelled... the law changed and it all had to be identified. Relatively easy as it was between buildings, switch of a breaker and 15 seconds later the phone went : “we have no power.. oh, where are you?” That was that breaker labelled... Good fun, but we did not mention those power cuts were deliberate... :)