Telephone unit test circuit - has additional switch which prevents double connection to test instrument in sequential testing

摘要

The system is to prevent double connection in checking several similar units on an instrument in a telephone exchange system, in which the units contain a test relay in the test current circuit which is initially short circuited and which is released in a controlled sequence. A first switch (G1) in the test loop is in series with the test relay (PN) and its parallel bridging contact (k). This first switch closes the test circuit if the voltage applied to its terminals reaches a predetermined value and it remains closed until a minimum current level is reached.