Gas discharge tubes (GDTs) are used for applications up to 1 000 V a.c. or 1 500 V d.c. in communication or signalling circuits. They are defined as a gap, or series of gaps, in an enclosed discharge medium other than air. They are designed to protect apparatus or personnel, or both, from high transient voltages. This standard does not specify requirements applicable to complete surge protective devices, nor does it specify total requirements for GDTs employed within electronic devices, where precise coordination between GDT performance and surge protective device withstand capability is highly critical. This part of IEC 61643 - deals with GDTs having two or three electrodes; - does not deal with mountings and their effect on GDT characteristics. Characteristics given apply solely to GDTs mounted in the ways described for the tests; - does not deal with mechanical dimensions; - does not deal with quality assurance requirements; - may not be sufficient for GDTs used on high-frequency or multi-channel systems; - does not deal with electrostatic voltages; - does not deal with GDTs connected in series with voltage-dependent resistors in order to limit follow-on currents in electrical power systems; - does not deal with hybrid or composite GDT devices.