Case Study of Malfunction in Metallic Subscriber Loop Carrier Equipment

(1) Traces of electrical discharge were observed on the primary contacts and back of the enclosure of the ELB for power reception (Fig. 2).

(2) Traces of electrical discharge were observed near the primary contacts and on the back of the enclosure of the ELB for wireless equipment (Fig. 3).

(3) Regarding the surge arrester of the UMC power-supply unit, we measured the operating voltage of the varistor with an arrester DC tester (Model 4K1M). The measurement results indicate that the varistor located between the surge arrester and ground was in an open state, i.e., not operating properly. An X-ray inspection system (Shimadzu Xslicer SMX-1020) was then used to look through the internal wiring condition of the surge arrester. The X-ray results indicate that the wiring of the varistor located between the surge arrester and ground was disconnected (Fig. 4).

(4) Rust and deterioration of the waterproof seal were observed on hinges of the service-entrance switchboard (Fig. 5).

(5) Traces of water penetration were observed at the bottom of the service-entrance switchboard (Fig. 6).

(6) To confirm the installation environment of the equipment, we referenced the data of wind direction and speed over the past 10 years at that remote island (Fig. 7). Figure 8 shows the equipment-installation location and wind directions. Since the location is subjected to strong winds from the northeast and northwest, and since the hinges of service-entrance switchboard are on the northeast side, it was presumed they are affected by sea breezes.

(1) During a large-scale power outage (September 2022), the first malfunction occurred. It was presumed that the switching surge at the time of power restoration passed through the power equipment and flowed into the ELBs for power reception and for wireless equipment. Therefore, a large current flowed between positive and negative power lines in the ELB for wireless equipment, which tripped the ELB for power reception (malfunction cause (1) shown in Fig. 9). It was also presumed that at that time, electrical discharges occurred on the primary contacts of the ELB for power reception and inside the ELB for wireless equipment.

(2) Frequent malfunctions of the ELB for power reception occurred after the first malfunction. The insulation resistance between the primary lines of the ELB for wireless equipment was reduced by half from the required 0.1 to 0.05 MΩ. It was thus assumed that the ELB for wireless equipment was prone to current leakage from positive power line to negative power line, and the ELB for power reception was prone to being tripped by the leakage current. The decrease in insulation resistance may also be partly due to water intruding into the service-entrance switchboard, as indicated by traces of water penetration in the switchboard (malfunction cause (2) shown in Fig. 9).

(3) It is unknown when the wiring of the varistor between the power line and ground for the UMC surge arrester became disconnected; however, it was estimated that the switching surge during the first malfunction was also a cause of the subsequent malfunctions (malfunction cause (3) shown in Fig. 9).

(4) The deterioration of the hinges of the service-entrance switchboard and water penetration to the switchboard are assumed to have occurred for two reasons: (i) the hinges and seals on the right side (northeast side) were more sensitive to deterioration due to sea breezes from the northeast than other areas of the switchboard and (ii) strong winds and rain from the northeast created an environment in which moisture can easily enter the switchboard.