When a large number of triggered spark-gap switches are used in parallel, as in many thermonuclear fusion experiments, it is essential to detect when one fires late. A further requirement is that any spark-gap which breaks down spontaneously will be indicated, even if other gaps fire in consequence. Both functions are performed by the 40-channel equipment described, lamp indication being used for early and late firing, with a time bracket adjustable from 10 to 50 ns. The system used is insensitive to interference and poor wave-shape.