It's not just RF power amps that have parasitics, and sometimes they can
cause people to die. The Association for Computing Machinery's newsgroup
on the risks associated with computing and electronics published this
fascinating article today:
: Date: Tue, 22 Sep 2009 23:36:06 -0400 (EDT)
: From: "David Lesher" <wb8foz@panix.com>
: Subject: More on the DC Metro collision 22 June 2009 (RISKS-25.73)
:
: The NTSB has issued an urgent interim recommendation re: the fatal Metro
: collision in June.
:
: <http://www.ntsb.gov/recs/letters/2009/R09_15_16.pdf>
:
: The letter discusses the failure they found:
:
: "Testing found that a spurious high-frequency modulated signal was being
: created by parasitic oscillation from the power output transistors in
: the track circuit module transmitter. This spurious signal propagated
: through the power transistor heat sink, through the metal rack
: structure, and through a shared power source into the associated module
: receiver, thus establishing an unintended signal path. The spurious
: signal mimicked a valid track circuit signal. The peak amplitude of the
: spurious signal appeared at the correct time interval and was large
: enough to be sensed by the module receiver as a valid track circuit
: signal, which energized the track relay. This combination -- of an
: alternate signal path between track circuit modules and a spurious
: signal capable of exploiting that path -- bypassed the rails, and the
: ability of the track circuit to detect the train was lost."
:
: and makes recommendations for WMATA, and other involved parties.
:
: Comment: Attention has long focused on the track signaling circuit that
: inexplicably failed to detect the stopped train. What was not know was
: why it failed; when AC track signals have been in use for a century.
: After a great deal of work on the scene and off, NTSB has part of the
: answer.
:
: The RISK here is this was and is the classic "all the eggs in one
: basket" protection scheme. It was a very sturdy basket, but...
:
: Now the issue is how to retrofit real redundancy into this system...and
: how to pay for it.
:
: [Doug Hosking noted a CNN item. PGN]
:
http://www.cnn.com/2009/US/09/22/transit.rail.alert/index.html?iref=mpstoryview
--
Mike Andrews, W5EGO
mikea@mikea.ath.cx
Tired old sysadmin
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