School networks and electrical equipment damage

T Gillett tgillett at
Thu Jun 6 20:40:31 EDT 2013

Hi All

As James has pointed out, there are many ways to fry a network.
But having a telephone line connected to a modem which is in turn connected
to LAN cables is one of the most problematic arrangements.

I used to work for a telco where one of our jobs was providing phone
services into power substations where both lightning strikes and power
ground faults produce similar problems.

The problem is that the telephone line represents an earth at a point a
long way from the actual installation, nominally at the telephone exchange.

So a lightning strike anywhere in the area can produce a large voltage
differential between the phone line and the local equipment which is
effectively tied to the local earth (even if the mains is not plugged in).

A three element gas arrestor fitted to the incoming phone line and
**effectively earthed** to the centre electrode can be effective in
clamping the phone line to the local ground, thus preventing the equipment
seeing the differential.

But "effectively earthed" is the key phrase. The earth connection must be
low resistance and low inductance - so no long earth wires, the gas
arrestor has to be almost at the earth stake. In practice it is quite
difficult to get these installations done correctly so that they are really

One approach for 'repeat offenders' in problem areas, in addition to
fitting gas arrestors to the phone line and surge suppressors to the mains
to deal with as much of the problem as possible, is to isolate the ADSL
modem from the rest of the local network. A wifi link is one effective way
to do this.

Have no LAN cables at all connected to the modem. Use a wifi link from the
modem to the network switch, then run cables from the switch.

In the event of a lightning strike, the modem is sacrificed, but the high
voltages from the phone line can't get into the rest of the equipment. (And
keep a spare modem, pre-configured in the cupboard).

A similar issue exists with running LAN cables between buildings on a
campus. Long LAN cables make great aerials as James points out, and will
pick up a lot of energy from nearby lightning strikes. One solution is to
use wifi to link buildings rather than cables. Fibre optic links are also
very effective.

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