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.