The Relay Control Unit is a single Box of metal chassis enclosure, provided with sufficient ventilation.  It has knock�out holes (dia 25mm) for cable entry conduits.  All control PCBs are installed and inter connected in this box.  All PCBs are interconnected using Flat cables and standard berge connectors.  It is easy to replace PCBs with spares.  Entire connections to and from this unit are with Cat5 and Cat3 cables.