IEEE Std 1692-2011 pdf download – IEEE Guide for the Protection of Communication Installations from Lightning Effects.
5.1 Surge protective devices (SPD) and wire-line The standard surge protective devices (SPD) in the telecommunications industry, for the termination of communication wire-line services is the gas discharge tube (GDT). GDTs are also called gas tubes. GDTs can be found on virtually every telephone pair terminated in homes, buildings, and similar locations. GDTs are designed to shunt most current to ground. If the magnitude shunted does not exceed a certain threshold the SPD will help protect equipment, and personnel, from harm. Most shunting devices, however, do not fully protect network electronic equipment from a GPR or “outgoing current,” whether induced from lightning or from a faulted power line. When shunting devices are connected to an elevated ground (outgoing current) during a GPR event, they merely offer an additional current path off the site to remote carth (the other end). When SPDs (GDTs, MOVs, ABDs, SCRs, SADs, SASs, etc.) are used as ground shunting devices, they will not protect equipment from GPR. These devices merely offer an additional path to remote earth through the communication pairs for any and all outgoing currents. When there is a GPR event the SPD provides a connection of the communication path in the reverse direction from which they were intended to operate and increases the possibility of equipment damage to telephone and power installations. The most susceptible locations are those where the equipment is located near, or under, towers and/or are located at a higher altitude than the surrounding area.
In some situations, economic considerations based on the cost of the telecommunication equipment to be protected may be sufficient to justify the cost of the special protection devices. However, isolation does not solve grounding deficiencies inside the telecommunications room. Isolation devices can be damaged on the service and/or power supply side due to improper grounding design. For critical services and sensitive equipment protection through isolation, using optical isolators or isolation transformers is recommended. Isolators provide a path for a signal, using either optical or electro- magnetic coupling, but do not provide a dc current path. If there is no path for incoming or outgoing currents to flow, there will be no current flow. The risk of harm to equipment, cable, or associated working personnel will be greatly reduced. In the design of the isolation installation, intrabuilding GPR problems must be addressed (see Cohen [B7], and Ma and Dawalabi [B29]). The“station” ground of the isolation device must be at the same ground reference with the protected equipment and ac power neutral and dc ground reference. W ithout proper grounding arrangement in the telecommunication room, equipment can be damaged no matter the kind of protection used (SPD, optical fiber, isolation). Figure 3 and Figure 4 show very simplifed8 installation examples depicting intrabuilding GPR and the : impact of the ground installation.