  We identify a topological Z index for three dimensional chiral insulators
with P*T symmetry where two Hamiltonian terms define a nodal loop. Such systems
may belong in the AIII or DIII symmetry class. The Z invariant is a winding
number assigned to the nodal loop and has a correspondence to the geometric
relation between the nodal loop and the zeroes of the gap terms. Dirac cone
edge states under open boundary conditions are in correspondence with the
winding numbers assigned to the nodal loops. We verify our method with the
low-energy effective Hamiltonian of a three-dimensional material of topological
insulators in the Bi$_2$Te$_3$ family.
