  The superconducting gap structure of a topological crystalline insulator
(TCI) candidate ZrRuAs ($T^{\rm on}_{\rm c}$ = 7.9(1) K) with a
noncentrosymmetric crystal structure has been investigated using muon spin
rotation/relaxation ($\mu$SR) measurements in transverse-field (TF) and
zero-field (ZF) geometries. We also present the results of magnetization,
electrical resistivity and heat capacity measurements on ZrRuAs, which reveal
bulk superconductivity below 7.9~K. The temperature dependence of the effective
penetration depth obtained from the analysis of the TF-$\mu$SR spectra below
$T_{\rm c}$ is well described by an isotropic $s$-wave gap model as also
inferred from an analysis of the heat capacity in the superconducting state. ZF
$\mu$SR data do not show any significant change in the muon spin relaxation
rate above and below the superconducting transition temperature indicating that
time-reversal symmetry is preserved in the superconducting state of this
material.
