In normal service, the exchange automatically changed all three sections from one side to the other every eight minutes. If Call Control detected eight failures to set up calls within that eight minutes, then it force-changed all the side-conscious units onto the other side, locked that side in service and raised a prompt alarm. In periods of very low traffic, there would typically be fewer than eight call set-up attempts on the exchange in eight minutes and this would have prevented the above security system from working. The exchange was therefore provided with an automatic Test Call Unit, which originated a test call every 30 seconds. As well as thus enabling Call Control to detect eight failures in less than eight minutes (if all call-attempts were failing), the Test Call would raise its own prompt alarm if it detected 31 sequential call-attempt failures, indicating that neither security side of the exchange was able to connect calls.