sentences:  If this occurred in one of the common-control areas of the exchange, it could give rise to the exchange becoming isolated (being unable to set up any calls) for as much as several hours.These faults were very difficult to locate and in the end, the problems were only resolved by a fairly substantial re-reeding programme carried out on the common-control units of the early Plessey exchanges.The STC reeds proved to be more reliable, but, if they failed, they tended to stick or short-circuit.