When I turn on my electric shower water starts leaking out from the bottom of the unit, why?

If water is leaking from a clear plastic tube or small plastic elbow,a small red rubber ball in Mira shower units then the Pressure Relief Device (PRD) has been activated.

The PRD is a safety device designed to relieve abnormal pressure from within the heater assembly of an electric shower when a restriction occurs.

The most common reasons for the PRD to activate are that the showerhead has become blocked or there is a restriction in the shower hose, if you squeeze or kink the shower hose while on, the pressure build up could blow the PRD.