  We show that spin-singlet $s$-wave multi-band superconductors have a
topological phase protected by rotation symmetry and time-reversal symmetry
without spin-orbit coupling in two and three dimensions. This topological
phase, an example of a representation-protected topological phase, has a
$\mathbb{Z}_2$ topological index and is stable as long as the bands at the
Fermi energy are formed by a doublet of orbital states with finite angular
momenta. In the limit of weak superconducting pair potential, the
$\mathbb{Z}_2$ index gives a Fermi-surface formula and is related to the
winding number of three-dimensional strong topological superconductors of class
CI. We present a model of a topological $s_{\pm}$-wave superconductor that has
gapless surface states with a quadratic dispersion and suggest a connection
with iron-based superconductors.
