Mitigation method/Results/Conclusion It is assumed that a lightning surge on the mains exceeds the breakdown voltage of the power supply transformer (8 kVd.c.) and injects current limited surges into the payphone 21 Va.c. power supply. This type of damage was not detected during testing of the mains input at 6 kV 10/700 μs (enhanced inherent mains port test). The implemented solution is shown in Figure 2.6-2. A gas discharge tube (GDT) has been added to the low-voltage power connection to the electronics. This GDT is bonded to the frame earth, the same earth as the exchange line surge protective device (SPD), to reduce the level of stress which can occur between the low-voltage port and the telecommunication line port. Payphone cabinet Double-insulated ps 21 V a.c. Exchange line Power earth 10's to 100's metres from payphone Earth bar Miti(08)_F2.6-2 New GDT, shown in red, added to reduce port-to-port stress. Local earth electrode Figure 2.6-2 – Implemented solution It is necessary to be careful when using a GDT on an a.c. circuit as it may not switch off after being triggered by a lightning surge. If the GDT continued to conduct a.c. current, the power supply and the payphone electronics could be damaged, due to overheating of the GDT. There are a number of ways to overcome the problem of the GDT conducting a.c. current: • Use a special GDT for a.c. circuits. • Use a positive temperature coefficient thermistor (PTC) or similar between the GDT and the transformer. • Use an alternative mitigation technique. An alternative solution is shown in Figure 2.6-3 below. The GDT does not conduct a.c. current in this case, due to the low voltage (21 V) involved. Payphone cabinet Double-insulated ps Mains SPDs Exchange line 21 V a.c. Power earth 10's to 100's metres from payphone Earth bar Miti(08)_F2.6-3 Local earth electrode Mains SPDs, shown in red, added to reduce port-to-port stress. Figure 2.6-3 – Alternative solution References Recs ITU-T K.21 and ITU-T K.44. Case study 2.6 – 2 –