On Wed, 3 Dec 2008 00:19:41 -0000, wrote:

"D7666" wrote in message
...
On Nov 30, 4:47 am, Charles Ellson wrote:

relays drop out causing an alarm;

reset up to three times in
succession before someone is sent

Nope. Respond at first alarm with an emergency fault. At least thats
how we do it on ''my'' bit of underground infrastructure.

--
Perhaps there is a risk that something happened, and people might be on or
around the tracks? You don't want to accidentally fry them.

Going back to the ancient version that I experienced, the "reset up to
X times before sending someone out" was to allow for transient faults
caused by wind-blown metallic debris, flowing/pouring water etc.

There should not be anyone on the tracks unless train crew have let
them out after taking appropriate measures which would include
application of a SCD; this should result in immediate re-tripping when
it is attempted to reset the supply breaker rather than the delayed
tripping which might be caused by debris or dangling bits of train.
AFAIR (this was before radio was in use) evacuation was allowed to
take place as immediately as necessary in the absence of
communications after X seconds had passed since the application of the
SCD to give time for the "reset X times then leave the power off and
investigate" procedure to be achieved; otherwise if communication was
available and/or there was no immediate need to evacuate then things
proceeded as required/appropriate.