The alarm bell is connected to negative battery (-24V DC) by the grey cable. These batteries are in the nuclear free accu case; +12V is used for the LSD indicators and otther control systems; I’ve added a -24V DC supply for some analogue telecoms kit I am experimenting with.

So the blue cable can be used to check if items are solidly bonded to frame earth (earth ground). I only suggest you do this if you are defnitely sure your audio kit can handle DC offsets and interference from the motor driven bell…