[AMPS] Another arc question

Ian White, G3SEK G3SEK@ifwtech.demon.co.uk
Fri, 8 May 1998 13:51:52 +0100

Rich Measures wrote:
>>I disagree. A microamp hipot tester will not be capable of any plasma
>>generation..if you want to try and duplicate the fault conditions.
>My highpot tester has plenty of current capability to create a gas plasma 
>in gassy tubes.  The bluish plasma glows plainly in a darkened room.    

But that's only a high-voltage, low-current glow discharge. The highpot
tester cannot create a low-voltage, high-current arc. Only the B+ supply
has the muscle to do that.

As far as I can see, the damage mechanism has to involve an arc powered
by the energy stored in the B+ supply. We can be sure that the filament
doesn't have the emission capability to supply enough electrons for the
current involved, and the space charge in the vacuum doesn't store
enough electrons or enough energy.

An arc can start from an extremely small release of gas. It then feeds
itself from ions ripped off the metal surface, but after a short time
these will 'plate out' on the inside of the tube.

There should be some visual evidence of surface erosion caused by an
arc, but there is no chemical reason to expect a vacuum arc to cause
blackening. It could be a very small spot. Maybe it's also more visible
to people who want to see it, than to people who want not to...
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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