Grounding-system Evaluator for Preventing Lightening Damage to Telecommunications Center Buildings

2.1 Configuration-visualization function

The configuration-visualization function is illustrated in Figure 3. The existing data of wiring connection, which are formatted by excel or csv, can be imported from the equipment database. This function enables users to reduce the time to input the grounding-system configuration and makes it possible to easily visualize the status/condition of telecommunication facilities, such as equipment, cables, and grounding cable, in telecommunications center buildings. It is also possible to visualize the results obtained through on-site investigations via a graphical user interface (GUI) and reflect them in the above visualization.

Fig. 3. Configuration-visualization function.

2.2 Function for automatically determining the status of a grounding configuration

The function for automatically determining the status of a grounding configuration is illustrated in Fig. 4. This function visualizes whether the status of the grounding configuration is correct. The evaluator investigates the 30 items for ideal grounding system described in the document of investigation and improvement procedures and methods written by TASC. If a fault is found, the evaluator shows the details of this fault. This means that users can easily find the areas to be improved. Thus, this function is not only useful for maintaining grounding systems but also makes design work more efficient.

Fig. 4. Function for automatically determining the status of grounding configuration.

3.1 Failure status

Due to lightning, the Subscriber Lines Cross-Connect Module (LXM) system failed and many customer services stopped for a considerable time. Although the system was recovered by restarting the LXM packages, TASC was asked to investigate the cause of the failure and propose countermeasures to prevent the lightning damage.

3.2 On-site investigation and simulation using the grounding-system evaluator

The status of a grounding-system configuration obtained from the on-site investigation is shown in Fig. 5, and points to be addressed are listed in Table 1. Figure 6 shows the simulation results of the grounding-system evaluator, focusing on the fiber termination module (FTM) on the second floor of the new building and Figure 7 shows details of wiring errors. The simulation results show that the wiring between the FTM and SSF1 (a power supply distributor) is incorrect, and the FTM should be wired to Interface – B (IF-B).

Fig. 5. On-site investigation of the building and identified configuration points.

Table 1. Wiring status of key points.

Fig. 6. Simulation based on on-site investigation results.

Fig. 7. Details of wiring errors determined using the grounding-system evaluator.

3.3 Estimation of causes

On the basis of the results obtained from the investigation, the estimated invading route of the lightning surge that caused the failure is shown in Fig. 8 and Table 2. Due to the increase in the ground potential caused by the lightning strike near the telecommunications center building, a potential difference occurred between the building ground and tension-member ground. Thus, a lightning surge current was generated and propagated through cables, i.e., the lightning-surge current flowed from the ground cable into the LXM via the metal cables and flowed out to the ground cable and clock cable.

Fig. 8. Estimated lightning-surge route.

Table 2. Estimated lightning-surge route.

3.4 Lightening countermeasures

The countermeasures that were proposed against the above lightning damage in this case study are shown schematically in Fig. 9 and described as follows.

Fig. 9. The proposed lightning-damage countermeasures.

Countermeasure (1): To pass the lightning surge directly to the optical-cable tension member of the FTM, the ground cable between SSF-1 and the FTM should be removed, and the FTM should directly connect to IF-B in the new building.

Countermeasure (2): To pass the lightning surge flowing into the LXM from the clock cable, two IF-Cs are inserted into the clock cables crossing the buildings from the DSM and CSM, and the ground terminal of the IF-Cs should connect to IF-B in the old building.

Countermeasure (3): To make the SSF-2 and LXM equipotential, SSF-2 should connect to the LXM instead of SSF-1 and LXM.