Bang & Olufsen BEOLINK Handbook page 45

All products must be disconnected during the test, as it would otherwise be
impossible to circuit test the cables due to short circuits in the products.
Connect the main box to the main room, for example, and then proceed to
connect the LED box to all Master Link connections one by one until you
locate the junction box causing the problem (in this example there are only
junction boxes). The small extra cables supplied can be used if you suspect
that a cable between a product and a wall socket is faulty.
LED tester
main box
MASTER LINK 41