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Original poster: "Scott Hanson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

The addition of GFI's (ground fault interrupters) to neon sign transformers is
to achieve compliance with the 1999 National Electrical Code and UL standard
2161. These standards require certain transformers and power supplies to be
equipped with secondary circuit ground fault protection (SCGFP).  This system,
somewhat similar to the ground fault interrupters used for household AC
outlets, senses leakage current between the transformer secondary and ground,
and interrupts power to the transformer primary when the leakage current
exceeds a specific value.
The stated reason for this change is to prevent fires caused by arcing of high
voltage secondary currents to ground. 
(Several years ago I was on a cross-country trip which passed through Las
Vegas, Nevada. I was beginning my quest for 60 ma NST's for my first Tesla
coil. As Las Vegas is the "neon capital of the world", I had visions of piles
of used 60 ma NST's behind every sign shop. In truth, I found that everyone had
stopped using 60 ma NST's  years ago, specifically because of the long history
of sign fires cause by the higher-current transformers.)
Scott Hanson