Recommendation ITU-T K.99 describes the construction, characteristics, ratings and application examples of gas discharge tubes (GDTs) intended for the protection of exchange and outdoor equipment, subscriber or customer equipment and telecommunication lines from surges. The 2017 edition of this Recommendation has added four informative Appendices " Appendix I Durability test using "Fast" GDTs " Appendix II Spark-over dark effect " Appendix III GDT Component Form Factors " Appendix IV 3-Electrode GDT operation in Ethernet circuits