callahan@netman.DEC (JOE CALLAHAN TWO/E17 DTN247-2843) (08/02/85)
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Several recent entries have referred to damage done to modems, terminals and
computer hardware by voltage surges (apparently from lightning) on telephone
lines, and have asked about or suggested sources for protective devices. Some
facts (now fifteen years old but probably still correct) to keep in mind when
trying to deal with this type of problem are:
- telephone lines (at least the ones connected to aerial cables) are normally
connected to surge arrestors at the point where the line enters a building, but
these protectors (gas discharge tubes or the much older carbon block spark
gaps) do not fire until the conductor to ground voltage reaches 300 VOLTS!
This has come as a rude surprise to the designers of many a solid state
telecom device.
- lightning is not the only source of such surges. If you are located within a
few hundred yards of an electric power generator or sub-station, a major fault
on any of the high-voltage lines emanating from it can cause local earth
potential to change dramatically relative to that of your (presumably remote)
telephone office and cause a significant current to try to flow from your
carefully "grounded" equipment through the telephone wires to the "ground" at
the C.O. The voltages involved are high enough to be a serious safety hazard,
and telephone companies have special isolation or neutralizing transformers
they install on their drops to power stations for this reason. (I wonder what
those inductors do to data?)
(Usual disclaimers re: these being my own and not my employer's opinions)
/s/ Joe Callahan
(DEC Easynet) NETMAN::CALLAHAN
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