Knowing Subsurface Safety Valve- API14A

An SCSSV is operated remotely through a control line that hydraulically
connects the safety valve, up and through the wellhead, to an emergency
shutdown system with hydraulic-pressure supply. The design is fail-safe:
through the control line, hydraulic pressure is applied to keep the valve open
during production. If the hydraulic pressure is lost, as would occur in a
catastrophic event, the safety valve closes automatically through the action of
an internal power-spring system—a normally-closed fail-safe design. With an
SCSSV, activation no longer depends on downhole flow conditions. External
control also allows the valve to be tested when desired, an important
improvement for a device that may be installed for years before its primary
use is required.
Charlie Chong/ Fion Zhang
https://www.slb.com/~/media/Files/resources/oilfield_review/ors02/win02/p52_64.pdf