a well installation includes a metallic structure having downhole metallic tubing for conveying product towards the surface , a sub surface safety valve disposed in the downhole metallic tubing for interrupting flow of product towards the surface when closed and allowing flow of product towards the surface when open . also , valve control means are disposed downhole and arranged for detecting electrical signals in the downhole metallic tubing and for holding the valve open while receiving an electrical signal , and surface signaling means for applying an electrical signal to metallic structure of the well for transmission via the downhole metallic tubing towards the valve control means so as to hold the valve open . the system can be made more robust by ensuring that the control means will only hold the valve open while receiving a signal of given characteristics to help prevent stray currents erroneously holding the valve open .