Improvements in Cable Surge Protectors

1. Introduction

Cable surge protectors are typically used to reduce lightning damage to telecommunication facilities. Nevertheless, the work required to install these devices is time-consuming. Moreover, ensuring there is sufficient installation space within terminal closures is required because the number of cable surge protectors to be installed depends on the total number of core wires in the target cable. We have therefore developed a new, more compact cable surge protector that improves connection workability. The following sections introduce this new surge protector.

2. Lightning damage to telecommunication facilities

Tall antenna towers are built on telecommunication buildings, and consequently, these towers are often struck by lightning. Metallic cables are connected to the telecommunication buildings that accommodate subscribers or have telecommunication or broadcasting towers, and also to wind farms or other structures. These cables are vulnerable to the surges caused by direct lightning strikes flowing into the telecommunication facilities. An example of damage to telecommunication facilities caused by lightning surge current is shown in Fig. 1.

Fig. 1. Example of damage to telecommunication facilities caused by lightning surge current.

Surge current caused by direct lightning strikes on steel towers or windmills flows into metallic cables. This current often damages the telecommunication equipment installed in NTT buildings, the telecommunication devices in customer premises, and also the metallic cables themselves and their connection points. Lightning damage in metallic cables is caused by an insulation breakdown between core wires or between a core wire and the cable’s aluminum sheath. In addition, indirect lightning surge current can occur in metallic telecommunication cables when lightning strikes nearby power lines or trees, or when inter-cloud discharges occur.

3. Overview of cable surge protector

A cable surge protector can bypass surge current to a ground electrode through multiple connections to the core wires of a metallic cable. The connection configuration of the cable surge protector is shown in Fig. 2.

Fig. 2. Connection configuration of cable surge protector.

4. Issues with existing cable surge protectors

PAT connectors are generally used to connect the core wires of metallic cable. However, such connectors may be damaged in the locations where surge current due to direct lightning strikes occurs. A cable surge protector must therefore be connected to each core wire by hand soldering, which is very time-consuming. Moreover, since all core wires must be connected, the number of cable surge protectors to be installed depends on the total number of core wires in the target cable. Therefore, it is necessary to ensure there is a sufficient installation space within a terminal closure. When the cable surge protectors cannot be installed in a terminal closure, it is possible to use cable surge protectors that are enclosed in a dedicated box for external use, which are installed on a utility pole. A dedicated-box-type cable surge protector is shown in Fig. 3.

Fig. 3. Dedicated-box-type cable surge protector.

In summary, cable surge protectors can be very effective to prevent lightning damage to metallic cables. The two key issues are that the connection task is very time-consuming and that sufficient installation space is needed.

5. Improvements to cable surge protector

6. Functional verification

As described above, the improved cable surge protector adopts U-slit structures in the connecting section, a multi-electrode configuration, and a downsized enclosure. Therefore, there were concerns that this new structure might have less resistibility to lightning surge current than the conventional product. Accordingly, the resistibility to lightning surge current of the improved cable surge protector by using a prototype unit and the conventional one were compared. Specifically, we tested the maximum resistibility to surge current using a direct lightning strike surge current generator between all telecommunication line ports and the ground port, between each telecommunication line port and the ground port, and between telecommunication line ports themselves.

The results of these tests showed that no sparks or other problems occurred at the U-slit connection sections, contact points, or unused points, as a result of enclosure downsizing, and that the maximum resistibility to surge current was nearly equivalent to that of the conventional cable surge protector. These results indicate that an effective new cable surge protector that improves connection workability and downsizes the unit enclosure was achieved. The resistibility to lightning surge current of the new cable surge protector is the same as that of the conventional cable surge protector.

7. Conclusion

This article introduced a new downsized cable surge protector that improves workability. The plan going forward is to carry out trials in the field and to commercialize a surge protector product reflecting the results of these trials. The EMC Engineering Group of the Technical Assistance and Support Center is committed to the smooth provision of telecommunication services and to achieving prompt solutions aimed to reduce the amount of lightning damage. To this end, we intend to actively focus on technology development, technology collaboration, and technology seminars.