[en] The causes of serious metallic melting and denting due to discharges in the insulating gas of the Daresbury tandem are examined. In order to explain the melting, high oscillating currents have to flow in the spark channels, and those can only be expected when return strokes excite local electromagnetic modes at the site of the origin of breakdown. In the case of intershield to tank sparks, evidence that magnetic confinement of the spark channel produces denting is presented. A model which accounts for the differences between sparks at Oak Ridge and sparks at Daresbury is also presented. (author)