Motor Caravan electrical systems can be split into two sections. Mains hook-up (220 - 240 volts in UK and Europe) and low voltage systems (generally 12 volts). These can be fitted as two separate systems or as a combined unit. The mains system comprise of a hook-up lead, inlet socket, consumer unit and internal outlet sockets. The hook-up leads are available in various lengths, generally 10, 15 or 25 Mtrs. The consumer unit should have a RCD, one, two or three double pole MCB and a polarity light. These differ from domestic consumer units in several ways to protect the van and occupants from dangerous situations. The 12 volt system is powered mainly by a 12 volt battery which can be used even when hook-up is not available. the battery can be charged from the vehicle alternator while traveling or from a charger while hooked-up. The 12 volt system is protected through inline fuse holders or a block fuse holder. All earths and 12 volt negatives can be fixed to the vehicle body or chassis.