  Protection of accelerator equipment is as old as accelerator technology and
was for many years related to high-power equipment. Examples are the protection
of powering equipment from overheating (magnets, power converters, high-current
cables), of superconducting magnets from damage after a quench and of
klystrons. The protection of equipment from beam accidents is more recent,
although there was one paper that discussed beam-induced damage for the SLAC
linac (Stanford Linear Accelerator Center) as early as in 1967. It is related
to the increasing beam power of high-power proton accelerators, to the emission
of synchrotron light by electron-positron accelerators and to the increase of
energy stored in the beam. Designing a machine protection system requires an
excellent understanding of accelerator physics and operation to anticipate
possible failures that could lead to damage. Machine protection includes beam
and equipment monitoring, a system to safely stop beam operation (e.g. dumping
the beam or stopping the beam at low energy) and an interlock system providing
the glue between these systems. The most recent accelerator, LHC, will operate
with about 3 x 10$^{14}$ protons per beam, corresponding to an energy stored in
each beam of 360 MJ. This energy can cause massive damage to accelerator
equipment in case of uncontrolled beam loss, and a single accident damaging
vital parts of the accelerator could interrupt operation for years. This
lecture will provide an overview of the requirements for protection of
accelerator equipment and introduces various protection systems. Examples are
mainly from LHC and ESS.
