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. This was also a cause of isolations early on, but a simple modification restricted the most serious type of failure to a small part of the exchange. The GEC/MOV reeds proved to be the most reliable of all. Once the teething troubles had been largely dealt with, which was not until about 1974, the TXE2s realised more of their expected benefits and it was eventually not uncommon for one Technical Officer to maintain the operation of three of these exchanges, serving perhaps some 5,000–6,000 customers in total. Preservation In the summer of 2005 a demonstration rack of TXE2 equipment was transferred to the Connected Earth collection at Milton Keynes Museum. There is a working MXE2 (mobile variant) at Avoncroft Museum. It can be used to make calls within the museum. Many of the MXE2s ended up in Northern Ireland. Only one of these ever had to be used 'in anger'. This was at Castlewellan about 1990, when the exchange was blown up by terrorists. The typical set-up time for an MXE2 was about six weeks, but at Castlewellan, full telephone service was restored using an MXE2 (and the additional use of a mobile transmission unit designed by Northern Ireland staff) within one week of the bombing. It did, however, take a lot of subsequent work by the switch maintenance Technical Officers to get the exchange up to an acceptable standard of service, as it had been standing idle for several years. TXE3 TXE3 was a cost reduced and improved version of TXE1 designed for exchanges with more than 2,000 subscribers. The same three companies that built the TXE1 developed TXE3, namely STC, AEI and AT&E, and it was intended to be the standard BPO system for large exchanges. The prototype exchange was built and tested in the Circuit Laboratory at Armour House. The trial period was for 200 subs, 100 were for senior engineers within Telecoms HQ the remaining 100 subs were