The need of logical relay as a robust way to control a PV System it is very much in demand and allows some of the more complex circuits to be  correlated, we talk about PV Solar Modules, Start Generators, Battery Banks, Battery Charge Controller, Load Controllers and Alarms (see the picture on the left hand side). This Logical Relays are logic modules which provides control functions such as high/low voltage alarms, load control and generator start for 12-, 24- or 48-volt battery systems. What make them to be in demand is the simple way to connect and correlate all the elements together  offering basic monitoring features.