neons for phase sequence?!

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http://en.wikipedia.org/wiki/Industrial_&_multiphase_power_plugs_&_sockets
The top receptacles shown in the figure, are fitted with neon night lights to indicate phase sequence, for triplex loads where proper phase sequence is desired.
200px-Triplex-electrical-socket-configuration.jpg

anyone know how neons can possiblly show phase sequence?
 
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By having an R-C between any two phases, and a neon between the junction and the centre point of an R-R between two more. The neon is out if the RC-phase advance cancels the phase advance between the other two, and bright if it is anti-phase, thus indicating rotation.
Choose R and C to give a 60 deg shift at 50 or 60Hz aas appropriate of course.


Clever juxtopostion of multipes of this circuit allow self rotation numbering lights to be produced.
Note that the sockets with the neons in are not wired to the phases directly but to the phase shift Rc networks that makes it all happen.
Let me know if I need to draw it as a schematic.
regards mike p-j
 
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