Abstract: Apparatus for blocking a d.c. component of a signal, comprises an electrically conductive signal path (1, 2) having a gap (3) in it preventing direct current flow across the gap, and an electrically conductive element (4) spaced from the path by a body (5) of dielectric material, the element (4) being located and dimensioned such that in use an a.c. signal is coupled from the signal path (1) into the element (4) at one side of the gap, and from tie element (4) into the signal path (2) at the other side of the gap. The apparatus optionally includes a quarter wavelength earthing strap between the signal path and ground potential. The apparatus includes means for providing an a.c. signal feed into a hazardous environment in an intrinsically safe manner.