ring main unit is distributing the power supply to meter cubical

A ring-main unit (rmu) is a piece of switchgear that is used to connect a transformer's primary winding to a high-voltage ring main. It consists of three load-breaking switches, each with three positions: 'on', 'off', and 'earth'. They enable the transformer to be connected/disconnected to the ring; the ring to be 'broken'; or the transformer to be connected to either half of the broken ring. The 'earth' positions allow the transformer's secondary to be earthed, or either half of the (broken) ring to be earthed.