What is a roofline system? Roofline systems are made up of high-voltage components such as switchgear, insulators, intercar cables, surge arrestors, bushings and downleads. They also include simpler connectors like lengths of busbar or braided conductors, as well as the support structures and even nuts and bolts that hold the connections in place. In the high-voltage world, roofline systems are alone in the extreme conditions they face. High air speeds, shock and vibration are just the starting point – the operating environment also exposes roofline systems to pollution, humidity, airborne sea salt, rapid air pressure and temperature changes, driving rain and on some lines, high altitudes. All of these factors can affect the performance of high-voltage systems. Because every railway line has its own environmental conditions, and train operating requirements train manufacturers must create bespoke designs to avoid failure through flashover. Also known as arc flash, flashover describes the short circuit when the current jumps through the air from the electrical conductor to the roof of the train. It leads to loss of power and has the potential to cause major disruption to timetables and services, with loss of reputation for the train operator.
components, but also the simpler connectors and supports that can be easily overlooked. Flashover happens when the electric field around a high-voltage conductor exceeds the insulating limits of air. One technique is to increase the physical clearance and create larger air gaps, which also increases the electrical clearance. But component choice also has a big impact. Invisible to the naked eye, sharp edges and pointed components focus and concentrate electric fields, whereas smooth curved surfaces spread out the charged particles in the air.
False economy of low cost components Good design will reduce the impact of arc flash – not just in selecting high-quality 63
Quality of materials is also important. Inconsistencies in the tightness of a braided copper conductor could lead to loose strands, causing highly localised concentrations in the electric field. Nuts, bolts and support steels selected on price could be vulnerable to corrosion, especially in polluted or humid air. Flakes of corroded material will then spread over the surfaces of insulators, reducing their performance. Brass should be used as the first material to connect to copper conductors – we have found some cases where low-grade steel was used instead, which