[Arran]

In this set, the 42-PT-7, the common negative (electrical ground) or B- wiring is routed back to the power switch where it says 15A on the schematic, you can trace out where it goes from that point. Philco was rather kind in the way this schematic was drawn in that they show the actual electrical connections of the B- wiring, other manufacturers would leave this out and denote what is the chassis ground verses the B- wiring by using a rake styled symbol for the B- connection, and an arrow looking symbol for the chassis connections. However B- or electrical ground, and chassis ground are not completely isolated in most cases, in this set part #7 couples the B- and the chassis together, it's a coil and .2 mf capacitor connected in series, other sets have a capacitor and a high value resistor connected in parallel between the B- and the chassis. I don't know if this explanation helps but if in doubt just print off the schematic and traces out the lines going back to the power switch with a high lighter pen, at any point where two lines meet and there is a black dot that's a connection point, if there is no dot there is no connection. 
Regards
Arran