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.
It must also be remembered that telephone apparatus at this time was still very simple and were fitted with robust components. It was not until semiconductors were introduced that lightning protection was reconsidered.

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.
BT introduced a self restoring gas discharge tube that is fitted to all master LJU sockets.  These act as a short circuit for transient voltages and not lightning.  These protectors, known as Gas Discharge Tube 11B acts as a short circuit to high voltages but are ineffective with lightning, which should be shunted to earth.
On telephone systems the gas discharge tubes are connected to earth and thus shunt high voltages to earth.  Some telephone systems have very low value resistors in series with the line and these rupture in the event of a high voltage.  Many modems are fitted with this sort of protection and in my experience are very effective, although they do have to be replaced.

What can you do about it?
Lightning has a nasty habit of blowing semiconductors and low voltage components to bits. It can also degrade chips and cause intermittent faults.  I have seen circuit cards with flame marks across them and components blown off the board and this was not a direct hit, but induced voltages in an external cable!
Do not be under the allusion that the lightning protector in master sockets will be your saviour - it won't. This is because the device acts as a short circuit and not a shunt. Power can still get through!
If you have a Network Termination Equipment 5A (shown to the right), as fitted to many customer lines, then the Gas Discharge 11B which is fitted this socket is a waste of time.  The socket can be modified by connecting a good earth to the E terminal and installing a Gas Discharge Tube 21A in place of the 11B (the backplate of the socket has to be removed to expose the line terminations and discharge tube).  Even so - lightning can still get through if it is a nearby strike.

In problem areas I suggest the fitting of a fuse barrier (cheap - low current 10 ohm resistors will do) and specialist lightning arrestors or Discharge Tubes 21A. Better still disconnect your equipment during storms (power and telephone lines) remembering that lightning can jump distances if it sees a better earth path (this is why you should not stand under trees)!

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!
The system was eventually covered by an earthed wire mesh cage (a Faraday cage) and since then does not fail every time.

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!

 

 
 
BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: December 21, 2010

FM