The European component level standard for gas discharge tubes is used by the manufacturers who build surge protective devices on this component, and its value to them is that it makes the behaviour of a purchased tube predictable enough to design around. A tube remains effectively open circuit until the applied voltage causes the gas to break down, then conducts at a low arc voltage and extinguishes when the current falls, and the quantities a designer needs, the sparkover voltages, the time lag to breakdown and the arc voltage, are measured under conditions that this document fixes so that data from different suppliers can be compared. The specification that matters most in practice is the follow current, the power frequency current that continues to flow after the surge, and whether the tube can interrupt it, because that single property decides whether the component may be used on a power circuit or only on a signal line.