Re: What's that noise?

[craigd@lios.apana.org.au (C. Dewick)]

In <373AF121.2E4A8EE5@ozemail.com.au> David Johnson <trainman@ozemail.com.au> writes:

>Gareth Lumsden wrote:

>> So basically it is the simple theory of electrons flowing, then 'jumping'
>> across the line-breaker when it is opening.  Much the same thing when a
>> tram is powering over a cut off point - A large arc 'jumping' between the
>> contact wire and the overhead.  Comeng trains just make a louder noise
>> because there is more voltage.  Have I got it right?

>More voltage, but more specifically more current.  Some high current DC CBs have
>a blast of air to blow the arc out an arc chute which also makes noise.

Which is what made the old red single-deck sets here in Sydney so much fun,
because the main CB's were *huge*, and sat under the middle of the floor of
the motor cars with the arc chutes facing out to the sides. 

When they tripped (such as when the driver shut off suddenly during a heavy
powering manouvure), you'd see a huge flash and an arc (much like a Jacob's
ladder, but controlled) writhe out of the body of the CB via the arc chute
as the air blast blows out the ionising path to break it quicker, and
prevent contact wear due to arc burning.

Regards,

Craig.

-- 
            Craig Ian Dewick            |       Stand clear - jaws closing
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