Abstract: Modular electronic trunk selectors are known which have a number of incoming trunk modules and a common control module. Incoming trunks are connected to outgoing trunks by the common control module. The invention disclosed is a trunk selector of this type but it includes automatic testing and fault location. Periodic tests are randomly made of incoming trunks to see if correct signals are present on certain leads between the incoming trunk module and the common control module. Checks are also made for continuity and open circuit conditions on the switching paths at appropriate times. Tests are also made when regular calls are being set up. Detection of a fault initiates a "loop-back" in the common control module to see if the fault is in the incoming trunk module or the common control module and an appropriate alarm is activated.