Gas discharge tube (GDT) – These rely on a gas trapped between two electrodes that is ionized by the high voltage to conduct electrical current. GDTs can conduct more current for their size than other components. Like MOVs, GDTs have a finite life expectancy, and can take a few very large transients or a greater number of smaller transients. GDTs also take time to trigger permitting a higher voltage spike to exist before the GDT conducts significant current. It is not uncommon for a GDT to let through pulses of 500V or more of 100ns in duration. In some cases additional protection is necessary to prevent damage due to this effect. GDT create a short circuit when triggered, so that if any electric power (spike, signal, or power) is present, the GDT will short this, and will continue conducting until all electric current sufficiently diminishes. Unlike other protector devices, a GDT will conduct at a voltage less than the high voltage that ionized the gas. Gas arrestors are often used in telecommunication equipment. Due to an exceptionally low capacitance, GDTs are commonly used on high frequency lines.