The 1996, 1999 and 2002 editions of the National Electric Code (NEC) require the use of neon transformers with secondary-circuit ground-fault protection (SGFP).  The intent of the NEC is to reduce neon fires by requiring a transformer to turn off if it detects a significant current to ground.  In 1996, UL published the new standard for neon transformers (UL2161).  This standard defined the SGFP transformer performance requirements mandated by the NEC. 

For clarification . . .

WHAT IS SECONDARY GROUND FAULT?

An abnormal condition causing current (defined by UL as 15mA or greater) to flow from the secondary circuit (high voltage sign components) to earth ground.

SGFP(SECONDARY GROUND FAULT PROTECTION)

SGFP requires that many neon transformers utilize a secondary ground fault interrupter (SGFI).  If a secondary ground fault occurs, the SGFI detects the fault and interrupts the high voltage output.  In other words, it turns the sign off to prevent a fire or any other safety hazard in the sign.