Voltage surges, often referred to as spikes or transients are typically caused by switching of fluorescent lights, fuses blowing or nearby lightning activity. The worst voltage surges are usually caused by lightning and can reach up to 6000V, with current surges of over 3000A. The maximum size of the voltage and current surges depends on the location within the building s wiring system. BS7671: 2011 Amendment 1 Sections 443 and 534 cover risk assessment and provides guidance on protection of electronic equipment for installation engineers to follow. The fullest information on lightning and surge protection comes from the BSEN 62305 series of standards.