and the subscriber put into a parked condition. This was possible because each subscriber had a dual armature line relay, and in the parked condition only the low-current armature was operated. The scanners scanned the subscribers seeking out the ones that had initiated a calling condition by lifting their handset, ignoring any in a parked condition. The scanners were mounted in the racks of the associated switching units and fed back information so that a register could be switched to the subscriber to provide dial tone. There were three outgoing junctions per shelf, and they could be busied by using the keys that can be seen in the photograph. The TXE1 required power supplies of −18 V, +50 V and −50 V DC. These were provided by lead–acid batteries charged from the mains supply, backed up by a diesel generator. Reliability and maintenance The exchange proved reasonably reliable, although it did have a few outages. Most of these were caused in the common control area. The common control equipment was divided into functional units, and each unit was duplicated, an A side and a B side, and each section was isolated by means of reed relays. Under either fault conditions or manual control or the predetermined time, the unit indicated would change over to its partner. The changeover of the relays was controlled by a series of reed relays whose reed inserts were wetted with mercury. Periodically over some weeks, the mercury would migrate to the point of contact of the blades, leaving a mercuric bead giving “ON” and both A and B sides into service. The confusion generated caused the exchange to be isolated. There were also some problems with the ASY63 transistors that had nickel–iron connecting wires and would not take the solder, causing dry joints to the circuit cards. This happened over all sections of the electronic equipment in the common control area. The remedy to this problem was to re-solder connections with a solder with a stronger flux. For maintenance, the registers were hinged and could be lowered for easier access. These units, unlike the common control, were hard-wired. However, a unit could be changed by breaking straps at the rear and then rewiring them. A subscriber was connected to the local register using the normal reed switching as the local registers were connected to the C switches. However, they were hard-wired to the common control