IEC 61643-311 is the component level standard for gas discharge tubes used in low-voltage surge protective devices. A gas discharge tube is the component that provides the largest discharge capability in a surge protective device, and its behaviour is fundamentally different from that of a metal-oxide varistor: it remains open circuit until the applied voltage causes the gas to break down, after which it conducts with a low arc voltage, and it extinguishes when the current falls. The standard defines the electrical characteristics that describe this behaviour, including the d.c. sparkover voltage, the impulse sparkover voltage measured with a fast rising impulse, the time lag to breakdown, the arc voltage, the insulation resistance and the capacitance, and it specifies the test circuits and the methods used to determine each quantity. Particular attention is given to the follow current, the power frequency current that flows through the tube after the surge has passed, and to the ability of the tube to interrupt it, because this determines whether the component can be used on a power circuit. Mechanical and environmental requirements complete the specification.