LIGHTNING and TELEPHONE EQUIPMENT
|LIGHTNING CAN DAMAGE YOUR WEALTH
A small section on what lightning can do and how to try and avoid too much damage.
In the early days of Telecoms all the telephone instruments had a simple lightning protector on them. These consisted of two pieces of metal aligned closely to each other. The customer would place a metal tipped plug between the two metal plates during storms. Considering the robustness of telephones in those days, one wonders why they did this. The answer is that with long lines made of large diameter iron or copper wire (especially in America), these were an ideal electrical conductor in a lightning storm and as the wiring was so substantial it was not likely to fail after being hit. Small gauge wire would act like a fuse and rupture. There was also a likelihood of the wires coming into contact with tram lines or the newly installed mains electrical system.
Early this century the protection fitted to telephones was discontinued and the protection was fitted externally to the telephone. At both ends of the line fuses and mica protectors were fitted. This involved an earthed protector unit with fuses on the customer premises and in the exchange, fuses, protectors and heat coils. The heat coils were to guard against low voltage contact (250 volts).
These were a liability and needed maintenance after storms. The BPO decided in
the 60's that lightning protection could be dispensed with as the cost involved in
replacing protection was greater than the damage to apparatus. They replaced fuses
with dummies and removed mica protectors. The heat coils were left in place as these
did not blow with high voltages.
Semiconductors do not like high voltages as this will degrade the internal electrical
junctions of the device. In many cases the chips survive the voltage but do not
operate to their original specification. Transistors can also suffer as I once
replaced every Trimphone in an area a quarter of a mile diameter.
What can you do about it?
I know of one site where the system processor blew at every storm. The building
was metal clad and acted as a conductor for the lightning and in one storm the lightning
discharged from the apex of the roof (25 ft approx.) to the floor. Two employees in
that area were taken by surprise as they watched the discharge 15 foot away from them,
inside the building!
It's generally better to stop than shunt.
If you fit a Gas Discharge Tube No. 21 to a NTE No. 5 - remember to connect an earth to the E terminal.
Strowger of course is highly resistant to lightning, but remember that a close hit may take out an AC relay coil! Now, you didn't want that to happen - did you!
Last revised: December 21, 2010