Re: Class 76 on 25kv ac

On 19 Apr, 11:25, Cathode Ray <RBCat...@xxxxxxx> wrote:
On 17 Apr, 22:38, Charles Ellson <char...@xxxxxxxxxxxxxxxxxx> wrote:



Isn't that counteracted by an AC arc having a tendency to
self-extinction 100 times a second ?

The circuit under consideration will almost certainly have reactance,
so the current will lead or lag the voltage so that current zero will
occur at a point on the voltage waveform where the voltage will tend
to sustain the arc.

To extinguish an arc in a air break circuit breaker, you need to
physically lengthen the arc to increase its resistance. The arc is
drawn away from the contacts by the magnetic interaction between the
fields of the arc and the conductors of the switchgear. In lengthening
the arc, it gets cooled and so the ionisation of the arc gases is
decreased, increasing the resistance of the arc and so on until the
current extinction conditions are met.

The arc extinction principle applied equally to DC and AC currents. AC
currents are more easily extinguished than DC current becuase of the
current zero as you have mentioned, but to some extent this is offset
by the reactive nature of the AC circuit which tends to encourage
restrikes within the arc chute.

Until the advent of vacuum interrupters for use in heavy duty
switchgear used for direct on line motor starting for power station
and process plant that required numerous switchgear operations every
day, air break switchgear was the preferred choice at voltages up to
3.3kV AC as it offered high speed operation and low maitnance when
compared to an oil circuit breaker. Very little of this AC switchgear
is now sold and the vast majority of air break switchgear is now sold
for DC traction purposes and for the likes of electrolysis plants.

The highest available voltage for air break switchgear is 3kV DC, so I
wouldn't fancy its chances in attempting to break a 25kV AC current.

Ray - Current zero is for wimps

P.S. If the NRM are willing to provide their 76, I'll provide the 25kV
plus the fire brigade. Short circuit tests are always such fun.


Hello,

As I seem to recall, most of the internal equipment has been removed
from E26020.

Very Best Wishes,
Bob.
.

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