School networks and electrical equipment damage

[John Watlington]

I considered sources such as James' theory, as well as someone
connecting one of the ethernet cables to line voltage, and neither
accounted for the level of damage you described.

But I can't agree more with James' point about building from the ground
up.   The first thing we used to wire up in a computer room was the
frame grounds --- with modern SOHO gear that all comes through
the grounded power plug.   But it has to be plugged in to be grounded
(i.e. protected).

Coincidentally, today I checked out the earth ground in the new hardware office
and wired up the workbench grounding in OLPC Boston's new digs in Davis Sq.

wad

On Jun 6, 2013, at 6:51 PM, James Cameron wrote:

> On Thu, Jun 06, 2013 at 01:58:58PM -0600, Daniel Drake wrote:
>> And the most surprising thing - we had not even turned on the network
>> yet, pending some electrical work. Everything was connected up except
>> one crucial link - the UPS was not plugged into mains power. So all of
>> this damage happened without any of the devices having a connection to
>> the mains.
> 
> Actually not plugged in?  The whole network was therefore either
> isolated from building electrical earth ("ground") or had a series of
> surreptitious connections, the critical one being the telephone line.
> 
> Hmm.  Lightning wasn't necessary.
> 
> Here's my theory:
> 
> Each long run of ethernet twisted pair becomes one side of a capacitor,
> the other side being the building wiring, piping, or structure.
> 
> The long run of telephone wire picks up static charge from wind,
> lightning ground currents, test currents from the telephone exchange
> or line workers, or induced currents from other subscribers or power
> network switching.  Even turning on many lights.
> 
> http://www.pololu.com/docs/0J16/all can give you an idea of what can
> happen.  Any of these events will induce a starting pulse of DC in the
> telephone wire, which is analogous to the length of DC power cable in
> the Pololu explanation; the inductor.  The low equivalent series
> resistance (ESR) capacitor can be thought of as the long cabling
> against the building.
> 
> As a result of the capacitor and the inductor, the voltage is
> amplified until it reaches the breakdown voltage of whatever is
> connected.
> 
> Having the UPS plugged in might have prevented this voltage from
> finding a route through something more precious.  Instead, it might
> have found a route through a series of surge protection devices in the
> UPS, and then the only damaged equipment would be the ADSL modem.
> 
> The convention is to build from the ground up.  Don't plug the cables
> in until the ground is available, and then plug them in in strict
> order.  People get away with not doing this because the damaging pulse
> isn't constant.
> 
> (Reminds me of the time that I pulled a UPS input power plug out
> instead of just turning it off.  A bad idea.  The last pin to separate
> was active.  The connected equipment lost ground reference, the only
> ground reference that remained was through a device, so it took a full
> hit and died.  The resulting current passed into the building ground,
> and triggered the earth leakage breaker on the circuit that the UPS
> was originally connected to.  Other equipment connected to that
> circuit powered down.)
> 
> (Reminds me also of working for a cable contractor in the 1980s,
> looking after their cable management system on a VAX ... they were
> putting millions of cables into a power station, and the build was
> done from the ground up; that is cables were tracked as to whether
> they had been terminated yet, and the list of unterminated cables was
> a special report from the database that they always wanted to see.)
> 
>> I have seen that some UPSs (unfortunately not these ones) allow a
>> phone line to be passed through them, supposedly offering some
>> protection. Would such a system protect against a lightening bolt,
>> assuming thats what happened here?
> 
> Yes, but only if the UPS was earthed.  It would also protect the ADSL
> modem.  It would also protect from most other causes of a current
> pulse arriving on the phone line.
> 
> -- 
> James Cameron
> http://quozl.linux.org.au/
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