  H.E.S.S is an array of atmospheric Cherenkov telescopes dedicated to GeV-TeV
gamma-ray astronomy. The original array has been in operation since the
beginning of 2004. It is composed of four 12-meter diameter telescopes. The
installation of a fifth 28-meter diameter telescope is being completed. This
telescope will operate both in stereoscopic mode and in monoscopic mode i.e.
without a coincident detection on the smaller telescopes. A second-level
trigger system is needed to supress spurious triggers of the 28-meter telescope
when operated in monoscopic mode. This paper gives the motivation and principle
of the second-level trigger. The principle of operation is illustrated by an
example algorithm. The hardware implementation of the second level trigger
system of H.E.S.S. phase 2 is described and its expected performances are then
evaluated.
